﻿Stingless bee classification and biology (Hymenoptera, Apidae): a review, with an updated key to genera and subgenera

﻿Abstract Stingless bees (Meliponini) are a ubiquitous and diverse element of the pantropical melittofauna, and have significant cultural and economic importance. This review outlines their diversity, and provides identification keys based on external morphology, brief accounts for each of the recognized genera, and an updated checklist of all living and fossil species. In total there are currently 605 described extant species in 45 extant genera, and a further 18 extinct species in nine genera, seven of which are extinct. A new fossil genus, Adactylurina Engel, gen. nov., is also described for a species in Miocene amber from Ethiopia. In addition to the systematic review, the biology of stingless bees is summarized with an emphasis on aspects related to their nesting biology and architecture.


Introduction
In the tropical and subtropical environs of the world, one of the predominant lineages of social bees is the tribe Meliponini ( Fig. 1). They are popularly known as indigenous bees or stingless bees due to the atrophy of the sting, which is no longer functional as a defensive weapon. At around 605 species the stingless bees are the most diverse lineage of the corbiculate bees, a clade that includes the most iconic groups of bees throughout the world: honey bees (Apini), bumble bees (Bombini), orchid bees (Euglossini), and, of course, the stingless bees (Meliponini). They are managed for their honey, second only to the honey bees, and are growingly used for agricultural purposes (e.g., Heard 1999;Slaa et al. 2006;Jha and Dick 2010). Meliponiculture, just like apiculture and the burgeoning area of bombiculture, is a growing industry in tropical countries and aside from pollination services, bee products such as honey, propolis, resin, and collected pollen are all key to human food, health, and food security. Stingless bees, like honey bees, are also key to the cultural and religious practices of many ancient and current indigenous peoples, further emphasizing how the bees are key to the everyday lives of those living in the tropics. The purpose of this chapter is to briefly summarize the phylogeny and evolution, current classification, and general biology of stingless bees. Naturally, these subjects could occupy entire books in their own right and it is therefore impossible for any of these topics to be afforded sufficient justice or depth as to satisfy most readers. Therefore, the present effort merely attempts to whet the appetite of the mind and direct the reader to where more thorough information may be sought. In this regard, we would be remiss if we did not mention the recent and excellent tome by Christoph Grüter that covers the biology, evolution, and ecology of Meliponini in greater depth than we could ever hope (Grüter 2020). Beyond this book, excellent reviews are by Michener (2007aMichener ( , 2013, Melo (2021), and Quezada-Euán (2018), and although now somewhat dated the review of meliponine nest architecture by Wille and Michener (1973), the review of stingless bee sociobiology by Michener (1974), and those on their biology and evolution by Schwarz (1948), Wille (1979Wille ( , 1983, and Roubik (2023) all remain indispensable resources.

Phylogeny and evolution
Stingless bees are a long-recognized lineage of apine in the superfamily Apoidea, and belong to the clade of corbiculate tribes. The corbiculate bees are so named for the possession of a metatibial corbicula in females of non-parasitic forms (Fig. 2). In common parlance the corbicula is the proverbial "pollen basket", used for the transport of pollen wetted by nectar and saliva. The structure consists of a broadened, depressed, smooth, largely glabrous area on the apical prolateral surface of the metatibia, typically fringed by long setae that help to create and define a negative space in which the resources are held. Aside from pollen, the corbicula can be used to transport other materials, such as resins or mud.
The Meliponini are universally recognized as a monophyletic group, supported by a large number of specializations relative to other corbiculate bees. Some of the key traits distinguishing Meliponini include: the presence of a penicillum, lack an auricle ("pollen press") proximally on the metabasitarsus, the simple pretarsal claws, alar venation reduction, and, of course, the general reduction of sclerites associated with the sting complex. Other features that in combination help to define Meliponini but are found in other combinations among the remaining corbiculate tribes are the general reduction of outer grooves on the mandibles, the loss of the metatibial spurs, the presence of arolia, the absence of a supra-alar carina, and the presence of a jugal lobe on the hind wing (Michener 1990(Michener , 2007aEngel et al. 2021aEngel et al. , 2021bEngel and Rasmussen 2021). Systematic coverages of meliponine immature stages have been provided by Rozen (2021), Rozen et al. (2019aRozen et al. ( , 2019bRozen et al. ( , 2021, and Rozen and Smith (2019).
Naturally, one of the hallmark traits of stingless bees is the largely vestigial sting apparatus. The various structures associated with the sting complex are present in Meliponini, but they are reduced and generally nonfunctional, although those of the African Meliponula Cockerell are comparatively well developed along with an enlarged poison gland (Kerr and Lello 1962), suggesting the potential for some minor functionality as a defensive apparatus. The reduction of the sting perhaps reflects a history in which the lineage underwent significant body size diminution, a "miniaturization bottleneck", with a sting no longer serving as an effective deterrent for vertebrate predators (Wille 1979). This has been lent some added credence by the early diverging position of Trigonisca Moure, a group of entirely minute stingless bees (Rasmussen and Cameron 2010), as well as the general loss of wing venation concomitant with a more enlarged pterostigma, which is another suite of modifications found among minute insects. While the sting was rendered inert, the nests of stingless bees gained considerably in their defensive qualities, picking up where the potency of a sting was removed. Nonetheless, while vestigial sting sclerites are distinctive for meliponines and inaccessible and tough nest architectures prevent many predators from interfering, stingless bees are not defenseless (Roubik 2023).  (Sakagami & Inoue) from Sulawesi. Individual images from Engel (2022c), Engel et al. (2021a), and . All images M.S. Engel (used with permission).
The closest relatives of Meliponini are the species of the extinct tribe Melikertini (Engel 1998(Engel , 2001a(Engel , 2001bSchultz et al. 2001), a group of corbiculate bees that disappeared around the time of the Eocene-Oligocene mass extinction event, approximately 35 million years ago (Engel 2001a(Engel , 2001bEngel and Davis 2021). Melikertines were globally distributed and seemingly tropical, subtropical, and paratropical eusocial bees, but unlike Meliponini possessed the complete complement of wing venation, had a well-developed and insertable sting, possessed a functional pollen press, and had metatibial spurs, albeit quite reduced relative to those of other bees (Engel and Davis 2021). Like Meliponini, melikertines seem to have commonly collected resin, presumably to be used similarly in nest construction (Engel and Davis 2021). Melikertines and meliponines coexisted for a considerable period of history before the disappearance of the former at the end of the Eocene.
There has been considerable interest in the phylogeny of stingless bees, not only in terms of their classification but also for understanding their biogeography, behavior, physiology, and nest architecture, among other phenomena. An understanding of relationships among the lineages of Meliponini has shifted considerably over the years. Earlier authors presented a wide number of hypotheses, typically with small differences but sometimes wholly incompatible, all based on different interpretations or analyses of morphological and/or biological data (e.g., Wille 1983), although some also incorporated or were based upon small swaths of DNA sequences (e.g., Costa et al. 2003). More recently, however, a detailed and comprehensive exploration of molecular data has helped to provide some greater clarity, and a pattern of overall relationships that is robust and simultaneously consistent with several morphological, behavioral, and biogeographic patterns (Rasmussen and Cameron 2010). These relationships, in turn, have helped to refine elements of the classification as well as formulate new hypotheses regarding meliponine evolution and are summarized in Figs 3, 4.

