Abstract
External and internal structures of the thorax of the myrmecophile beetle Claviger testaceus (Clavigeritae, Pselaphinae) were examined and documented with state-of-the-art visualization techniques. Following a general trend in the omaliine lineage (Staphylinidae), the skeletal elements of the pro- and pterothorax in Claviger reach a maximum degree of compactness, with largely reduced inter- and intrasegmental sutures and skeletal elements linked with the flight apparatus. The musculature, especially metathoracic direct and indirect flight muscles, also shows a high degree of reduction. Two forms of wings were found among individuals of C. testaceus, both non-functional and representing an advanced stage of reduction. However, that wing vestiges are still present and the metanotum, only slightly reduced, suggests that loss of flight in this species is likely the result of a young evolutionary process. Several structures are linked with myrmecophilous habits: small body size facilitates transportation of beetles by ant workers and makes it easier to move inside nest tunnels; the remarkably compact body and mechanically robust appendages make the beetles less vulnerable to attacks by ant mandibles; the improved elytral interlocking mechanism and unusually expanded epipleura enhance the protection of vulnerable dorsal parts of the pterothorax and anterior abdomen; and glands associated with trichomes on the posterolateral elytral angle produce secretions attractive for ants. Various modifications of the thorax and anterior abdomen lead to an optimization of intimate associations with ants. The morphological syndrome enabling these beetles to cope with life in ant colonies evolved in several steps. This is suggested by an increasing solidification of the thoracic skeleton in related non-myrmecophilous groups and also by less modified related clavigerites;for instance, ant-associated tropical species are still able to fly.
Highlights
With a world-wide distribution and more than 10,000 described species (Yin et al 2019), Pselaphinae ranks as the second largest subfamily of the megadiverse polyphaganStaphylinidae
The protocol recommended by Schneeberg et al (2017) was modified to clean the beetles: the specimens were transferred from FAE into 70% ethanol, followed by 0.5% Triton X100 (14 h), 5% KOH (14 h), glacial acetic acid (3 × 15 min), distilled water, and 70% ethanol
A remarkable degree of morphological modification of the thorax of Clavigeritae leads to an optimization of intimate associations with ants, keeping the ecological coexistence stable and sustainable
Summary
With a world-wide distribution and more than 10,000 described species (Yin et al 2019), Pselaphinae ranks as the second largest subfamily of the megadiverse polyphaganStaphylinidae (over 64,000 described species, Fikáček et al 2020). One of the most successful myrmecophilous radiations in Coleoptera took place in this subfamily, with far-reaching morphological modifications (Thayer 2016; Parker 2016a). Apparently linked with a far-reaching social integration in ant colonies, evolved in Clavigeritae, one of the six pselaphine supertribes with more than 300 described species (Parker and Grimaldi 2014). Most studies were focused on structural features of the head and abdomen, such as the distinctly reduced mouthparts and specialized armatures of abdominal and elytral trichomes (see Hermann 1982; Parker 2016a; Jałoszyński et al 2020), and on appeasement glands (e.g., Cammaerts 1973, 1974). The trichomes on the abdominal tergites are a presumptive synapomorphy of the
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