Insect form and function is a vast field of knowledge that covers the relationship between the structure and physiology of insects and how they interact with their biotic and abiotic environment to survive and reproduce. Foundational textbooks in this area have been written by Snodgrass (1935) and Wigglesworth (1950) and comprehensive updates have been written by Klowden (2013) or incorporated within contemporary general entomology textbooks, such as by Gillott (2005). In the last thirty years, insect physiology has increasingly been studied through the lens of molecular biology, genetics and genomics (Hoy 2018). It is obviously too large a body of knowledge to address in depth in a single chapter concerning (or in a book on) forest entomology and the reader is invited to consult these works for a deeper understanding of the topic.

Advances in insect physiology have been dependant on the study of a few “model” insect species that provide plenty of biological material at a reasonable cost, usually reared under controlled laboratory settings. As very few insects relevant to forestry fit these requirements, the physiology of insects is largely known from agriculturally- and medically-important species. Nevertheless, the major organs and mechanisms by which specific insect physiological systems operate are conserved across species, such that generalizations can be made.

2.1 Insect Development

Developmental trajectories in insects and related hexapod taxa can be classified along three main types, depending on the degree of morphological difference between immature and adult stages, and the presence or absence of metamorphosis. In ametabolous hexapods, larvae reach the adult stage through a series of molts without exhibiting significant morphological changes, except for the presence of genitalia in adults. Metamorphosis is absent in these insects and molting can continue to occur in adults. This type of developmental program is restricted to the basal hexapod orders Protura, Diplura and Collembola, all soil-dwelling organisms.

Hemimetabolous insects also grow through a series of molts in which the nymphs (immatures) are morphologically similar to the adults. However, adults gain their wings and genitalia after a single molt. Hemimetabolous insects are represented by eleven recognized orders (Song et al. 2016), including some of the largest such as Hemiptera (true bugs, scale insects, aphids) and Orthoptera (crickets and grasshoppers).

Holometabolous insects display radically different morphologies between immatures and adults. The transformation between immature and adult takes place in what is called the pupal stage, a generally inactive stage where extensive organ and tissue remodeling occurs. Complete metamorphosis has been deemed an “evolutionary innovation” among insects that enabled the occupation of different ecological niches by adult and immature forms, and explains some of the evolutionary success of the major modern insect orders i.e. Coleoptera, Lepidoptera, Hymenoptra and Diptera (Ureña et al. 2016).

2.1.1 Eggs

In most insects, progeny are deposited in the environment in the form of eggs, a reproductive pattern known as oviparity. Insect eggs consist of a developing embryo accompanied by yolk, a maternally-secreted substance rich in proteins that fuels growth until hatching. The embryo and the yolk are enclosed in protective layers originating from two sources: a maternally-derived eggshell synthesized before fertilisation and two epithelia, the amnion and the serosa, produced by the embryo. The eggshell is comprised of an innermost vitelline membrane on top of which sits the chorion, a multilayered, proteinaceous cover. In some species a wax layer is present between the vitelline membrane and the chorion to prevent desiccation (Klowden 2013). Egg chorions vary in the number of layers and internal architecture between species, but the presence of meshwork of airspaces between the inner and outer chorionic layers is common. These pockets of air facilitate gas exchange for the developing embryo and are connected to the outside world via openings called aeropyles. The eggshell also presents a micropyle, an opening that enables access of the sperm to the egg (Zeh et al. 1989). The embryo-derived amnion envelops the ventral side of the embryo, while the serosa surrounds both the yolk and the embryo, just underneath the vitelline envelope (Panfilio 2008). The amnion and the serosa provide additional protection against desiccation, act as a barrier against microbial pathogens and in many insects one or both layers can synthesize a chitinous cuticle to enhance mechanical rigidity (Jacobs et al. 2013; Rezende et al. 2008.).

Eggs often require further maternally-derived products to ensure their survival. Glue-like secretions from the ovipositor can be added by the female to attach the egg to a substrate, such as the abaxial surface of leaves. Some Lepidoptera also cover their eggs with hairs or scales to deter potential predators (Floater 1998). The forest tent caterpillar (Malacososma disstria) attaches eggs in masses around branches of host trees, using a foamy substance called spumaline that also protects the eggs against parasitism (Williams and Langor 2011).

2.1.2 Viviparity

In some insects, embryonic development proceeds inside the female and free-living nymphs or larvae are laid instead of eggs. This reproductive pattern is known as viviparity and is classified in four types depending on the amount of yolk, the body cavity where the embryo is incubated, and the manner in which supplementary nutrients are acquired if yolk is absent or reduced. Ovoviviparity and pseudoplacental viviparity are by far the most common types in insects of relevance to forestry. Ovoviviparous insects produce embryos covered with a thin and elastic eggshell that also encloses yolk. Eggs hatch in the uterus after a period of incubation. Species from various lineages employ this reproductive pattern, including thrips (Thysanoptera), Lepidoptera, Coleoptera, Hymenoptera, Homoptera and several families of Diptera such as parasitic tachinid flies (Hagan 1948). In pseudoplacental viviparity embryos also develop in the reproductive tract but a significant amount of nutrients is acquired through placenta-like structures of maternal or embryonic origin, in the latter from the amnion or serosa. All aphids (Aphididae) and some other Hemiptera reproduce in this fashion, along with a few species of barklice (Psocoptera). The other two types of viviparity are hemoceolic and adenotrophic viviparity. In hemocelic viviparity, embryos develop in the mother’s hemocoel and absorb nutrients from the hemolymph. This viviparity type is found among the parasitic Strepsiptera and in some gall midges (Cecidomyiidae). In adenotrophic viviparity, hatched larvae feed on nutritive secretions produced by maternal glands, and this form of viviparity is found only in some families of dipteran parasites of mammals (e.g. Tsetse flies, Hagan 1948).

2.1.3 Post-embryonic Development and Larval Morphology

Insect post-embryonic growth proceeds in discrete steps marked by the shedding of the exoskeleton, an event known as molting. The form assumed by immatures between two molts is called a larval or nymphal instar (or simply instar) preceded by a number identifying its order of appearance after egg hatching (e.g. 1st larval instar or 1st instar) (Chapman and Chapman 1998). The term “stage” generally refers to the major ontogenetic divisions of the life cycle (larval, pupal and adult stages), but here again the numbering system can be applied for nymphs and larvae (e.g. 1st larval stage). The term “stadium” is applied strictly in reference to the duration of an instar (Carlson 1983).