Classification
For more than half of the time since the start of formal binomial nomenclature in 1758, the stingless bees were largely classified, with some minor exceptions (e.g., the earliest species were placed in Apis Linnaeus or Centris Fabricius), in either a single genus, Melipona, or into two genera, Melipona Illiger and Trigona Jurine. It was not until the works of Heinrich Friese (1860Friese ( -1948 and Theodore D. A. Cockerell (1866A. Cockerell ( -1948 that serious alterations to the classification were initiated, although most species were still placed in a massive, ill-circumscribed Trigona. These two authors were then followed by the extensive and detailed studies of Herbert F. Schwarz (1883Schwarz ( -1960 and Jesus S. Moure (1912Moure ( -2010, who effectively provided new interpretations for natural groupings of stingless bees, with the latter establishing the foundations for our current system of Meliponini. The growing biological data available for many species also served as a new source of character information for establishing groupings of these bees (e.g., Schwarz 1948). Subsequent to Moure, most changes to the supraspecific classification consisted of the addition of new taxa, largely through the efforts of João M.F. Camargo (1941 and his collaborators in the New World and Shôichi F. Sakagami (1927Sakagami ( -1996 in the Old World (e.g., Sakagami 1975Sakagami , 1978Sakagami , 1982. Contra these systems, Charles D. Michener (1918Michener ( -2015 (Michener 1990(Michener , 2007a and Alvaro  (Michener & Grimaldi) in Maastrichtian New Jersey amber, Proplebeia silacea (Wille) in Miocene Chiapas amber. Images of fossil bees from Engel (2000a) and Engel et al. (2021a). All images M.S. Engel (used with permission). Wille (1928Wille ( -2006 (Wille 1963(Wille , 1979 tended to reduce the number of recognized genera, but the phylogenies of Rasmussen andCameron (2007, 2010) corroborated many of the conclusions originally raised by Moure (1946Moure ( , 1951Moure ( , 1961Moure ( , 1971, thereby necessitating a considerable reconsideration of their organization.  Rasmussen and Cameron (2010) Engel and Aber (2022), Engel (2001a), and . All images M.S. Engel (used with permission).
Here we present a brief overview of the revised supraspecific classification of Meliponini (M.S. Engel in Engel et al. 2021a), including keys to the genera and subgenera by region and summaries of distribution and identification tools for each genus, along with a checklist of currently recognized species. The three keys presented here allow for the identification of all currently recognized genera and subgenera of Meliponini. The keys are adapted from recent works that relate to the new organization of the tribe Engel 2019;Engel et al. 2021a) and are organized by geographic region, in the same manner as was done by Michener (2007a), although with a considerably different arrangement of genera. Naturally, there will always be disagreement over the recognition of particular groups and at what rank they should be classified, and different systematists will advocate for slightly different systems depending on their concepts of the groups. Even the authors of this chapter do not agree on all points of the current classification (e.g., whether Leurotrigona Moure should be removed from Trigonisca as its own genus, whether Dolichotrigona Moure should be recognized as a valid subgenus independent of Trigonisca s.str., whether Mourella Schwarz should be accorded generic rank, etc.). These finer points are all worthy of merit and undoubtedly will be revised in time. For now, we have followed the system as recently outlined in Engel et al. (2021a), with the exception of elevating Mourella as a genus and some additions from the last year (e.g., subgenera of Geotrigona Moure, Scaptotrigona Moure, and Scaura Schwarz) and herein (one new genus). Although we allude to some disparities and future subjects to be resolved, we do not intend here to emphasize justification or alternative interpretations, which are more fully explained in some of the recent publications cited herein. Regardless, the revised classification is based on comprehensive phylogenetic hypotheses for the tribe (Rasmussen and Cameron 2010).
Morphological terminology for the keys generally follows that used in major works on bees (e.g., Engel 2001a;Michener 2007a), although the terms of orientation for the legs follow that outlined by Engel et al. (2021a) and wing venation terms are from . Following the keys are brief summaries for each genus, along with a list of those currently recognized species in each taxon (subspecies and synonyms are not listed; species lists are current as of 29 June 2023).

Neotropical Meliponini
In the New World, stingless bees are found from 34.89°S in Uruguay (Montevideo) and Argentina (Buenos Aires) up to 27.03°N in Mexico (Álamos, Sonora), at exceptional elevations up to 4000 m.a.s.l. in Peru and Bolivia (Camargo and Pedro 2007;Roig-Alsina and Alvarez 2017;Roubik and Vergara 2021). Meliponini are not native elsewhere in the Greater or Lesser Antilles, although extinct species of the genus Proplebeia Michener are known from the Miocene of the Dominican Republic and Chiapas, México. Camargo and Pedro (2007) and Camargo et al. (2013) provided a catalog of the Neotropical Meliponini. All Neotropical stingless bees belong to the subtribe Meliponina (Rasmussen and Cameron 2010;Engel et al. 2021a), a group not found in other regions of the world ( Table 1). As of this writing we recognize 474 extant species in the New World and 26 extant genera.

Genus Asperplebeia Engel
This is a recently established genus for two species of minute stingless bees formerly included in Plebeia Schwarz and occurring from southern Mexico to Costa Rica. The species look like smaller species of Plebeia (e.g., subgenus Nanoplebeia Engel), at only ~ 3 mm length, but can be distinguished by the generally more matte integument and coarser sculpturing. Nesting biology has only been studied for Asperplebeia tica (Wille), which nests in tree cavities and, unlike the superficially similar Plebeia, builds brood clusters rather than combs, although species of Nanoplebeia also build brood clusters (Roubik 2021).

