The pupa of Chironomus decorus Johannsen, 1905 – additional description and an unusual sexual dimorphism

In his original description of Chironomus decorus, Johannsen included a brief description of the pupa but with insufficient detail to differentiate it from other North American species, particularly those of the de-corus-complex. In this note, further information of pupal characters, mostly derived from specimens reared from egg masses, are given which allow separation of these pupae from those of some other species, e


Introduction
In his original description of Chironomus decorus, Johannsen (1905) included some information on the pupa, e.g., length (7-8 mm), color (dusky greenish brown, the colors of the imago showing through the integument), black chitinised lateral spur prominent and without teeth (i.e., a single terminal spine).He also included a figure of a pupa, but not detailed enough to permit identification, and a figure of the shagreen pattern on abdominal segments five and eight.Wülker et al. (2009) noted the identity of species 3a of Martin et al. (1979) as C. decorus Johannsen and figured the frontal apotome of a female pupa in their figure 6b to contrast the presence of frontal warts (larger than normal for the species with height equal to width at base) in C. decorus in contrast to C. bifurcatus where they are lacking.This account expands the description of the pupal exuviae of males and females, mainly reared from egg masses.One of these characters, the cephalic tubercles, which are usually larger in males of Chironomus, is further differentiated in C. decorus and allows identification of the male pupae as belonging to this species.

Material and Methods
The material examined here included 4 females and 2 males all reared from two egg masses collected in Madison, Wisconsin, USA in May 1978 (UWI.5.1 Em#1 and UWI.6.3Em#1).The identity of these egg masses was confirmed from cytology and morphological features, and for the latter egg mass by the mtCOI barcode sequence of a female larva .Other specimens included a reared male from Belzoni, Humphries Co., Mississippi (UMS.2.1 reared male DNA1) for which a mtCOI sequence is also in the Barcode of Life Data Systems database (BOLD CoTW095-20).Both specimens are in BOLD BIN: BOLD:AAB7030.
In addition to the reared specimens, I also examined pupae in the collection of Jim Sublette in New Mexico, material that is now in University of Minnesota Insect Collection, but not yet curated.Specifically, I have photographs of pupal spurs of a female and an unsexed specimen from New Mexico, as well as the cephalic tubercles of a male specimen from Yankton, South Dakota which are illustrated below (Figure 1).I have photographs of pupal spurs of a female and an unsexed specimen from New Mexico, as well as the cephalic tubercles of a male specimen from Yankton, South Dakota which are illustrated below.

Results and Conclusions
Many of the morphological characters are summarized in Table 1.These data indicate that while many characters have different mean values between males and females (usually higher in males), there is considerable overlap of the ranges.The main exception is in the ratio of length to basal diameter of the cephalic tubercles where the males have a higher value.This difference is further reflected in the presence of a secondary tubercle, with a small seta, in males (see Figure 1), but not in females.Further, C. decorus is the only species from North America with such a secondary tubercle, permitting the male pupa to be readily identified.
For females, only the presence of frontal warts provides a potential means of eliminating a number of species where frontal warts are known to be absent.The difficulty is that very few pupal descriptions make any mention of them.This may be in large part due to the statement in the Holarctic pupal diagnosis of the genus Chironomus (Pinder and Rice 1986) that frontal warts are absent in the genus.Aside from the present species, there are only two species for which their presence is confirmed: C. melanescens (Keyl 1962) (Martin 2015, Fig. 1b), where their length of about 55 µm will probably identify the pupae of that species, and C. decorus group species 2 of Butler et al. 1995 in which they are slightly larger in the females but largely overlap with the size range in C. decorus and can only be separated if there are more than 6 spines (up to 11) on the spurs.The pupal key of Langton and Visser (2003) makes it clear that they are commonly present in the subgenera Chaetolabis and Einfeldia, but that pupae of those subgenera have other differences to those of Chironomus (s.s.).A further impediment to clearly identifying, particularly the female pupae, is that there is no description of the pupae of at least 12 Nearctic species, and only a note of the number of spines on the pupal spur for a further 11 species -of which only C.

Figure 1 .
Figure 1.Cephalic tubercles and frontal warts of a male pupa of C. decorus from Yankton, South Dakota.The small secondary tubercles, with a small subapical seta, are indicated by arrows.

Table 1 .
crassicaudatus (9-19 spines, Sublette & Sublette 1971) does not overlap with the number in C. decorus.However, they can be separated from C. anonymus, C. bifurcatus, C. blaylocki, C. staegeri, C. stigmaterus, C. dilutus, and C. pallidivittatus, which are confirmed to lack frontal warts.Summary of the mean and range of some morphological characters of the pupae of Chironomus decorus.