As mentioned previously, hemimetabolous nymphs molt progressively into adults, however in some taxa there are deviations from this basic growth pattern. In the Plecoptera, Odonata and Ephemeroptera, the aquatic nymphs (also called naiads) have gills that are lost at metamorphosis. In the thrips (Thysanoptera), whiteflies (Alyrodidae) and male scale insects (Coccoidea), the transition from nymph to adults is interrupted by immobile non-feeding stages which functionally resemble holometabolous pupae. There can be up to three such stages in thrips of the suborder Tubulifera, named propupa and pupa I and pupa II (Moritz 1997).

The larvae of holometabolous insects display a variety of morphologies but convergence of form is observed for many distinct taxa that feed on the same food type. In general, cryptic feeders such as leaf miners, skeletonizers, wood borers, gall-forming insects and endoparasitoids show the simplest overall shape, are most often apodous with greatly reduced sensory appendages and cephalic structures. Extreme minimalism is observed in Dipteran larvae, such as the Agromyzidae (leaf miners) and Tachinidae (endoparasitoids) where the only distinguishing feature of the maggots is the sclerotized cephalopharyngeal skeleton (mouth hooks) (Feener and Brown 1997; Teskey 1981). In parasitic Hymenoptera (e.g. Ichneumonidae), young instars are also apodous but the terminal segment can extend as a tail in which case larvae are called “caudate” or in addition show developed mouthparts, in which case they are termed “caudate-mandibulate”. The first instar of some species, particularly in the Cynipidae and Figitidae are termed “eucoiliform” for the long flexible processes that they present on their ventral side, whose function is unknown. Eventually these hymenopteran larvae transition to featureless, grub-like “hymenopteriform” shapes in later instars (Gordh et al. 1999).

The immatures of woodboring insect taxa are also generally larviform, but harbor more robust cephalic structures to consume hard woody substrates. In the Cerambycidae, larvae are cylindrical and slightly dorsoventrally compressed with a sclerotized head capsule retracted within the prothorax, with mouthparts oriented forward (prognathous, e.g. Eutrypanus dorsalis, Casari and Teixeira 2014). Buprestid immatures are typically elongate and the dorsoventral flattening is more pronounced (e.g. emerald ash borer Agrilus planipennis, Chamorro et al. 2012) while Curculionids are compressed along the antero-posterior axis (Chamorro 2019). The thoracic legs of woodborer larvae are usually small or absent, but in the latter case locomotion can be aided by protuberances present on the abdomen or the thorax, called ambulatory ampullae.

Soil dwelling insects that feed on roots or rotten wood adopt two distinctive larval shapes, the scarabeiform or elateriform-type. Scarabeiform larvae are comma-shaped, with highly sclerotized head capsules and developed thoracic legs, taking its name from immatures of the scarab beetle family. Elateriform larvae are slender, heavily sclerotized with powerful legs and mouthparts, adaptations which allow them to move rapidly in the soil and cope with abrasion stress.

Larvae from holometabolous insects feeding on aerial plant parts have the familiar “caterpillar” shape, also known as eruciform (latin eruca-: caterpillar). Lepidoptera, Trichoptera, some species of the basal Hymneopteran suborder Symphyta (sawflies), and Chrysomelidae adopt this form. Eruciform larvae are characterized by elongate and cylindrical bodies, three pairs of segmented thoracic legs and a variable number (2–5) of pairs of unsegmented abdominal prolegs, adaptations that allow them to move rapidly between patches of food (Kou and Hua 2016). They also have a head capsule and highly sclerotized mandibles to crush foliage or other plant structures (e.g. seeds, buds, cones). The mouthparts can be prognathous (oriented forward) or hypognathous (oriented ventrally).

The larvae of highly mobile insects often display a form called campodeiform, characterized by an overall flattened shape and well-developed legs and antennae (Krafka 1923). This shape is more often associated with the obligate or facultative predatory lifestyle of certain families in diverse orders (e.g. Carabidae and Staphylinidae in the Coleoptera, Chrysopidae in the Neuroptera, Winterton et al. 2018), but is also encountered among filter-feeding insects, such as in the Trichoptera.

2.1.4 Molting and Metamorphosis

Insects benefit from the presence of a chitinous exoskeleton, which acts primarily as a barrier against external biotic and abiotic insults. However, this barrier is incompatible with continuous growth. Insects, as well as all the arthropods, have solved the growth-protection conundrum by introducing molting as an elaborate mechanism to ensure that the exoskeleton is replaced rapidly. The cellular and molecular aspects of molting have been studied in representative species of several insect orders, particularly the Diptera, Coleoptera and Lepidoptera and the topic has been extensively reviewed elsewhere (Truman and Riddiford 2002; Belles 2011). Molting is a chain of events that culminates in the synthesis and tanning of a new cuticle. Two hormones orchestrate this process: the steroid 20-hydroxyecdysone (20E) and the sesquiterpenoid juvenile hormone (JH). 20E is normally present at low levels throughout the immature stages, but its titers increase and decrease rapidly as a “pulse” before each molt. Therefore, 20E determines the timing of the molt. JH for its part is present at high levels throughout the larval stages, but disappears as the larva reaches the species-specific critical weight necessary for metamorphosis (Nijhout and Callier 2015). Under high JH, a pulse of 20E directs the larva to molt into another larva, but in the absence of JH, a pulse of 20E will direct the larva to molt into a pupa and a pupa to molt into an adult.

The increase in 20E titers originates from the activation of neurosecretory cells in the brain that release the prothoracicotropic hormone (PTTH). PTTH activates the production of the 20E precursor ecdysone in the prothoracic glands and, upon reaching peripheral tissues (e.g. the epidermis), ecdysone is converted into 20E, the active version of the hormone. The physiological conditions that trigger the molting cascade vary between insects and can involve multiple stimuli announcing the need to “change suit”. Some hemipterans use the stretching of the abdomen that occurs after feeding as a cue to molt. In the hornworm Manduca sexta, the sensing of oxygen limitation to growing tissues is also a trigger, since the chitin-lined tracheal system becomes unable to adequately facilitate gas exchange as the larva grows in volume (Callier and Nijhout 2013).

Metamorphosis involves a much more extensive remodeling of the body plan unfolding over two consecutive molts (larval-pupal and pupal-adult). In Manduca, two pulses of 20E occur in the last larval instar. The first one, called the “commitment peak”, is a brief low amplitude elevation of 20E titers that irreversibly changes the gene expression program of epidermal cells from larval to pupal. The second peak, much larger, directs epidermal cells to synthesize pupal cuticle (Riddiford 1976). The morphogenesis of the adult appendages and internal organs during pupation varies substantially between endopterygote insects. In the Diptera, Hymenoptera and Lepidoptera, most larval tissues are completely dissolved (histolysed) while adult appendages such as the wings, legs, antennae and eyes, arise from the rapid growth and differentiation from clusters of cells of embryonic origin called imaginal discs. Likewise, many of the adult’s internal organs in these orders are formed from undifferentiated cells, the histoblasts. In the beetles (Coleoptera), metamorphosis is more progressive and is reminiscent of the changes observed in exopterygotes, with the most notable change being in the development of the adult flight mechanism (Gillott 2005).