Genus Cephalotrigona Schwarz
Cephalotrigona Schwarz includes modestly large bees (8-10 mm), which are generally dark brown to black with faint yellow marks. Noteworthy for the genus is the carinate to lamellate preoccipital ridge, the abundant facial punctation, the long legs, and spatula-or racket-shaped metatibiae with broad corbiculae and only simple setae on the retromarginal edge. Nests are built in tree hollows, and the entrances are a bee-sized hole on a short, rounded platform, built only with cerumen and dark solid materials. The honey is of good quality, and they usually store propolis in abundance. Colonies are large but they are not easy to manage in meliponiculture. This genus occurs from Colima and Jalisco, as well as in Tamaulipas, Mexico to Santa Catarina, Brazil and Missiones, Argentina. The most common species between Mexico and Colombia is Cephalotrigona zexmeniae (Cockerell), similar in appearance to Trigona (Koilotrigona) fulviventris Guérin-Méneville since both are black with a reddish orange metasoma but is larger, while the most common in South America is C. capitata (Smith). Currently, there are only keys for the species from Mexico and Central America (Ayala 1999) and a revision of the genus is needed.
The two species of Duckeola Moure are robust bees of 8-9 mm length, found in Brazil, Ecuador, French Guiana, and Colombia. Perhaps the most striking feature of Duckeola is the considerably depressed parocular area between the compound eyes and the vertex, which elevates the vertex noticeably. In addition, the metatibia is noticeably claviform and lacks plumose setae on the retromarginal surface, and the mesotibial spur is absent (Oliveira 2002). No key to species is available but the two forms are quite distinct. Brood cells are arranged in clusters, although the nesting biology remains to be studied in detail.
This is a genus of tiny bees, 3 mm in length, which look much like Plebeia, but have a reticulated matte integument, conspicuous abundant whitish pubescence, particularly on the face, and almost no yellow maculation on the face. The bees occur in southern Brazil and build irregular combs without an involucrum.
• F. schrottkyi (Friese) Genus Frieseomelitta Ihering The genus Frieseomelitta Ihering includes slender species of 4-7 mm in length, ranging from Sinaloa and Veracruz, Mexico to Mato Grosso do Sul, Brazil. Recognition of the genus is aided by the presence of yellow marks on the face bordering the compound eyes on the paraocular area and genae, the absence of the mesotibial spur, the enlarged and inflated metatibia with a small corbicular depression restricted to the apical third, and an overall spatulate or racket-shaped to claviform metatibia with plumose setae on the retromarginal edge. The shape of the metasoma is subtriangular when constricted, in dorsal view, in the varia species group, while the metasoma is elongate, even when contracted, owing to broader terga II and III (this is the situation in the nigra and portoi species groups). The nigra species group has a claviform metatibia, while the portoi species group has a baseball-bat-shaped metatibia. Adults soon turn black on the head and mesosoma after emergence but the metasoma remains whitish for more than a week. The milky white wing tip in most species is another distinctive feature. The nests are distinctive for the arrangement of brood in clusters. New species are being described and a key to species developed by FFO (pers. obs.).

Genus Geotrigona Moure
As the name of the genus suggests, species of Geotrigona nest in cavities in the ground. The species are generally robust, 5-7 mm in length, with a short broad metasoma. The genus is distributed from Michoacan, in the central Balsas River depression in Mexico to Santiago de Estero, Argentina. Camargo and Moure (1996) and Gonzalez and Engel (2012) provided keys to the species.

Genus Lestrimelitta Friese
This is the most diverse genus of robber stingless bees, occurring from Nayarit and San Luis Potosi, Mexico to Argentina. Species of Lestrimelitta Friese are cleptobiotic (Roubik 1980;Bego et al. 1991), maintaining their nests by "stealing" the resources of other Meliponini and introduced Apis, often causing losses to stingless beekeepers. Lestrimelitta primarily rob brood provisions from their hosts, most frequently species of Nannotrigona, Scaptotrigona, Plebeia, and Melipona (Sakagami et al. 1993). During an attack on the host bees' nests, most Lestrimelitta release a pheromone reminiscent of lemon (citral), which confuses the defense communication between workers and guards of the host (Breed et al. 2004.). Keys to identification are presented by Camargo and Moure (1989), Oliveira and Marchi (2005), Marchi and Melo (2006), Gonzalez and Griswold (2012), and Guevara et al. (2020). The shape of the propodeal spiracle (Ayala 1999) and the dimensions of the mesotibial spur (Oliveira 2002) are two important characters that allow, together with the pattern of body setation, for the separation of species.

Genus Melipona Illiger
Melipona includes almost all of the most massive meliponines. These robust bees, 9-14 mm in length, with abundant plumose pubescence on the mesoscutum have some superficial resemblance to the largest African Meliponula. The wings are usually short and only reach the posterior end of the metasoma or slightly exceed it. The integument is generally black, but in some species brown or pale brown, with yellow, ivory, or brown areas, while the mesosoma has pale, brown, or dark pubescence, particularly the mesoscutum. The genus can be found from Sinaloa and south of Tamaulipas in Mexico to northern Argentina. Many species are used in meliponiculture, and these were the first 'semi-domesticated' bees, subject to multiplication and husbandry in the prehispanic Mayan culture. Nest products such as cerumen and honey have been extensively used through the ages. Melipona is now the largest genus in the tribe, but modern keys are lacking. A key to the species was provided by Schwarz (1932) and for Mexican species by Ayala (1999), with a recent reassessment of species by Camargo and Pedro (2007 (Camargo and Wittmann 1989). The head and mesosoma have conspicuous yellow maculation, while the remainder of the integument is weakly metallic, and the mesoscutum is largely shiny with distinct piligerous punctation. The genus is sometimes considered a distinctive subgenus at the base of Schwarziana (e.g., Engel et al. 2021a).
• M. caerulea (Friese) Genus Nannotrigona Cockerell This genus includes small bees (4-4.5 mm) with largely black integument and areas of yellow maculation on the mesoscutum, mesoscutellum, and legs. The

Genus Nogueirapis Moure
Nogueirapis Moure differs from the closely related Partamona in the presence of abundant yellow markings; only slightly spoon-shaped, not greatly enlarged metatibia of the worker; as well as the smaller body size (3.5-5.5 mm), superficially resembling species of Plebeia. Like Partamona, there are one or two elongate bristles arising from the corbicular surface. Species nest in ground cavities but some also occasionally nest in tree hollows. A key to the species was presented by Ayala and Engel (2014) and Nogueira et al. (2020).

Genus Oxytrigona Cockerell
This genus includes the infamous "fire bees", so named because of their characteristic defense system. Workers have well-developed mandibular glands that produce a secretion containing formic acid, and which can inflict significant burns to the recipient of an attack. The bees are orange to black in color, with the integument of the head quite smooth and polished, sometimes with a vitreous appearance. Overall, the bees are typically 5-6 mm in length, with the head large, wider in comparison to the mesosoma, with an enlarged malar space and an interocular distance greater than the length of the compound eye itself. Species range from Chiapas, Mexico to southern Brazil (Santa Catarina). A key to the species was provided by Gonzalez and Roubik (2008).

Genus Paratrigona Schwarz
Species of Paratrigona Schwarz are small, between 3.5-5.5 mm in length, and are generally black with well-delimited yellow markings. The mesoscutellum projects posteriorly over the metanotum as a plate, and the metasoma is typically robust, almost as wide as the mesosoma. The genus occurs from Veracruz, Mexico to northern Argentina (Salta), and southern Brazil (Rio Grande do Sul). Some species nest on the ground, while others build nests in various substrates, including wood and termite mounds and often on vines and within epiphytic plants. A key to the species was provided by Camargo and Moure (1994).

