Discothyrea traegaordhi, Santschi, 1914
Hita-Garcia, Francisco, Lieberman, Ziv, Audisio, Tracy L., Liu, Cong & Economo, Evan P., 2019, Revision of the Highly Specialized Ant Genus Discothyrea (Hymenoptera: Formicidae) in the Afrotropics with X-Ray Microtomography and 3 D Cybertaxonomy, Insect Systematics and Diversity 5, pp. 1-84: 9-13
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Discothyrea traegaordhi Species Complex
Frontal carinae fused for most of their extent, reduced to a narrow medial lamella, platelike to triangular in profile, apex rounded to acute; linear in frontal view; antennal scrobes absent; AT3 never as long as in D. oculata complex ( ASI 85–183; Fig. 2C and D View Fig ); eyes variably developed: either absent, a pigmented spot, or if larger, ommatidia flattened, hence eye not protruding from head; sculpture variable; ventrolateral surface of head without distinct processes; petiole variable, dorsally attenuated anteroposteriorly or broadly cuneate; not disciform or hemispherical in profile, outline usually pentagonal in anterior view, sometimes rectangular or hexagonal or dorsally rounded.
[See Fig. 5 View Fig E–H for illustrations of diagnostic characters given above.]
The traegaordhi complex is comparatively species-rich with 18 of the 20 known Afrotropical Discothyrea . Compared to the two species of the D. oculata complex, most species in this complex tend to have rather restricted distribution ranges, with some species apparently endemic to single localities ( Fig. 4 View Fig C–T). The few species that are widespread still have significantly smaller ranges than the two members of the D. oculata complex. Based on a preliminary assessment, the majority of species worldwide appear to belong to this complex. If true, then this might be the most widespread complex within the genus, representing all of the New World species, most of the Afrotropical fauna, as well as most species from Australia and Oceania. Its members, however, are apparently absent from the Malagasy, Oriental and Indomalayan regions.
Synoptic List of Afrotropical Species
Discothyrea oculata Species Complex
Discothyrea traegaordhi Species Complex
Morphological Characters of Taxonomic Importance
Despite the previous considerable lack of useful taxonomic characters for Discothyrea , our data show that the Afrotropical fauna is especially rich in morphological characters of high diagnostic value. All body parts investigated have numerous external morphological characters that varied moderately to greatly among species while being highly stable intraspecifically ( Figs. 6–14 View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig ). Overall, we observed a remarkable diversity in the shape of mandibles ( Fig. 8 View Fig ; see section about Mandibles below), the cephalic capsule ( Fig. 6 View Fig and 7 View Fig ), the mesosoma ( Figs. 9 View Fig and 10 View Fig ), and the metasoma ( Fig. 12 View Fig ). Furthermore, some surface areas that are commonly covered by hairs or difficult to examine, such as the anterior clypeus ( Fig. 6 View Fig ) or the propodeal declivity ( Fig. 11 View Fig ), proved to have some informative variation in character states. The complete removal of pilosity through ‘virtual shaving’ also showed that there are noticeable differences in surface sculpturing between species, although it must be noted that there is some considerable intraspecific variation in some species. Despite most species possessing a dense pelt of pilosity/pubescence, which appears to be very species-specific, there was some significant variation among several species, especially with the expression of standing pilosity ( Fig. 14 View Fig ).
In the following, we present numerous diagnostic image plates displaying this morphological diversity ( Figs. 6–12 View Fig View Fig View Fig View Fig View Fig View Fig View Fig ). Each body part was virtually sectioned and dissected from the remainder of the specimen to permit a better examination. These diagnostic plates not only illustrate and aid the identification key provided below, but also serve as a general overview of morphological characters of taxonomic importance and can be used by future taxonomists, parataxonomists, or ecologists to compare whole specimens or body parts in order to quickly gain a better understanding of the species studied.
The main groundplan within Discothyrea seems to consist of an edentate triangular mandible with a pronounced blunt carina along its masticatory margin, as can be seen in most African species. Prior to this study, the lack of dentition was considered an autapomorphy of Discothyrea ( Bolton 2003). However, unexpectedly, we found that two species actually possess teeth/denticles on the apical third of the masticatory margin, namely D. chimera ( Fig. 8E View Fig ) and D. gryphon ( Fig. 8I View Fig ). While the tooth on the mandible of D. chimera is very pronounced and rather large it is definitely smaller and more of a denticle in D. gryphon . Furthermore, D. chimera also has a small but acute prebasal denticle at the corner between the basal margin and the masticatory margin. This prebasal denticle is also conspicuously visible in D. damato ( Fig. 8F View Fig ), D. dryad ( Fig. 8G View Fig ), and D. schulzei ( Fig. 8Q View Fig ). Several other species, such as D. athene ( Fig. 8D View Fig ), D. hawkesi ( Fig. 8J View Fig ), and D. kalypso ( Fig. 8K View Fig ), have something like a blunt prebasal protuberance on the masticatory margin, located basally close to the corner to the basal margin of the mandible. The masticatory margin appears to be variably shaped from species to species with a more or less pronounced ectal carina that may run the whole length of the margin (in most species) or be interrupted, as in D. traegaordhi ( Fig. 8R View Fig ) and D. wakanda ( Fig. 8T View Fig ). The latter species also possesses a second, more irregularly shaped carina basally and medially. Another feature not expected before this study is the variety in overall shape and aspect ratio, since some species, such as D. mixta , have rather elongated mandibles with a short basal margin and a long masticatory margin, whereas other species, such as D. maia , possess a shorter mandible with a long basal margin and a short masticatory margin. There seems to be gradual cline leading from the former to the latter. This higher than expected diversity of mandibular shapes presents the opportunity to apply additional taxonomic characters of high diagnostic value for the species descriptions and the identification key.