2.2 Sensory Perception

Sensory perception involves the detection of electromagnetic radiation (vision), diverse chemicals (olfaction, gustation), temperature (thermoreception) and changes in mechanical pressure on or distortion of (touch, proprioception and hearing) the cuticle. Some insects, particularly eusocial species, can also use the earth’s magnetic field as a sensory input during foraging (Wajnberg et al. 2010; Fleischmann et al. 2018). Signals are detected by specialized sensory neurons that convert the stimulus into an electrical response (signal transduction) carried from the peripheral to the central nervous system (CNS) (Torre et al. 1995). The CNS integrates all these diverse sensory modalities to drive physiological and or behavioral responses.

The physiological and molecular features of sensory perception in forest insects have received a great deal of attention, particularly olfaction and vision in moths and beetles. In many insects, mating partners must be located over long distances (relative to insect body size) and, in herbivores, acceptable host plants must be found among a diversity of non-host species. Much of the impetus for the study of insect sensory physiology emerges from practical considerations. Senses that can detect stimuli over long distances, such as vision and olfaction, can be exploited for the survey of insect spatial and temporal abundance via visually attractive and/or semiochemical-baited traps (Grant 1991; Brockerhoff et al. 2006; Cavaletto et al. 2020; Thistle and Strom 2006). Additionally, pest management tactics that directly suppress insect populations based on these stimuli also exist (e.g. pheromone-based mating disruption).

In adult insects, olfaction is mediated primarily by antennae. Antennae are composed of three sections: a basal segment, the scape, anchoring the rest of the antenna to the cranium. The next is the pedicel, which acts as like a hinge joint between the scape and the last section, the flagellum. The flagellum is constituted of units called antennomeres (Minelli 2017). Beyond this basic segmental arrangement, antennal morphology is extremely varied among taxa, the result of natural and sexual selection (Elgar et al. 2018).

Insect antennae are populated by microscopic protruding structures, called sensillae, which serve to detect odor. A typical olfactory sensilla consists of a fine and porous hair-like extension (seta) rising from the antennal cuticle. The pores serve to let volatile odors in, where they will be dissolved in an aqueous fluid (the sensillar lymph) before reaching the plasma membrane of sensory receptor neurons located inside. As many volatiles are hydrophobic, their transport within the sensillar lymph is mediated by specialized odor-binding proteins (odorant-binding proteins, OBPs and chemosensory proteins, CSPs, Pelosi et al. 2018). Sensillae have been classified depending on their external appearance and internal morphology, such as the number of pores and seta shape. Sensilla type, location on antennae and differential abundance between the sexes are important pieces of information in order to understand the chemical ecology of a given insect species.

The ultrastructure of the antennae has been characterized using electron microscopy techniques in numerous forest pest insects of economic importance. Examples include the Dendroctonus bark beetle species complex [D. valens (Chen et al. 2010), D. frontalis (Dickens and Payne 1978) and D. ponderosae (Whitehead 1981)], the emerald ash borer Agrilus planipennis (Crook et al. 2008), the eucalyptus borer Phoracantha semipunctata (Lopes et al. 2002) and the brown spruce longhorned beetle, Tetropium fuscum (Mackay et al. 2014). Likewise, similar attention has been given to antennal sensillar structures in lepidopteran forest pest species. They include the teak skeletonizer Eutectona machaeralis (Lan et al. 2020), the spruce budworm Choristoneura fumiferana (Albert and Seabrook 1973) and the Chinese pine caterpillar Dendrolimus tabulaeformis (Zhang et al. 2013). A comparative analysis of the antennae of Trichoptera, and basal and derived Lepidoptera species, revealed that a relationship exists between the proportions of certain sensilla types, and the type of sex pheromone used (Yuvaraj et al. 2018).

Gustation is used to perceive surface chemicals that can mediate acceptance or rejection of food sources. In herbivore species, many types of sugars act as phagostimulants while plant secondary metabolites can act as deterrents. Gustation is mediated by structures analogous to the olfactory system, i.e. via sensilla housing gustatory receptor neurons. Most gustatory sensillae are located on dedicated sensory appendages around the mouth (labial and maxillary palps) and inside the mouth itself, but can be found in other locations including the tarsi and ovipositor (Seada et al. 2018). Gustatory sensillae have been studied in larvae of the spruce budworm, Choristoneura fumiferana. The L1 sensilla and the lateral styloconic sensilla (LST) located respectively at the tips of the maxillary palp and on the galea, enable the detection of sugars. While the L1 sensillum detects furanose sugars, LST detects pyranose-type sugars (Hock et al. 2007). Interestingly furanoses, either as monosaccharides (fructose) or as subunits of disaccharides (e.g. sucrose) are indicators of plant stress. Thus L1 sensilla may assist in the identification of vulnerable host trees (Albert 2003).

Insect vision is accomplished by two types of ocular structures: simple eyes and compound eyes. Two types of simple eyes are further recognized: the ocelli and the stemmata. Ocelli are located dorsally on the head and are present in many insect orders in both adults and larvae, although they are absent from holometabolous larvae (Stehr 2009). In Drosophila, ocelli are composed of a corneal lens located above a thin layer of corneagenous cells (which secretes the lens), itself located above a group of 80 photoreceptor cells (Sabat et al. 2016). In general, ocelli cannot form images at high resolution and serve mostly to perceive rapid and slow (e.g. day/night cycles) changes in light intensity.

Stemmata are simple eyes located on the lateral sides of the head of holometabolous insect larvae. Like ocelli, they are composed of a corneal lens and a layer of photoreceptor cells, but also present a transparent crystalline cone as an intermediate layer. The structural organization of stemmata is reminiscent of the ommatidia of compound eyes, and indeed good molecular evidence suggests that stemmata are derived from a compound eye ancestor existing before the split of the holometabola and hemimetabola lineages (Buschbeck 2014). In insects with rudimentary stemmata, these simple eyes fulfill a light intensity detection function similar to the ocelli, but on the horizontal plane rather than above the head. However, in some predatory insects such as tiger beetle larvae, stemmata are sophisticated enough that they can be used to locate and capture prey (Buschbeck 2014).