Genus Paratrigonoides Camargo & Roubik
This genus superficially resembles Paratrigona or Plebeia with a dull integument, but has the keirotrichiate area of the metatibia not depressed on the superior margin; has yellow markings on the paraocular area, frontal median line, and spots below the lateral ocelli; and the upper part of the preoccipital ridge lamellate and bordered by a row of robust setae. The genus includes a single species of ~ 4.7 mm in length and is currently known only from Colombia. The nesting biology remains to be documented.

Genus Partamona Schwarz
Species of Partamona are usually 5-6.5 mm in length, with a smooth and shiny integument, which can be black to orange-yellow, depending on the species, with vitreous yellow paraocular markings. The metatibia is quite large and broadened, making it distinctively spoon-shaped, and lacks plumose setae on the retromarginal edge. Species build semi-exposed nests on either natural or human constructions or on trees, as well as in the ground. Nest entrances are built of a material similar to hardened mud, often with a wide entrance.
Partamona are not appropriate for meliponiculture as they are quite defensive and challenging to work with, in addition to insignificant amounts of stored honey. The genus occurs from Sonora, Mexico to southern Brazil (Rio Grande Do Sul). The species with testaceous bodies were revised by Camargo (1980), and the entire genus by Pedro and Camargo (2003

Genus Plebeia Schwarz
The genus Plebeia is a diverse group of often small and medium size bees (2-7 mm), with shiny integument bees and prominent yellow or white maculation. The metatibia is triangular shaped with only simple setae on the retromarginal edge, and the keirotrichiate zone of the retrolateral surface extends to the superior margin (without a shiny depressed rim). Included herein is the former genus Plectoplebeia Melo, an apparent synonym of the large subgenus Plebeia s.str. and representing merely larger, higher-elevation, seemingly cloud-forest specialized species of the subgenus (Engel 2022a Engel 2022a). A key to the species occurring in Mexico and Central America was provided by Ayala (1999) and for Argentina by Alvarez et al. (2016). Species mostly nest in tree cavities and build brood combs. Two species nest exclusively in internodes in Cecropia Loefl. (Urticaceae), made available by ants, and interact with obligate inquiline scale insects that provide wax and honeydew (Roubik 2021(Roubik , 2023. A key for the subgenus Nanoplebeia was provided by Engel (2021c

Genus Ptilotrigona Moure
This genus greatly resembles Tetragona Lepeletier & Audinet-Serville in the presence of yellow maculation on the face and the velvety setation of the gena, the presence of a mesotibial spur, and the plumose setae on the retromarginal edge of the metatibia but can be distinguished by the setose basal area to the propodeum, the calviform metatibia with the proximal third more plump, and the larger mandibular teeth. The bees are 7-9 mm in length and extend from Costa Rica, with a noteworthy gap through Panama, thence to Colombia, central Brazil, and Peru. A key for the species was presented by Camargo (1996) and Camargo and Pedro (2004

Genus Scaptotrigona Moure
Scaptotrigona is a distinctive group of small to medium-sized bees, 4.5-9 mm, which range in color from orange to black and lack yellow markings except for those on the postgena or elsewhere in one species of the subgenus Sakagamilla Moure [Scaptotrigona affabra (Moure)]. The preoccipital ridge is carinate to minutely lamellate and with three distinctive pits dorsally, and often with an interruption laterally. The metatibiae are also quite characteristic, subtriangular in shape and lacking plumose setae on the retromarginal edge, but bordered by an abundance of rather thick, curved bristles. Like Nannotrigona Cockerell, the mesoscutellum has a characteristic longitudinal depression or groove extending from anterior margin medially, but the integument is not as coarsely punctured. While the genus is easy to distinguish, the species are quite complex with considerable variation and the identification of the species can be difficult. The genus was recently organized into a series of subgenera, including a key to these subgeneric lineages (Engel 2022d). A key to species of the subgenus Sakagamilla is presented in Engel (2022b); to those of Eoscaptotrigona Engel, Gymnotrigona Engel, and Baryorygma Engel in Engel (2022d); to the species of Astegotrigona Engel by Engel (2022e). A partial key to one group of Scaptotrigona s.str. has been published (Engel 2022c) and other new species presented by Engel (2022f), but further work remains to be completed on the Central American fauna (in prep.) as well as Scaptotrigona s.str. in South America. The genus occurs from central Mexico to northern Argentina and is frequently found in meliponiaries. The nest entrance is a long trumpet or tube, sometimes greatly expanded, and usually with small uniformly spaced holes and much flexibility. Species nest almost solely in living trees (except when they live in buildings) and always build regular horizontal combs. A notable exception is S. subobscuripennis (Schwarz), of Costa Rica and western Panama, which nests in cavities in the ground, and apparently never uses tree cavities. The honey has good flavor and is appreciated by many, and propolis from Scaptotrigona is widely used for medicinal purposes.

Genus Scaura Schwarz
This is a genus of small, 4-6 mm long, bees with slightly opaque black integument, without yellow markings, and superficially resemble darker species of Plebeia. The metabasitarsi are large and dilated, wider than the corresponding metatibiae and are used for rubbing floral structures to mop up loosened and scattered pollen. Nests are in tree cavities or even within the arboreal nests of nasutitermitine termites, and the bees build brood combs, except for S. latitarsis (Friese) and the species of Schwarzula Moure who build brood clusters and S. longula (Lepeletier) that builds simple vertical, double-sided appressed cell combs (Oliveira et al. 2013;Nogueira et al. 2023). The species of the subgenus Schwarzula further depart from this biology in that they tend scale insects that have an obligate nesting association and share wax and honeydew (such an association also exists independently in two species of Nanoplebeia: Roubik 2021Roubik , 2023. Information on the two species of this subgenus is provided by (Camargo and Pedro 2002a

Genus Tetragona Lepeletier & Audinet-Serville
The bees of the genus Tetragona superficially resemble species of Frieseomelitta, as both include relatively long-legged bees, differing mainly by the presence of the mesotibial spur, the velvety pubescence of the gena, the yellow maculation of the head in most species (not extending to the top of the compound eyes and absent on the genae; Tetragona essequiboensis (Schwarz) lacks yellow maculation entirely and species of the handlirschii group only have reddish yellow-brown areas on the clypeus), and the shape of the metatibiae, which are narrower. Species of Tetragona are ~ 5-8 mm in length and occur from Tabasco, Mexico to Uruguay. Species are used in meliponiculture and for supplies of sticky resin materials stored in nests. A key is provided by Nogueira et al. (2022) to species of the clavipes species group.