Our results show that in contrast to the external morphology, the internal structure of Discothyrea antennae is consistent and provides a reliable antennomere count. However, as expected, this value often differs from that obtained through surface examination and, in at least D. mixta and D. poweri , from that given in the original description ( Table 2). In the generalized state, each flagellomere has a simple, internal sclerotic ring at its base which connects to a simple internal preapical ring toward the distal end of the prior flagellomere. Externally, adjacent flagellomeres overlap only at the junction of the internal rings, which usually constitute around one-third at most of the subsegment’s length and are numerically congruent with the external flagellomere count ( Fig. 15 View Fig ). Internally, the sclerotic annuli form a cylindrical channel through which the two branches of the antennal nerves run. The flagellomeres are apparently connected by membranes between the distal and proximal outer walls of adjacent subsegments, by conjunctiva of the internal rings, or by both.
In Discothyrea , the flagellomeres take two distinct forms. The proximal flagellomeres are extremely simplified, with a single internal annulus per flagellomere, i.e., the basal and distal rings are not disparate. Two or three of the distal flagellomeres proximal to the club, which itself comprises a single hypertrophied subsegment, have the internal annuli deeply invaginated, while the entire flagellomere is reduced in length, so that most of the outer wall of a flagellomere rests within a fold of the prior subsegment ( Fig. 15D, E View Fig , and G–I). The internal channel is therefore highly imbricated, unlike in other ants ( Fig. 16 View Fig ). These distal flagellomeres are typically relatively distinct and countable externally. The simple, proximal flagellomeres are tightly fused at their apical and distal margins, and the external plates of these subsegments overlap strongly, such that a single externally apparent subsegment may actually correspond to several internal divisions ( Fig. 15H and I View Fig ). All Afrotropical Discothyrea , as well as D. banna from and D. diana from China, and an undescribed species from Samoa, possess the deeply infolded structure of the distal 2–3 antennomeres and the fusion of the proximal flagellomeres. In D. gryphon , which with six antennomeres has the fewest in the Afrotropical fauna, the infolded subsegments plus the club comprise the entire flagellum ( Fig. 15I View Fig ). We therefore hypothesize that reduction in antennomere count in Discothyrea takes place through the progressive fusion of subsegments proximal to these modified flagellomeres.
Identifying the homology of elements of the frontoclypeal structure of Discothyrea is complicated by its extremely derived morphology. External anatomical landmarks used to differentiate the frons and clypeus in nearly all other ants—namely, the antennal insertions, the anterior tentorial pits, and the epistomal sulcus—are concealed, lost, or highly modified in Discothyrea . The frontal region bears the antennae, which are inserted posterad or in line with the anterior tentorial pits in all genera except Discothyrea (and the proceratiine Problomyrmex ), in which the antennae are located far anterad the anterior tentorial pits. The anterior tentorial pits themselves are located in the usually clearly defined epistomal sulcus corresponding to the internal epistomal ridge connecting the anterior tentorial arms; in Discothyrea this sulcus is completely absent. In contrast to the challenges of interpreting frontoclypeal morphology based on external characters, virtual dissection enabled clarification of the structure’s composition through comparison of the preoral and pharyngeal skeletomusculature. Despite the high degree of fusion, migration, and modification of the frons and clypeus, the endoskeletal and muscular elements are quite consistent and can be easily identified in the context of the Hymenopteran groundplan.
The frons is intimately fused to the clypeus and anteriorly prolonged over the clypeo-labral articulation. In most species, the clypeus itself is reduced to a thin, sinuate strip that externally contains the anterior tentorial pits and constitutes the anteriormost portion of the shelf overhanging the mandibles. Internally the anteromedial disc of the clypeus is located by the elongated dorsal cibarial dilators, which arise solely from this disc and insert on the distal wall of the cibarium ( Fig. 17 View Fig ). The frontal region is located by the antennal insertions and by the origin of several muscle groups, described in detail below. The prominent, posteromedial portion of the frontoclypeal structure, variously produced as a lamella in the traegaordhi complex, as a broad rhomboid platform in the oculata - complex, and as a triangular to rhomboid swelling in most Asian species, belongs to the frons; because the structure is not derived from the toruli, it is interpreted to comprise the modified, anteromedially fused frontal carinae. The medial portion of the shelf, which usually projects anteriorly past the fusion of the frontal carinae, is also formed by the frons, which is delimited anteriorly and laterally by the antennal sockets. An anterior section of the head shows both the narrow space left between the ventral face of the frons and the dorsal face of the clypeus and the anterior fusion and posterior expansion of the frontal carinae ( Fig. 17G and H View Fig ).
Three nonclypeal muscle groups are identified which locate the frons. The dorsal pharyngeal dilators originate just anterad the retractors of the mouth angle which arise from the cephalic dorsum ( Fig. 17B, C View Fig , and F). The posterior labral retractors, located distal to the dorsolateral arms of the sitophore, originate around the midpoint of the elevated frontal carinae in the oculata -complex ( Fig. 17D View Fig ), and posterior to the carinae in the traegaordhi- complex ( Fig. 17E View Fig ). The frontal lamella apparently lacks musculature ( Fig. 17E and F View Fig ).
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