Compound eyes are present in adult insect species and in immature hemimetabolous species. They occupy the lateral portion of the head, and in some good fliers (e.g. Tabanid flies, species in the order Odonata) they can extend to meet on the dorsal section of the head. The basic functional unit of compound eyes is the ommatidium and its architecture is well conserved among insects (Friedrich et al. 2006). The external facet of the ommatidium, also called the corneal lens, is made of transparent cuticle. Situated directly underneath is another transparent structure called the crystalline lens, flanked by four secretory cells, the Semper’s cells. Both lenses form the dioptric apparatus that refracts incident light toward a layer of eight photoreceptor cells occupying the basal section of the ommatidium. The dioptric apparatus is sheathed by primary pigment cells, and secondary and tertiary pigment cells can occur in some species to surround the photoreceptors. Light sensation is concentrated in an area around the central axis of the ommatidium where the cell membrane of each photoreceptor come in close proximity, called the rhabdom, each cell contributing a “rhabdomere”. Rhabdomeres display dense microvilli and are enriched in opsins, the visual pigments responsible for the conversion of light into an electric signal. Insects active during the day and the night show important structural differences in their compound eyes. Daytime active insects have apposition compound eyes, where photoreceptors only receive light penetrating through the lens directly above them. In these insects the ommatidial pigment cells are optically opaque to light coming from neighboring ommatidia. In contrast, nighttime active insects have superposition compound eyes, where the walls of each ommatidium are made of transparent pigment cells. Superposition eyes enable the collection of a much larger number of photons by the photoreceptor cells (Warrant 2017).

Insects show enormous species-to-species differences in their ability to perceive the various properties of light, such as light intensity, spectral composition and polarization. Herbivorous insects present a variety of adaptations of their visual system, ranging from eye structure, compound eye facet arrangement and opsin gene content that match the requirements to select acceptable hosts. For instance, scolytine beetles display reduced numbers of compound eye facets, indicative of a secondary dependence on visual cues compared to olfactory ones (Chapman 1972). In contrast, buprestid beetles show extremely good visual abilities, mediated in part by opsin gene sequence diversity and expression patterns in photoreceptors (Lord et al. 2016). Several reviews on the anatomical and molecular adaptations of insect visual systems have been published elsewhere (Briscoe and Chittka 2001; Egelhaaf and Kern 2002; Warrant and Dacke 2011; Cheng and Frye 2019) and provide comprehensive treatments of this most complex sensory organ.

2.3 Food Acquisition, Consumption and Utilization

Food intake, processing and utilization take place along the subdivisions of the insect alimentary canal. Physical breakdown of the food into smaller particles is helped by the crushing action of the mandibles in leaf- and wood-feeding species, while obviously little such modification is necessary in sap-feeders. The extraction of nutriments then proceeds along the three main regions of the gut: the foregut, midgut and hindgut. An example of these broad alimentary canal divisions in the larva of a xylophagous insect, the Brown Spruce longhorned beetle (Tetropium fuscum), is provided in Fig. 2.1. Organic polymers such as proteins and cellulose are broken down into their respective amino acids and sugar units through the action of digestive enzymes. In some xylophagous insect species, cellulose degradation requires a supply of enzymes secreted by microbial symbionts, harbored in specific regions of the gut (Martin 1991).

Fig. 2.1
A 3-D image of a beetle's digestive tract displays the following components: proximal midgut, distal midgut, Malpighian tubules, and hindgut.

Photo credit: Susan Bowman, Natural Resources Canada

Morphology of the alimentary canal of the brown spruce longhorn beetle (T. fuscum) larva. Two major divisions of the midgut, proximal and distal, form the majority of the digestive system in this species. The Malpighian tubule openings define the boundary between the midgut and the hindgut.

Full size image

In insects, five mouthpart regions are involved in food manipulation: (1) the labrum, (2) the hypopharynx, (3) the mandibles, (4) the maxillae, and (5) the labium. Borrowing from vertebrate anatomy, the labrum and labium can be thought as analogous to the upper and lower lips respectively, while the hypopharynx would be closest to the tongue. The pair of mandibles and maxillae are positioned caudal to the labrum and their task is food crushing (primarily the mandibles) and manipulation. The morphology of insect mouthparts, which is classically illustrated by using chewing insects as examples (e.g. orthopterans, Coleoptera) is actually remarkably varied (Labandeira 1997). Insects that feed on a strict diet of liquids show a range of mouthpart shapes adapted to facilitate liquid uptake. Nectar-feeding hymenoptera such as orchid bees have “lapping” mouthparts that have evolved by greatly extending the palps of the labium and the lobes (galea) of the maxillary palps. Joined together, they form the proboscis, which encloses a similarly extended lobe of the labium called the glossa (Düster et al. 2018). Adult cytclorraphan Diptera have a highly specialized “sponging” structure (the labellum) formed by the fusion of labial palps. In adult Lepidoptera, the coiled maxillary galea are joined in a proboscis.

The foregut and the hindgut of immature insects originate from the embryonic ectoderm and for this reason are able to secrete a cuticle, just like the epidermis (Reuter 1994). The midgut for its part develops from the endoderm (Stainier 2002). The foregut is subdivided into four anatomical regions: the pharynx, the oesophagus, the crop and the proventriculus. Collectively these regions serve to further breakdown food particles and regulate the flow of food before its entry in the midgut. The pharynx is populated with sensory neurons and assists in the decision of rejecting or accepting food, while the oesophagus is primarily involved in pushing food down the alimentary canal. The foregut crop is generally present as an enlargement where food is stored and can be further processed before entering the midgut. The proventriculus acts like a valve to regulate food entry into the midgut. Cuticular hair-like projections present on the crop side of the proventriculus help separate food based on particle size (Serrão 2005). In some species of ants, the proventriculus is also instrumental in preventing bacteria from entering the midgut (Lanan et al. 2016).

The midgut is in most insects the primary site for enzymatic breakdown of ingested food and the absorption of nutriments. The midgut epithelium is constituted of two types of differentiated cells, the columnar cells and endocrine cells, whose renewal is ensured by undifferentiated stem cells (Caccia et al. 2019). The side of the columnar cells facing the lumen of the gut (called the apical side) is extensively folded into microvilli to increase the effective surface of secretion and absorption functions. As their name implies, endocrine cells assist to regulate gut function via the secretion of peptide hormones. These hormones can affect nearby cells (paracrine signalling) or other organs in the insect. An important structure present in the midgut, but absent from both the foregut and the hindgut is the peritrophic matrix. The peritrophic matrix is composed of a thin layer of proteins attached to chitin fibers (Tellam 1996). The structure of the peritrophic matrix is highly variable among insects, and in some groups such as many sap sucking insects, it is totally absent. The peritrophic matrix has several protective functions on the midgut epithelium, mainly from physical damage by food particles, but also from digestive enzymes and it also prevents the entry of pathogenic microbes such as viruses (Hegedus et al. 2009).