• T. atahualpa Nogueira & Rasmussen • T. beebei (Schwarz) • T. clavipes (Fabricius) • T. dorsalis (Smith)
• T. essequiboensis (Schwarz) • T. goettei (Friese) • T. handlirschii (Friese) • T. kaieteurensis (Schwarz) • T. korotaii Nogueira (Friese) Genus Tetragonisca Moure Species of Tetragonisca Moure are small (4-5 mm), slender bees, with yellow maculation on the lower portion of the face in most species, plumose setae on the retromarginal edge of the metatibia, and have a mesotibial spur (resembling Tetragona). However, the species of Tetragonisca have a basal sericeous area (an oval area with matted keirotrichia) on the retrolateral surface of the metabasitarsus of workers and lack velvety pilosity on the gena. The metatibia is also rather inflated, with the corbicula reduced to the apical portion of the podite. The genus extends from Veracruz, Mexico to northern Argentina. It should be noted that the widespread and common Tetragonisca angustula (Latreille) appears to be a complex of species. The systematics of this species should be explored in depth. Nests are built in tree cavities or the ground. No key to species is currently available.

Genus Trichotrigona Camargo & Moure
This is a genus of enigmatic stingless bees from northern Brazil. The genus includes bees of 5-6 mm in length, and that superficially resemble Frieseomelitta but have conspicuous setae on the compound eyes (hence the generic name). Species of Trichotrigona Camargo & Moure are likely robber bees but seemingly undertaking isolated, rather than mass, raids. Pedro and Cordeiro (2015) provide a means to distinguish the two species.

Genus Trigona Jurine
This is a genus with bees of dramatically different proportions, ranging in size from 5-12 mm in length, with plumose setae on the retromarginal edge of the metatibia, velvety pilosity on the gena, a mesotibial spur (resembling Tetragona), and can be reddish orange to black in coloration, but always lack yellow maculation. The mandibles have well-developed, prominent teeth (either four or five). The metatibiae are claviform and in one subgenus (Necrotrigona Engel) the corbicula is not developed. Aside from the raised median keirotrichiate area on the retrolateral surface of the metatibia, there is also a basal sericeous area (an oval area with matted keirotrichia) on retrolateral surface of the metabasitarsus. The biology of Trigona is remarkably varied, perhaps more so than any other genus of Meliponini, and includes obligate necrophages that scavange from carcasses (Necrotrigona). Species nest in squirrel or bird nests as well as in tree cavities, while some nest on the ground among tree roots and others build exposed nests around tree branches. The largest nests are composed of the bees' own feces, from defecated pollen exines. This group now includes 30 described species but will likely be found to have more than twice as many, like Trigonisca (Rasmussen and Camargo 2008). The subgenus Nostotrigona was revised recently, and a key was provided to the species (Ribeiro et al. 2023

Genus Trigonisca Moure
The genus Trigonisca includes exclusively minute stingless bee species and is the earliest-diverging lineage of extant Neotropical Meliponini. The genus is one of the more widespread groups of New World stingless bees, as well as a relatively commonly encountered group of meliponines. The nesting biology was explored for Trigonisca mepecheu Engel & Gonzalez by Engel et al. (2019). A key to the species of Celetrigona Moure is presented by Camargo and Pedro (2009). The subgenera Leurotrigona and Exochotrigona Engel apparently form a grade at the base of Trigonisca s.l. and are classified by some authors as a separate genus. They can be easily distinguished from the other Neotropical minute bees by the smooth, polished, and shiny integument, the absence of a preoccipital carina, and the absence of yellow markings. Many new species have been encountered and are currently being described for the western Amazon (Roubik 2018). A key to the species was provided by Pedro and Camargo (2009: all species at that time classified in Leurotrigona). A revision of Trigonisca s.str. is needed, but a beginning was made by Albuquerque and . Camargo and Pedro (2005) provided a key to species for Dolichotrigona, although this group almost assuredly renders Trigonisca s.str. paraphyletic and is therefore not recognized herein. Nonetheless, for readers who prefer to recognize Dolichotrigona we provide the following couplet that distinguishes those species from Trigonisca s.strictiss.  (Camargo & Pedro) • T. hirsuticornis (Camargo & Pedro) • T. longicornis (Friese) • T. manauara (Camargo & Pedro) Subgenus Exochotrigona Engel • T. crispula (Pedro & Camargo) • T. pusilla (Moure & Camargo) Subgenus Leurotrigona Moure • T. gracilis (Pedro & Camargo) • T. muelleri (Friese) Subgenus Trigonisca Moure, s.str. (Camargo & Pedro) [Dolichotrigona] • T. buyssoni (Friese) • T. chachapoya (Camargo & Pedro) [Dolichotrigona] • T. clavicornis (Camargo & Pedro)

Afrotropical Meliponini
The fauna of stingless bees in Africa and Madagascar is the least diverse of any in the world. Afrotropical Meliponini are unique and found in a line from Senegal to Eritrea along the southern Sahel southward to KwaZulu-Natal, South Africa. The tribe is also found throughout Madagascar. Currently we recognize 33 extant species in the Afrotropical fauna and eight extant genera (Table 2).

Genus Dactylurina Cockerell
Dactylurina Cockerell is perhaps the most distinctive genus of African stingless bees. As the name implies, the metasoma is elongate, thin, and subclavate, giving it a finger-like shape. The genus occurs from Guinea eastward to the Congo and Uganda and thence southward to Angola. The genus is distinctive for building double vertical combs (e.g., Michener 1974;Njoya et al. 2016), much like Apis Linnaeus, and also shared by the Neotropical Scaura s.str. ).

Genus Hypotrigona Cockerell
Like Liotrigona Moure, Hypotrigona Cockerell includes minute stingless bees commonly encountered in Africa from Senegal eastward to Eritrea, and southward to northern South Africa. The genus is distinctive for the broadly rounded distal superior angle of the metatibia and the dull, matte, reticulate to micro-alveolate integument. The genus is also commonly encountered in East African copal and modern resins (Solórzano-Kraemer et al. 2022). Nest may be built in dry logs, rock or wall crevices, or even in pre-existing cavities of living trees, often with a protruding oval entrance tube. The brood are arranged in clusters or irregular layers (e.g., Portugal-Araújo 1955;Michener 1959;Kajobe 2007;Ndungu et al. 2019).

Genus Liotrigona Cockerell
This is a genus of minute stingless bees and occurs commonly throughout Madagascar as well as less predominantly on the African continental mainland from Liberia eastward to Ethiopia and southward to the northern half of South Africa. The species can be confused with Hypotrigona but differ in the smooth and shiny integument and the presence of a distinct superior distal angle on the metatibia. The monotypic subgenus Cleptotrigona Moure includes a species that is a robber bee on Hypotrigona and other Liotrigona. A single extinct species is also known from Early Miocene amber from Ethiopia (Engel and Aber 2022). Like most minute stingless bees, the brood cells are clustered rather than arranged in distinct combs (e.g., Brooks and Michener 1988;Hora et al. 2023).

