Therea Billberg 1820
Creators: name: Bohn, Horst
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Blattodea
Corydiidae
Therea

description: 3. The Therea -group of Corydiinae
The four genera of the Therea -group, Ergaula, Eucorydia, Homoeogamia and Therea, differ from all other genera of Corydiinae by a series of shared characters:
1. Tibia armament. Ventral surface of the fronttibia with one spine [3.6.1] (Table 2, col. 2–4), total number of spines at the fronttibia 10; apical whorls of all tibiae with only 6 spines; number of ventral spines on the midtibia always 3.—Remaining genera: Fronttibia at the ventral surface without a spine, total number of spines 8 (only Eremoblatta) or 9, in the latter case irrespectively of whether the apical whorl has six [3.6.0], seven [2.7.0] or eight [1.8.0] spines; whorls with 6 spines only present in Polyphagina (on all legs) and in Arenivaga (on mid- and hindleg); number of ventral spines on the midtibia usually 0-2, only exceptionally 3 (Polyphaghina).
The presence of one spine more or less might be judged as an unreliable character in the light of a rather great variability of spine numbers, for example, at the dorsal surface of the hindtibia (Table 1, col. 3). But no such variabilty is found in the fronttibia, its pattern has proved to be almost invariable in all species so far studied (see also under “Tibia armament”, p. 10 ff, and Table 1, col. 1).
The statement of the absence of a ventral spine on the fronttibia in other genera of Corydiinae seems to be in contrast with the spine formula of the fronttibia proposed by Chopard (1929) and Bey-Bienko (1950) for Polyphaga [1.7. 1] indicating (in bold) one ventral spine which, however, in my interpretation is dorsal. Even if their assumption is right, the position of the respective spine (with the roman number I in Fig. 1H) would be very near to the border between the ventral and dorsal surface and thus not be comparable with the ventral spine of Ergaula and relatives which has a mesally ventral position (spine designated with ve in Fig. 1A).
2. Specialised right tegmen. (Table 2, col. 8) Right tegmen in the area being overlapped by the left tegmen with a special surface structure allowing the fixation of the left tegmen in repose (see under Chapter I. 5. and Fig. 19E‒H).—Remaining genera: without specialised overlapping area, no structural differences between right and left tegmen.
3. Tegmina with subcosta lobe. The subcosta lobe is a lobelike elevation along the subcosta at the lower side of the tegmina, present in all four genera and their species (Table 2, col. 9).—Remaining genera: Subcosta lobe only present in three genera, in Eupolyphaga (all species), in Hemelytroblatta subg. Mollidentoblatta and in Psammoblatta (majority of species in the latter two).
4. Females with wings. In Eucorydia and Therea wings in both sexes similarly developed, considerably surpassing the abdomen; males in Ergaula and Homoeogamia similarly winged as preceding genera, females with shorter wings, but tegmina usually covering the whole abdomen.—Remaining genera: males winged, females wingless.
Characters (3) and (4) were already listed by Princis (1960), in his eyes justifying the establishment of a separate category (Homoeogamiidae) for the four (originally five) genera. The addition of two more specific characters seems to further confirm Princis’ decision. In the following, important aspects of the four characters and their occurrence in other subfamilies of Corydiidae will be discussed.
The genera of Princis’ family “ Euthyrrhaphidae ” studied for comparison have a very similar tibial spine armament: front tibia with 1 spine at the ventral surface, and 6 spines in the apical whorl of front-, mid- and hindtibia in most of the genera (Table 2, col. 2). A ventral spine at the fronttibia is missing in all other genera of Corydiinae; and apical whorls with 6 spines are only found in Polyphagina (tibiae of all legs) and Arenivaga (tibiae of mid- and hindlegs), in the other genera only whorls with seven or eight spines are present. This is also true for the genus Nymphrytria. Subgenus Mononychoblatta shows the presumed ground pattern of the genus [n.8.n][n.7.n][n.7.n]; the lower numbers in the subgenera Nymphrytria and Leiopteroblatta (Table 1, col. 1–3) are certainly due to secondary reductions (see under II. 1.).
The distribution of the spine patterns in Corydiinae and neighbouring subfamilies suggests that the state found in the Therea -group is primitive, plesiomorphic for the subfamily. The increase in the number of apical spines, often connected with an increase in spine size and strength, can be seen as an adaptation to the burrowing mode of life allowing more effective strokes of the legs in the loose sand. The apical whorls with six spines are certainly a very old heritage of Blattodea; but, apart from Corydiinae no other group of Blattodea seems to have evolved whorls with more than six spines, including also the Panesthiinae with their often burrowing mode of life.