The insect hindgut is generally divided in three sections called the pylorus, ileum and rectum. The pylorus typically regulates the flow of contents exiting the midgut by way of a muscular pyloric sphincter (Dallai 1976). It is also the section where in most insects the proximal ends of the Malpighian tubules connect with the digestive system. In several insect species the ileum section of the foregut is involved in ion and water transport. In termites, the entire hindgut, including the ileum, has been extensively modified to handle the digestion of cellulose. Five distinct proctodeal segments are present (P1 to P5), with the ileum being the first one (Rocha and Constantini 2015). The third proctodeal segment (P3) harbors most of the microbial symbionts responsible for the enzymatic breakdown of cellulose and the hydrolysis of xylan (e.g. Nasutitermes, Warnecke et al. 2007).

2.4 Nervous System

The nervous system and the endocrine system function as coordination centers of the insect. The insect sensory system receives stimuli from both the internal and external environments, integrates this information within the central nervous system, and processes the information to make the appropriate responses to the stimuli. The insect nervous system is comprised of nerve cells or neurons that are very similar in structure and physiology to vertebrate neurons. They are elongate cells that propagate a nerve impulse (a wave of electrical depolarization of the cell membrane) from one end of the elongate cell (the dendrite end) to the other end (the axon end). Information is coded in the frequency and temporal pattern of nerve impulses and by which neuron the nerve impulses originate (e.g. the same pattern of nerve impulses coming from the optic nerve are interpreted by the brain differently than the same pattern of nerve impulses coming from the auditory nerve). Neurons are arranged sequentially (end to end) so that nerve impulses can be transmitted from one neuron to the next. When the nerve impulse reaches the axon end of the neuron, it stops; however, it causes the release of a chemical neurotransmitter from the cell membrane which then diffuses across the very tiny gap called a synapse between the two neurons. When the neurotransmitter reaches the dendrites on the other neuron across the synapse, it stimulates an electrical nerve impulse to travel down the length of the next neuron.

There are several different types of neurons. Sensory neurons are afferent neurons that conduct nerve impulses initiated at sensory organs towards the central nervous system (CNS). Motor neurons are efferent neurons that carry nerve impulses from the CNS and transmit them to the effector organs (e.g. muscles or glands) to stimulate the effector organs to respond (e.g. muscles contract or glands secrete). Internuncial neurons are located entirely within the CNS and interconnect neurons with each other.

The insect nervous system is generally organized as a connected series of ganglia, a nerve cord, and peripheral nerves. Ganglia are groups of neuron cell bodies. The segmental ganglia (“segmental brains”) are the functional units of the central nervous system and individual segments often have their own ganglion. This reflects the ancestral trait organization inherited from the proto-annelid ancestors of arthropods. However, there is fusion of adjacent segmental ganglia into larger compound ganglia in many advanced insect taxa illustrating a higher degree of centralization (Niven et al. 2008). Synapses, the connections between neurons, occur only in ganglia; consequently, all communication between neurons takes place in the ganglia. The ganglia are where sensory information is received and processed and where the appropriate motor responses are initiated and coordinated. Segmental ganglia function much like “segmental brains”. The ventral nerve cord is a paired structure that connects ganglia with one another and allows ganglia to communicate and coordinate with each other. Peripheral nerves enter and leave the ganglia. These neurons innervate the various parts of the body (sensory organs, muscles, glands, etc.). They bring sensory information from the body into ganglia and transmit motor signals from ganglia out to parts of the body.

The insect central nervous system has a number of specialized ganglia. The brain is a fusion of ganglia of the anterior-most segments of the body. The brain controls movement of the antennae and labrum, receives some of the most important sensory information from the eyes, ocelli, antennae, and labrum, and is the most important ganglion for processing that sensory information and initiating the appropriate behavioral or physiological responses. The brain has considerable influence over other ganglia and plays a major role in coordinating the ganglia so that the insect functions as a unit rather that a collection of individual segments. The sub-esophageal ganglion is a fusion of mandibular, maxillary and labial segmental ganglia. It receives sensory information from these mouthparts and coordinates and initiates their movement. Thoracic ganglia receive sensory information from legs, wings, and other structures on the thorax, and coordinate and initiate their movement. The ancestral condition for the thoracic ganglia is one per segment but they are sometimes fused together as a compound thoracic ganglion in more advanced groups. Abdominal ganglia receive sensory information from abdominal structures, and coordinate and initiate their operation.

2.5 Epidermis and Cuticle

The integument is the outer body covering of arthropods and functions as an exoskeleton. It is one of the major reasons why arthropods are the most diverse and successful Phylum. The exoskeleton gives arthropods an enormous advantage over other invertebrates due to the efficient locomotion that a skeleton can provide. The lever-like mechanics of the exoskeleton allows a small muscle contraction to cause a large movement of an appendage. The arthropod integument serves as rigid skeleton, provides tough protective covering (armor), gives protection against water loss (an absolutely critical necessity for terrestrial organisms), and facilitates perception of the environment through sensilla embedded in the cuticle.

The insect integument has a characteristic layered microstructure. There are two major divisions: (1) the interior epidermis, the living, cellular part, and (2) the exterior cuticle, the non-living part secreted by the epidermis. The epidermis is a single layer of cells beneath the cuticle comprised of several cell types. The epidermal cells secrete the non-living cuticle and the dermal glands secrete defensive fluids, pheromones, etc. to the exterior surface of the body. There are also specialized epidermal cells that form at least part of many sensory organs located on the integument. The cuticle is non-living and comprises the bulk of the integument (Moussian 2010). The cuticle is initially secreted as procuticle that differentiates into the endocuticle and exocuticle layers. Endocuticle is located immediately above the epidermis and is tough and flexible. The major chemical constituent is chitin, a complex carbohydrate similar to cellulose that forms fibrils. The chitin fibrils of the procuticle are laid down in distinct layers with the fibrils within each layer oriented in the same direction, but the orientation of each layer is at a slightly different direction. This provides a lot of structural strength and is similar in principle to why plywood is so strong: the grain of each layer in plywood is oriented in a different direction. The exocuticle layer is above the endocuticle and is tough and rigid. In addition to flexible chitin, the other major chemical component is sclerotin (a protein that binds with the chitin fibers). The sclerotin bound to the chitin fibrils becomes cross-linked to each other by quinones; this prevents the chitin fibrils from moving relative to each other and thus the cuticle is no longer flexible, it becomes hard and rigid. The cross-linking process is referred to as hardening or as sclerotization. The third layer, the epicuticle, is the exterior covering of the cuticle. In insects, this layer is very thin, but very complex. It is composed of four separate layers including a waterproofing wax layer (a critical adaptation to terrestrial life) and a cuticulin layer that provides a critical protective barrier during molting.