Genus Meliplebeia Moure
Meliplebeia includes species superficially resembling the smaller Plebeiella and found from Gambia to Eritrea and Somaliland, and southward to Namibia and northern South Africa. Unlike Plebeiella, the basal area of the propodeum is pubescent. Nests are built like those described for Axestotrigona (supra).

Genus Meliponula Cockerell
The genus Meliponula includes a modestly large and robust species, 6-8 mm in length, found commonly throughout tropical Africa, from Guinea eastward to Kenya and thence southward to Namibia and Botswana. The genus is distinctive for the wholly declivitous propodeal basal area, dull and matte metasomal terga, and restriction of the corbicula to less than the distal half of the metatibia. Nests are constructed within pre-existing cavities in trees, and the brood are arranged in irregular layers (Portugal-Araújo 1955) or can also be within ground cavities. There is a pattern for nests in the highlands to always be belowground, while those of lower elevations are in tree hollows (e.g., Kajobe 2007).

Genus Plebeiella Moure
The genus Plebeiella includes small bees most similar to Meliplebeia but differing in the glabrous basal area to the propodeum. The genus occurs from Togo eastward to Kenya and southward to Angola and Zambia. Nests are built like those described for Axestotrigona (supra).

Genus Plebeina Moure
This genus superficially resembles the New World Plebeia and includes a small species (4-5 mm in length) that can be found from Senegal eastward to Ethiopia and then southward to Angola and northeastern South Africa. Nests are built like those described for Axestotrigona (supra).

Indomalayan, Papuasian, and Australian Meliponini
The fauna of stingless bees across South and Southeast Asia, through the Indomalayan and Papuasian regions, and into Australia is the richest in the Eastern Hemisphere, with a particularly interesting diversity extending across Indomalaya and Papuasia. A catalogue of the fauna was provided by Rasmussen (2008). All meliponines from these regions belong to the subtribe Hypotrigonina (Table 2), a group which also includes the African lineages as well as likely encompasses those species preserved in European and Asian amber sources (Engel et al. 2021a). Currently we recognize 98 extant species in the fauna and 11 extant genera.

Genus Ebaiotrigona Engel & Nguyen
The sole species of this genus of minute bees is found in Southeast Asia and was originally classified in Liotrigona. Recently, however, the discovery of the male demonstrated that the species was more dramatically different from true Liotrigona than originally surmised. Instead, the type species seems more similar to Austroplebeia and was therefore reclassified . Nests are built in rock crevices and the brood cells are built in irregular clusters .
• E. carpenteri (Engel) Genus Geniotrigona Moure Geniotrigona Moure includes large robust stingless bees with a prominent elevated ridge on the vertex posterior to the ocelli, a long malar space, and dense plumose setae on the mesosoma. The genus occurs from Southeast Asia through Malesia. A key to the species was provided by .

Genus Heterotrigona Schwarz
The  (2019), while a key distinguishing the two species of Sundatrigona was provided by Sakagami et al. (1990).

Genus Homotrigona Moure
Homotrigona includes those larger species with pronounced mandibular dentition. Some authors have afforded the individual subgenera generic rank (e.g., Moure 1961). To emphasize the close relationship of these groups we follow a more conservative approach whereby they are considered subgenera of a more inclusive and more readily circumscribed genus.  (Bänziger et al. 2011).
• P. pendleburyi (Schwarz) Genus Tetragonula Moure The genus Tetragonula Moure is the most diverse and widespread of all Old World stingless bees, extending from western India to central-eastern Australia. Many of the species can be exceedingly similar, differing in seemingly minor details. Simultaneously, individuals within any given species, even within a single nest, may also be quite variable in aspects of coloration and some proportions. There are regional keys to species (e.g., Sakagami 1978), but no comprehensive monograph and the identification of species in some areas of tropical Asia can be challenging given the absence of keys and modern evaluations of their circumscriptions. Ultimately, some synonymy may well be recognized, potentially older names may need to be resurrected from synonymy, and some new species may continue to be discovered. The genus is also quite old, with a species of subgenus Tetragonula s.str. preserved in mid-Miocene amber from Zhangpu, China (Engel et al. 2021a

Extinct genera
The remaining genera are exclusively known from fossil species, all preserved in ambers ranging in age from the Late Cretaceous (Maastrichtian) to the early Miocene (Burdigalian). Extinct species in genera that are still living (e.g., Tetragonula florilega Engel) are listed above under their respective clades. †Adactylurina Engel, gen. nov. https://zoobank.org/F0829A32-01BC-44C6-9F66-453E52DF900A Type species. Dactylurina aethiopica Lepeco & Melo, 2022. Diagnosis. This species in Miocene amber from Ethiopia was originally placed in the genus Dactylurina. It differs quite notably from Dactylurina and is therefore here removed to a new genus. The fossil genus differs from Dactylurina in the absence of a basal sericeous area on the retrolateral surface of the metabasitarsus (such an area is present in Dactylurina), the metasoma that is roughly cylindrical and tapers apically (metasoma greatly elongate, finger-like, and subclavate in Dactylurina), face not wider than compound eye length (wider than compound eye length in Dactylurina), and two preapical teeth of the mandible (unidentate in Dactylurina).
Etymology. The new genus-group name is a combination of the Ancient Greek alpha privative a-/ ᾰ̓ -, indicating negation, and Dactylurina Cockerell [itself a combination of the Latin adjective dactylus, meaning, "finger-like" (from Ancient Greek dáktulos / δᾰκτῠλος, meaning, "finger"), and the noun ūrīna, meaning, "urine" but also referring more generally to "genitals" or even metaphorically to the "tail end" through its Ancient Greek origins from the word ourā́ / οὐρᾱ, meaning, "tail"], the genus to which the species was originally placed. The gender of the name is feminine.

Genus †Cretotrigona Engel
This is an interesting fossil preserved in amber from New Jersey that dates from near the end of the Maastrichtian and is therefore the earliest fossil Meliponini and also the oldest definitive bee.

Genus †Exebotrigona Engel & Michener
This genus was described for a Trigonisca-like species of Meliponini in Fushun amber, but subsequent studied indicated that the fossil was not in Eocene amber from China but instead likely from the Baltic region. The provenance of the holotype needs considerable study.

Genus †Kelneriapis Sakagami
Kelneriapis Sakagami is known only from a single worker preserved in Eocene Baltic amber. Engel (2001a) revised the genus and species. This is a minute stingless bee similar to the extant African genus Liotrigona and the extinct genus Liotrigonopsis Engel from the same deposits.