The specialised overlapping area of the right tegmen obviously contains plesiomophic as well as apomorphic elements. A similar division of the tegmina into two very differently structured parts on both sides of a diagonal line crossing the anal field is also found in Holocampsinae (Holocompsa, Hypercompsa), and indications of such a diversification of the surface structures are also found on the tegmina of some species of Tivia and in the Australian genus Polyphagoides (figs. I/6,7 in Mackerras 1968). But in all these cases (inclusively Polyphagoides?) both tegmina are similarly differentiated; whether the differently structured areas have a function in the mutual adhesion of the tegmina remains to be clarified. Therefore, the development of differently structured areas on the tegmina is certainly an old heritage, but its specialisation and the restriction to the right tegmen can be seen as a synapomorphy of Corydiinae, which, however, only survived in the four genera of the Therea- group, a further proof for their basal position within the subfamily.
The subcosta lobe on the tegmina can also be interpreted as a synapomorphy of Corydiinae. The lobe may function as a stabilisator of the tegmen plain. Such a lobe is not found in any of the other subfamilies of Corydiidae; it may be dispensible there because of the smaller size of the species.
Presence of wings in females is certainly a plesiomorphic character of Corydiinae, and only found in the four basal genera. Holocompsinae and Euthyrrhaphinae also have winged females; females of the genus Zetha have strongly shortened wings, those of Tivia are wingless. Loss of the wings in females must have occurred independently in Corydiinae and Tiviinae; it was a frequent event in the evolution of Blattodea.
4. Diversification within the Therea -group
Among the four genera, Homoegamia is set apart by a number of characters in which it differs from the other three genera of the group. They are listed below, after a dash followed by the alternatives realised in the other genera.
1. Supraanal plate: relatively long, with rounded posterior border (Fig. 16A)—short, transversely cut shortly behind the insertion of the cerci (Fig. 16C,E,G).
2. Subgenital plate: well developed anterior apodemes, posteriorly more or less transversely cut, bristles only along the posterior and lateral margins (Fig. 16B)—apodemes weakly developed, posterior border convex, bristles distributed in the posterior half or third of the plate (Fig. 16D,F,H).
3. Cercal tricholiths, maximal number per anulus: 2 (Fig. 18C)—3 per anulus (Fig. 18G,H), in Eucorydia absent.
4. Subgenital plate of female: valvular—without valvular incisions.
The differences suggest, that there was an early branching in the ancestors of the Therea- group separating Homoeogamia from the three other genera (Fig. 24). As synapomorphies of the latter can be considered: (1) the loss of the valvular incisions in the female subgenital plate (char. 6)—valvular incisions also present in all females of “ Euthyrrhaphidae ”; (2) the shortened supraanal plate (char. 3)—in “ Euthyrrhaphidae ” of normal length or, rarely, slightly shortened; (3) the presence of three tricholiths per cercal anulus (char. 4). Outside the Corydiinae tricholiths are only found in Tiviella inconspicua and Euthyrrhapha pacifica, in the former (Fig. 18B) in a double row as in Homoeogamia; in the latter there are basally some pairs, but most anuli have only one of these organs (Fig. 18A). The agreement in the distribution pattern between Tiviella inconspicua and Homoeogamia suggests that the last common ancestor of the Therea- group had similarly arranged tricholiths, and that the pattern found in Ergaula and Therea was later developed after the splitting from the Homoeogamia branch. The loss of the tricholiths in Eucorydia presumably occurred still later, at least after the development of synapomorphies (1) and (2). As an autapomorphy of Homoeogamia may be considered the shape of the male subgenital plate with concave posterior border and bristles destributed along posterior and lateral margins. The proposed phylogeny implies, that the reduction of female wings and of part of the specialisation of the right tegmen must have occurred independently on both branches.
description: Published as part of Bohn, Horst, 2024, The spine armament of the legs as an important means for the characterisation of the genera of Corydiinae and their relationships (Blattodea, Corydiidae), pp. 1-79 in Zootaxa 5482 (1) on pages 15-16, DOI: 10.11646/zootaxa.5482.1.1, http://zenodo.org/record/13209117