The macrostructure of the integument is organized into sclerites and membranes, which join adjacent sclerites and enable the articulated movement of a rigid exoskeleton. Sclerites are hardened plates comprised of heavily sclerotized exocuticle. Membranes are flexible areas composed mostly of flexible endocuticle. The sclerites provide protection while the membranes provide flexibility of joints.

As mentioned earlier in this chapter, there are a number of advantages and disadvantages of having an exoskeleton relative to an internal skeleton. An exoskeleton is lightweight and strong; the tubular structure provides maximal strength with minimal skeletal material. The exoskeleton also provides a protective armor covering. However, the exoskeleton must be periodically shed (molted) in order to permit growth; even unsclerotized cuticle is relatively “unstretchable”. The necessity to molt, limits the maximum size that can be attained. Immediately after a molt, the integument is not yet hardened (because it must expand to stretch bigger than the previous integument); consequently, if the arthropod is too big and heavy, the integument will not be able to support the body weight right after a molt. It will bend, buckle and distort, and then eventually harden in a malformed shape. This limitation is more restrictive for terrestrial arthropods than for aquatic or marine arthropods.

The process of molting is critical in the life history of insects. At the start of molting, the epidermis separates from the endocuticle, a process referred to as apolysis, and epidermal cells divide to increase their number in order to accommodate the upcoming larger body size. The epidermal cells expand in size so that the new epidermis is now larger than it was before. However, in order to fit within the confines of the old cuticle, the newly expanded epidermis is folded and pleated beneath the old cuticle. Next, the epidermis secretes the cuticulin layer of the new epicuticle. The epidermal cells then secrete molting fluid that passes through pores in the cuticulin into the gap between the epidermis and the old cuticle; molting fluid contains enzymes that digest the old endocuticle. As the old endocuticle is being digested by molting fluid, the new procuticle is synthesized and laid down by the epidermal cells. Much of the breakdown products from enzymatic digestion of the old cuticle are absorbed by the epidermal cells and recycled for the synthesis of the new cuticle; this conserves a lot of the building blocks from the old cuticle, but it is not enough to complete the new cuticle, so additional building blocks are also synthesized by the epidermis.

The cuticulin layer plays a critical role as a barrier between the molting fluid and the newly forming cuticle; without the cuticulin layer, the new cuticle would also be digested by the molting fluid. Only the endocuticle of the old cuticle is digested by the molting fluid. The exocuticle resists digestion (probably due to the sclerotin) and remains as a thin shell enclosing the insect, which must be shed. Following the deposition of the new procuticle, the insect then expands its body, mostly by muscle contraction. This expansion causes the old exocuticle to split open, allowing the insect to escape the confines of its old cuticle.

The process of shedding the old cuticle is called ecdysis, and old cuticle which is shed is called an exuvium. The old cuticle splits along pre-formed lines of weakness, called ecdysial sutures, generally occurring longitudinally along the dorsal midline. Ecdysial sutures are simply a line along the cuticle where there is a sharp break in the exocuticle and the endocuticle fills this break; as long as the endocuticle is intact, this is not a weak spot, but when the endocuticle is digested during molting, this becomes a line of weakness that splits apart when the insect begins to expand its body. The insect then continues to expand its body by muscle contraction, pumping blood into its extremities (i.e. legs, wings, antennae) and swallowing air (or water for aquatic insects) until the folds and pleats in the new cuticle are all stretched out and the insect has reached its new, larger size. The procuticle is still completely flexible and non-rigid, so it cannot function as an exoskeleton; consequently, the insect is very vulnerable at this point. Shortly after the insect expands its new cuticle, quinones are secreted by the epidermis which crosslink the sclerotin-chitin complex in the exocuticle, making the exocuticle hard and rigid with all the functions of an exoskeleton and the insect can efficiently move around.

2.6 Neuroendocrine System

Insects have a complex endocrine system that regulates physiology and behavior. Hormones are chemical messengers secreted into the insect body (Gilbert 2011). They are released by cells in endocrine glands and cause a physiological response in target tissues. Many, but not all, of the endocrine glands are associated with neurosecretory tissues. They travel from endocrine glands to the target tissues by circulating in the blood within the body cavity. Hormones regulate many physiological and behavioral functions in insects. In addition to metamorphosis and molting described previously, other functions under hormonal control include heart rate, pigmentation, blood sugar level, egg development, water excretion, cuticle sclerotization, and diapause (insect version of hibernation).

2.7 Circulation and Immunity

The insect circulatory system almost never is involved in transport of respiratory gasses (O2, CO2) and there is nothing equivalent to our red blood cells in insects. The insect circulatory system is an open system. The body is a contiguous open cavity, called a hemocoel (“blood cavity”), from head to abdomen (Hillyer 2015). Blood is pumped from the posterior to anterior end of the body in the dorsal vessel. In the return trip (anterior to posterior), blood is not confined to vessels; it percolates through the hemocoel, bathing all the tissues and organs in blood. The dorsal vessel forms a long, narrow tube located along dorsal midline. The heart is the posterior part and is contractile, providing the pumping force to propel the blood forward. The aorta is the anterior part, and serves as an artery for blood to travel from the heart to the anterior end of the insect. When the heart fills with blood during the diastole phase of a heartbeat, alary muscles attaching the heart to the lateral body walls contract causing the heart to dilate. Blood flows from the hemocoel into the heart through a series of openings, called ostia, located on each side of the heart in the abdominal region. Ostia are one-way valves; they allow blood to enter the heart from hemocoel, but do not permit blood flow in the opposite direction from the heart out to the hemocoel. During the systole phase of a heartbeat, the muscles that form the wall of the heart contract, forcing blood out of the heart. Blood cannot be forced out of the ostia (one-way valves), so it must be forced out through one of the two ends of the heart (almost always the anterior end). Blood is pumped from the posterior to the anterior end of the heart by peristaltic waves of systole followed by diastole traveling the length of the heart from posterior to anterior end. This creates an area of high pressure in the head and an area of low pressure in the abdomen. After leaving the aorta at the anterior end of the body, blood simply flows through the hemocoel down a pressure gradient from head to abdomen, bathing the organs and tissues in a current of blood.

