Surface ultrastructure and evolution of tarsal attachment structures in Plecoptera (Arthropoda: Hexapoda)

Abstract

The plantar surfaces of the stonefly tarsomeres and pretarsus are examined chiefly using scanning electron microscopy (SEM) of 39 Plecoptera exemplar species representing each family within the two suborders Arctoperlaria and Antarctoperlaria. Features examined include the shape and size of the tarsomeres as well as external features of the plantar surfaces of the tarsomeres, unguitractor plate, claws, arolium and orbicula. Characteristics presumed to be part of the stonefly ground pattern were determined by mapping these features on a cladogram of stonefly phylogeny (Zwick 2000, Annual Review of Entomology 45:709–746). This analysis indicates that the following characters are part of the stonefly ground pattern: (1) cylindrical shape of the tarsomeres, (2) an elongate tarsomere 1, (3) a short tarsomere 2, (4) presence of hairy euplantulae on the plantar surfaces of tarsomeres 1 and 2, (5) absence of specialised attachment surfaces on the plantar area of tarsomere 3, (6) absence of setae on the arolium plantar surface, and (7) presence of setae on the orbicula surface. Hairs covering the euplantulae of tarsomeres 1 and 2 and the median longitudinal unscelerotised region of tarsomeres 1–3 are non-setal cuticular projections and appear to be acanthae. Hairy euplantulae are unusual and are only present in one other insect order, the Mantophasmatodea. Specialised tenent setae were not found. The presence of hairy euplantulae as part of the stonefly ground pattern contradicts the recent view (Beutel and Gorb 2006, Arthropod Systematics & Phylogeny 64:3–25) that stoneflies lack pad-like euplantulae and that the ancestral condition for tarsomeres 1 and 2 may be a narrow median longitudinal unsclerotised band such as that found in representatives of the Austroperlidae. The hypothesis that Plecoptera are the sister clade to the remaining Neoptera suggests that euplantulae might be a synapomorphy of both clades. On the other hand, placement of Plecoptera within a monophyletic Polyneoptera clade within the Neoptera suggests that euplantulae could be an autapomorphy of this subordinate clade.