Genus †Liotrigonopsis Engel
This genus, like the two preceding genera, is known only from middle Eocene Baltic amber. Currently, there is only a single worker known. The genus and species were characterized by Engel (2001a) and is a minute stingless bee similar in morphology to the extant African genus Liotrigona.

Genus †Meliponorytes Tosi
This genus is known only from Miocene Sicilian amber. The original material has not been re-examined since the end of the 19 th century and so it remains a poorly understood group, but likely belongs to the Hypotrigonina (Rasmussen and Cameron 2010;Engel et al. 2021a), based on the original description and figures.

Genus †Proplebeia Michener
This is a genus of Plebeia-like bees occurring in middle Miocene amber of the Dominican Republic and southern Mexico.

Biology
Stingless bees are, of course, eusocial (Michener 1974), and of the anchored grade (sensu Engel 2023). They live in perennial colonies, which range considerably in size and complexity, but always have males and females, the latter organized into distinct worker and queen castes, and the usual division of labor, age polyethism, and generation overlaps that come from these distinctions (Fig.  6). In addition, polyethism within the worker caste, most often based on age, is sometimes also based on subtle morphometric differences to produce, in some species, a functional soldier-like caste (Grüter et al. 2017). Workers dominate the population of a colony and undertake all of the work to maintain the nest and support the queen and males. Most notably they perform all of the foraging activities for food and nest materials, and throughout the active period of the day a nearly constant flow of foraging workers can be seen coming and going. While for most stingless bees this consists of collecting pollen, nectar, resin, and mud, there are exceptions, with some species harvesting honeydew and wax from scale insects (Coccidae), collecting from fungi, and others scavenging the decomposing carcasses of vertebrates (Camargo and Pedro 2002b;Oliveira and Morato 2000;Roubik 1982Roubik , 2021. Scavenging the carcasses of small vertebrates occurs, but larger vertebrates are used by obligate necrophages, who also utilize invertebrates like annelids, spiders and Orthoptera. Several species of Meliponini and other bees visit carcasses to collect salts, lipids, and water (Oliveira et al. 2013, Ribeiro et al. 2022), but only obligate necrophages consume animal protein and make honey from dead meat, and microbes make the glucose and amino acids (Roubik 2023). In Asia, minute species, where known, are lachryphagous, sucking the tears from eyes of various birds and mammals (Bänziger 2018). In the Neotropical region lachryphagous species of Trigonisca s.l. (typically the subgenera Leurotrigona, Exochotrigona, and some Trigonisca s.str.), are known as "lambe-olhos", "lameojos", or "chupaojos" (lick eye) (Camargo and Pedro 2005; pers. obs.), and some species when crushed leave a caustic secretion that can cause eye irritation (Carvalho et al. 1949;Villas-Bôas and Villas-Bôas 1994). Foraging for pollen may even be decoupled from visiting flowers in Scaura, where the bees use enlarged metabasitarsi to "mop" pollen that has fallen from flowers onto bordering leaves or other surfaces (Michener et al. 1978). A similar behavior was observed for Trigonisca (Trigonisca) tavaresi Camargo & Pedro whereby the bees collected pollen from the surface of flower petals that had been dropped by larger bees (Oliveira et al. 2013). At an even greater extreme, species of Lestrimelitta and Liotrigona (Cleptotrigona) are robbers that collect their materials from the nests of other meliponines, either by mass invasion, as in the former or by stealthy, spy-like gathering by isolated workers in the latter (Portugal-Araújo 1958;Sakagami et al. 1993). Trichotrigona apparently also does not store food and is a cleptobiotic social bee associated with Frieseomelitta (Pedro and Cordeiro 2015).
From the moment of eclosion to the adult, workers begin their life-long labors, with lifespans ranging from 30-40 days (Sakagami 1982). Initially they are teneral, appearing nearly white and with comparatively soft cuticle (called callows), but gradually the tanning and melanization of sclerites is completed. For the first phase of their lives, workers remain within the colony to care for the developing young, building brood combs and storage pots, process food resources, engage in hygienic behaviors, cleaning and repairing the nest, tending the queen, thermal regulation for the nest, and, when necessary, defense from invaders, among many other roles (Grüter, 2020). Workers within the colony may also lay eggs, which are either trophic eggs used to feed the queen or develop into males given that workers are unmated and their eggs are therefore unfertilized (Koedam et al. 1996). As they age, workers take on the more dangerous roles external to the nest, primarily foraging. Males will also forage for nutrients as they do not receive food within the nest, nor do they live within the nest. Foraging workers locate, harvest, and transport the many supplies needed to keep colonial life going (van Veen and Sommeijer 2000;Sakagami 1982). Additionally, during colony division such foragers will locate a new suitable nest site and begin construction of the new nest, with the materials coming from nature or even derived from the original nest. Eventually, that subset of workers that established the new nest will be followed by an unmated queen that will then copulate with males and assume the reproductive role for the new colony. This is, of course, in stark contrast to honey bees (Apini) in which it is the old queen that departs to establish the new colony (Michener 2007a). Gradually, interaction and exchange between the original and new colonies cease, which results in two distinct and functional societies.
Most colonies have a single physogastric queen (Fig. 6A), although there are uncommon cases of polygynous colonies occurring naturally (e.g., Velthuis et al. 2001). Queens are produced in larger brood cells, queen cells (Fig. 6), which are built on the periphery of combs and typically formed by combining two normal brood cells (Grüter 2020). In Melipona, however, the brood cells are of a uniform size. In Meliponini queens are determined by diet and a genetic-feeding mechanism (Kerr 1948), but in Melipona and some species that build brood cells in clusters virtually any emerging female is capable of becoming a potential queen dependent on what she is fed by the earlier generation of workers. Unmated queens can be found in colonies at nearly any time of year and may even be imprisoned by the workers in entrapment cells composed of cerumen or even in empty storage pots in the case of Melipona (Imperatriz-Fonseca and Zucchi 1995). The small population of unmated queens provides the colony a source of replacement queens should the current queen die unexpectedly. In meliponiaries there is in Melipona the phenomenon of parasitic virgin queens, which enter nests lacking queens, but might or might not do so in native forest environments (Roubik 2023). When colonies are about to divide, the production of unmated queens will increase. If too may unmated queens are present, then workers may expel them from the nest, kill them outright, or imprison them. Either way, the result is that such expelled queens will die in the absence of a nest and support from a retinue of workers. Often a queen will store considerable nutrients within her body, resulting in the expansion of her metasoma. Nonetheless, once mated the queen's metasoma will expand dramatically owing to the development of her ovaries and an increase in the number of ovarioles. Many mated queens will become so physogastric as to be unable to fly.
Males are usually the least common individuals within a nest. Males usually appear when the number of royal cells increases and there is a preponderance of food stores within the nest. The emerging males do little other than hang about the periphery of the nest or cluster around unmated queens, waiting for them to depart on their nuptial flight at which point they have a chance of mating. During times of scarcity, any males are often expelled and killed to avoid their wasting of resources.