The appendages (legs, antennae, wings, etc.) are not in the main current of blood through the hemocoel so an additional mechanism is needed to circulate blood through the appendages. In the wings, blood circulates through some of the wing veins. Septa divide most other appendages longitudinally into two channels but these two channels connect at the apex of the appendage. When muscles in the appendage contract, they put more pressure on one side of the septum than on the other side, depending which side of the septum the contracting muscle is located. This creates a pressure gradient between the channels on either side of the septum, and blood will flow from the channel under high pressure to the apex of the appendage and into the channel on the other side of the septum. Some appendages have accessory pulsatile organs at their base to assist circulation (Pass 2000). The accessory pulsatile organs are contractile organs that pump blood into one of the channels along the septum causing a circulation of blood down one channel and back out the other; this makes circulation through the appendage very efficient.

Insect blood (hemolymph) consists of plasma and blood cells (Mullins 1985). Plasma functions to transport nutrients, hormones, and waste throughout the body. The blood cells (hemocytes) function in clotting (wound healing), phagocytosis of histolysing tissue (clean up broken-down tissues), immunity to microorganisms (fight infections), and encapsulation of parasitoid eggs (a very important defense mechanism against parasitoids) (Hillyer 2016). In encapsulation, blood cells aggregate around the parasitoid egg, cutting it off from nutrients in the blood and from access to oxygen. The phagocytosis function is especially active during molting and metamorphosis when some tissues and organs from the previous developmental stage that do not occur in the next developmental stage are being broken down.

2.8 Respiration and Gas Exchange

Insects breathe using a tracheal system comprised of a branching network of cuticle-lined tubes, called trachea, that reach every cell in the body (Locke 1957). Spiracles are the openings of the tracheal system to the atmosphere. The maximum number is 10 pair; each pair is laterally positioned (left and right) on the meso- and metathorax and on abdominal segments 1–8. The openings have valves that can be closed to reduce respiratory water loss. The valves are also used in controlling ventilation or the movement of air through the body. Trachea are the main air tubes extending from the spiracles to all parts of the body. These tubes branch extensively and as they branch, their diameter generally gets smaller and smaller. Trachea are reinforced with taenidia, which are thickened cuticular rings in the tracheal walls that strengthen the trachea and prevent their collapse. However, trachea do not function in gas exchange with respiring tissues. Gas exchange with respiring cells occurs via the tracheoles, which are very narrow diameter tubular terminal ends of the tracheal system. Tracheoles are not lined with cuticle and are anatomically, functionally, and physiologically different from trachea.

Ventilation of the insect body occurs through the longitudinal tracheal trunks. These are wide diameter trachea that run longitudinally along the body, connecting the trachea that originate at the spiracles in each spiracle-bearing segment, and also extend into the head where there are no spiracles. Generally, there are three pair of longitudinal tracheal trunks: lateral, dorsal, and ventral. The tracheal trunks are interconnected with each other via additional trachea. Tracheal air sacs are parts of tracheal trunks that are very wide diameter, enclosing a relatively large volume of air. These play an important role in ventilation of the tracheal system. When muscles in surrounding parts of the body contract, the air sacs get compressed, forcing air out of the sacks; when these muscles relax, the air sacs return to their normal, wide-diameter shape, drawing air into the sacs. The abdomen can compress and decompress by contraction and relaxation of dorsal–ventral muscles and dorsal longitudinal muscles; thus compressing and decompressing the tracheal air sacs in the abdomen. Thoracic tracheal air sacs are often in close contact with flight muscles and compress and decompress as the adjacent flight muscles contract and relax.

Movement of respiratory gasses (O2, CO2) through the tracheal system is a combination of diffusion and active ventilation (Buck 1962). Transport of O2 and CO2 between spiracles and tracheoles is very dependent on diffusion. However, diffusion is a relatively slow process and is efficient only over short distances. Active ventilation (mechanical air movement) of the tracheal system can reduce the reliance on diffusion to move O2 and CO2 between spiracles and tracheoles. Compression and decompression of air sacs can move air at least the distance from the spiracles to the air sacs. The rest of the route for gas movement (air sacs to tracheoles) still relies on diffusion. If air sacs move air in and out of the same spiracles, there is not a 100% air exchange due to the residual volume of air in the sacs and trachea (i.e. the sacs and trachea cannot compress down to zero volume). However, timing the opening and closing of spiracular valves to coordinate with the compression and decompression of air sacs can result in nearly 100% air exchange throughout the tracheal trunks. The trachea from all spiracles are interconnected via tracheal trunks. During a ventilation cycle, the anterior spiracular valves close and posterior valves open, and abdominal muscles relax causing the abdominal air sacs to decompress (expand) causing air to be drawn into the tracheal system through the posterior spiracles. When abdominal muscles contract causing the abdominal air sacs to compress, the anterior spiracular valves open and posterior valves close forcing air out of the tracheal system through the anterior spiracles. By this rhythmic coordination of muscle contractions with the opening and closing of the spiracles, a steady stream of fresh air flows through the tracheal trunks without leaving any residual volume of “old air”. The rest of the route for O2 and CO2 (tracheal trunks to tracheoles) still relies on diffusion.

2.9 Locomotion

The structures associated with insect locomotion are located on the thorax. In the adult insect, there are three pairs of legs, one pair associated with each body segment. The legs have five components; each element is unsegmented except the most distal. Beginning at the body, they are the coxa, trochanter, femur, tibia, and tarsus. The tarsus can be made up of 3–5 segments called tarsomeres. While insect legs have the same structural organization, the legs have been modified in form through natural selection to adapt to a wide range of life history strategies. For example, long and slender cursorial legs are adapted for running, natatorial legs with expanded and flattened oar-like femur or tibia are adapted for swimming, raptorial legs are adapted for grasping prey, saltatorial legs have enlarged femur with powerful muscles for jumping, and fossorial have shovel-like shapes for digging.

Insects are the only group of invertebrate animals that have evolved the capability of powered flight. The evolution of wings gives insects a significant advantage in exploiting their environment. Wings, when present on the adult insect, are found on the meso- and meta-thoracic segments, but never on the prothoracic segment. They are composed of two layers of integument (exoskeleton) with heavier veins in the wings providing stability and rigidity (Wootton 1992). Veins contain nerves, trachaea and haemolymph. Some orders of wingless insects, the Apterygota, evolved before the advent of wings, and within the winged orders, the Pterygota, some orders have lost their wings through natural selection (i.e. Siphonaptera). Like the legs, the wings have been subject to intense natural selection for adaptation to specific life histories. Consequently, there have been significant modifications. The winged Diptera have a pair of mesothoracic wings, but the metathoracic wings have been modified into club-like halteres that have numerous sensillae that respond to body position in flight. The mesothoracic wings (the elytra) of many species of Coleoptera are hard and rigid, protecting the underlying metathoracic wings and abdomen from physical damage and enabling the insects to use a wide range of habitats or niches. The leathery mesothoracic wings (the tegmina) of many species of Orthoptera and related groups have a similar function. The wings of the Lepidoptera are covered with scales that are often colored and can provide crypsis or advertise their presence, while the wings of the Thysanoptera are narrow and covered with long hairs that provide surface area for lift. The tiny parasitic Hymenoptera have greatly reduced wing venation.