Nesting biology and architecture
The nests of stingless bees are more intricate than any other bees and represent a cornucopia of interconnected layers and structures, most of which are composed of cerumen (Wille and Michener 1973) (Figs 5-7). Cerumen is a substance produced by the bees through the combination of wax, secreted from the bees' tergal glands, with resin or other plant substances (plant resins stored by the bees are called propolis), or even enzymatic secretions (that is, some may add significant saliva) (Roubik 2023). Depending on the part of the nest, the cerumen may be further mixed with or bolstered by mud, feces, small pebbles, and/or sand. Mud and clay mixed with propolis by the bees is frequently referred to as geopropolis. Cerumens vary considerably, not only among bees, but for different parts of the nest and may therefore range from pliable to friable to exceedingly tough (Roubik 2023). Perhaps not surprisingly, tougher cerumen is often found toward the exterior, while more flexible materials, if present, are internal to the nest. The cerumen also embodies considerably antimicrobial properties, the result of both the bee secretions incorporated into the admixture or bestowed by the different plant resins (Shanahan and Spivak 2021;Roubik 2023). Thus, the chemical properties of the cerumen itself represents a form Figure 6. Nests of representative stingless bees A physogastric queen of Melipona (Michmelia) illota Cockerell from Peru B colony of Tetragonisca angustula (Latreille) from Colombia C, D colony with honey pots of Scaura (Scaura) latitarsis (Friese) from Peru E Nannotrigona (Nannotrigona) melanocera (Schwarz) from Peru F Melipona (Melikerria) beecheii Bennett from Mexico G Heterotrigona (Heterotrigona) itama (Cockerell) from Brunei H Geniotrigona lacteifasciata (Cameron) from Brunei. All photographs C. Rasmussen except F R. Ayala and G and H M.S. Engel. of colony defense from the most unseen of pathogenic invaders. An extensive line of research awaits comparative studies of the microbiomes of the unharvested resins, the unadulterated wax and bee secretions, the bees' intestines, and the resulting cerumen used in different parts of the nest.
Nests vary widely but can be generally summarized as follows (Fig. 5): At the center is a brood chamber composed of brood cells. Individual brood cells are oriented horizontally (except vertically in Dactylurina and Scaura (Scaura) longula; Darchen and Pain 1966;Nogueira-Neto 1992), house a single developing bee, and open upward when the adult emerges (Fig. 6F). Mass provisions are provided before the queen lays an egg within the brood cell, which is thereafter sealed and left undisturbed until the new adult chews her way out. These cells are not reused and are broken down once development is completed. The brood cells are arranged in characteristic patterns, many times in a spiral pattern to form a horizontal comb (Fig. 6B, E), although in some species the cells are more irregularly placed in clusters, sometimes quite loosely (Fig. 6C, D). Brood clusters are most often found in those species of smaller sizes that can build nests within irregular cavities, interconnected cavities, or tight crevices that otherwise do not lend themselves to the more rigid spatial parameters required by stacks of horizontal combs. This brood chamber is wrapped by an involucrum (Fig. 5), often made of several tightly adjoined layers and thus, laminate (Roubik 2006). The involucrum maintains a stable temperature for the brood chamber and is important for thermoregulation in many species (Roubik 2006). Outside of the brood chamber, and outside of the involucrum when present, is the storage area (Fig. 5). Within this space the bees construct pots in clusters or in layers, but not distinct combs (Fig. 6C, G). These pots are built with cerumen and used to store pollen and honey. Surrounding the storage space, and thus enveloping the entirety of the nest, are layers of batumen (Fig. 5). The batumen layers, which are wall-like layers of particularly thick and cured resin. The outer layer of batumen can nicely fit against and within irregular surfaces or spaces. When the batumen is used to close off the nest from the remainder of a larger cavity, the layers are thick and tough, concrete-like (Fig. 5), and referred to as geopropolis. Similar tough layers of batumen are used to form the outer walls of exposed nests. A tube is built that leads from the storage space through the layers of batumen to the exterior of the nest. Entrance tubes can often be unique, even diagnostic for given genera, subgenera, or species, and range from simple, thin tubes with a singular opening to elaborate branched or multi-opening structures (Fig. 7).
Nest sites are a limiting factor for most bees and the same seems to be true for stingless bees. Indeed, this may have also been a contributing factor leading to the miniaturization bottleneck for Meliponini. Many species build nests within pre-existing cavities in tree trunks or branches, with a preference for spaces that have narrowed openings with the surrounding environment, and which can be easily closed by the bees during nest construction (Wille and Michener 1973;Roubik 2006). The tree hollows used are often the result of some decay, which perhaps has also fueled the use of antimicrobially active cerumen for the bee colonies. Other forms of cavities may also be used, such as hollows in the ground, spaces within cliff faces, and in a wide range of human-produced spaces: in the walls or roofs of homes, in pipes, in electrical boxes, or even within furniture. Other stingless bees build their nests inside the exposed aerial nests of ants or termites, building an entrance tube on the exterior of the host nest and then gradually excavating into the side of the host and constantly enclosing the growing space so as to prevent their "hosts" from entering the newly founded bee nest (Camargo 1984). At the other end of the spectrum, some species of Partamona will construct their own "cavities" by building out from a starting point, usually against a tree or cliff, while some species of Trigona do similarly by wrapping batumen walls around a tree branch (semi-exposed nests) Rasmussen and Camargo 2008). Only two stingless bees, of all Apoidea, are known to nest in a single kind of plant, or have a host melittophyte. They are western Amazonian Plebeia that live in the upper branches of large Cecropia (Roubik 2021). A diversity of nests, both natural and managed, are illustrated in Figs 8, 9.
Although stingless bees have an atrophied (vestigial) sting and lack an ability to sting it does not make them defenseless. They exhibit different mechanisms for protecting their nests, ranging from camouflaging the nest entrance, nest construction in places difficult to access, to an active defensive behavior, sometimes quite elaborate. They can tangle themselves in hair and fur, pinching the skin of the aggressor or intruder with their jaws, which can even cause some injuries, enter the nostrils, and ears of intruders, as well as depositing plant resins or caustic substances on the intruder, this last specialized behavior observed with the mandibular glands of bees in the genus Oxytrigona (it also has to be wondered if Papuatrigona has similarly caustic mandibular secretions: MSE, pers. obs.). If grabbed, many stingless bees will bend the metasoma around to their attacker, mimicking the behavior of stinging, perhaps relying on a Batesian-like mimicry with stinging bees and wasps. Lastly, some species, such as Frieseomelitta silvestrii (Friese), the bees will play dead (thanatosis) when they encounter a large enemy (Ihering 1912;Michener 1974).