2.10 Excretion and Osmoregulation Systems

In most insects, the excretory and osmoregulation systems involve Malpighian tubules working in concert with the hindgut. However, other organs such as salivary glands play a role in excretion and/or osmoregulation in some insects. The Malpighian tubules are hollow, blind ended tubes extending from the digestive system near the midgut/hindgut junction. The walls are 1 cell thick the number of tubules can vary from 0 to 250. Malpighian tubules generally float freely in hemolymph where they filter out wastes from the blood (analogous to vertebrate kidneys). They remove nitrogenous waste (usually uric acid), salts, and water from the hemolymph and transport them (the primary filtrate) into the hollow lumen of the tubule (Beyenbach et al. 2010). The contents of the tubule lumen flow to the base of the Malpighian tubule and empty into the gut near the hindgut/midgut junction. The Maligian tubules also function in reabsorption of vital salts; in order to maintain proper osmolarity of the blood, water and salts are selectively resorbed from the primary filtrate. Reabsorption takes place in the hindgut, and in some insects, it also takes place in all or part of the Malpighian tubules. As water is resorbed, uric acid precipitates out as a solid because it is not very water soluble; the precipitated uric acid mixes with the contents of the hindgut and is passed out the anus with the feces.

The insect fat body can also be important for excretion and osmoregulation. It is a very diffuse, amorphous organ located throughout the hemocoel, primarily in the abdomen. It generally appears as a mass of whitish or yellowish globules that float in the hemocoel and is continuously bathed in hemolymph. The fat body serves many different physiological functions including storage of fat, glycogen, and protein. In some insects, specialized fat body cells store nitrogenous waste such as uric acid. It also serves as a metabolic center controlling intermediate metabolism (e.g. amino acid conversions, glycogen synthesis and breakdown, fat metabolism, etc.). The fat body can also provide functions analogous to the vertebrate liver by detoxifying poisons and metabolizing hormones (Li et al. 2019).

2.11 Reproduction

The female reproductive system is located in the abdomen. It includes a pair of ovaries each made up of one, a few, or many ovarioles. The ovarioles are elongate tubes that are the functional unit of the ovaries and produce the eggs (Hodin 2009). As eggs develop, they travel down the length of the ovariole from the distal end (the germarium) where meiosis and egg cell formation occurs to the proximal end (the vitillarium) where eggs grow, accumulate yolk, and mature before they leave the ovariole and enter the lateral oviduct. A specialized storage organ, the spermatheca also opens into the oviduct (Pascini and Martins 2017). The spermatheca stores sperm for days to years depending on the species of insect; fertilization does not necessarily occur shortly after copulation. It has a valve to let sperm in during copulation and to regulate the release of sperm when eggs are ready to be fertilized. This is a critical fitness advantage for haplo-diploid insects (see below) like some social and parasitic Hymenoptera that can control the sex of their offspring. Fertilization is regulated by females that can withhold or release sperm when an egg is present. In addition to the spermatheca, accessory glands also release a variety of secretions associated with oviposition into the oviduct. Secretions from the accessory gland can produce egg cases which enclose and protect a clutch of eggs produced by some insects from desiccation, predators, and disease. The accessory gland can produce adhesive for eggs to glue the eggs to a substrate and produce venom for bee and wasp sting. In these cases, the sting is a modified ovipositor.

The male reproductive system is composed of a pair of testes that produce sperm, the vas deferens, which are tubes to transport sperm from testes to the ejaculatory duct, seminal vesicles that store mature sperm, an ejaculatory duct through which sperm leave the body, and accessory glands. The accessory glands in the male reproductive system produce a variety of secretions associated with copulation including seminal fluid, which is a liquid medium for sperm motility, but may also provide nourishment for sperm. Accessory glands also produce spermatophores which are enclosed packets of sperm and are thought to be an early adaptation for fertilization by terrestrial arthropods. Spermatophores do not occur in all insects. In many early terrestrial arthropods and insects, males deposit a spermatophore on the substrate and then the female picks it up off the substrate with her genitals. In these species there is no copulation associated with sperm transfer. In more advanced groups, fertilization became more efficient by the male directly placing the spermatophore into the female’s genitals. More derived insects have lost the spermatophore altogether, and the male has a penis to deposit the sperm in a non-encapsulated form directly into the female’s genital opening. In a few insect groups, accessory glands can produce “mating plugs”. These are gel plugs that seal the female’s genital opening after copulation to prevent other males from copulating with her, thus providing a mechanism for ensuring paternity.

There are many examples of insect groups that reproduce through sexual reproduction or through parthenogenesis (reproduction without fertilization). Sexual reproduction is the ancestral means of reproduction and it is still the most common strategy. Nonetheless, parthenogenesis has evolved independently in many different groups of insects, in some cases multiple times within groups. Hymenoptera (bees, wasps, ants), whiteflies, scale insects, thrips, and a few others have a rather unusual reproduction process; female offspring are produced by sexual reproduction and male offspring are primarily produced parthenogenically. In these groups, females develop from fertilized eggs (fertilized by standard sexual reproduction) and are diploid (2n chromosomes) while unfertilized eggs develop into males which are haploid (1n chromosomes), known as haplo-diploid sex determination. As a consequence, in many species with haplo-diploid sex determination, the mother can choose the sex of her offspring according to current needs. For example, female insects generally are bigger and require more food to reach maturity than male insects; consequently, many Hymenoptera parasitoids deposit male eggs (unfertilized) in small hosts and female eggs (fertilized) in large hosts. In many social Hymenoptera, workers are all female. The queen produces male eggs only right before the mating season. The rest of the year, she produces only female eggs.

2.12 Conclusions

Insects have successfully adapted to virtually every environment on the planet. Their basic body plan and physiology has been modified through evolutionary selection to allow them to exploit a wide variety of habitats and the ecological niches within those habitats. Many of the insect groups have highly specialized feeding ecologies, while many others are extreme generalists in their requirements. Most importantly, they have demonstrated exceptional capacity to adapt to environmental change. This has served the insects, as a taxonomic group, very well in evolutionary history and suggests that they have the capacity to adapt to the current pattern of global change.