Hairy attachment structures in Reduviidae (Cimicomorpha, Heteroptera), with observations on the fossula spongiosa in some other Cimicomorpha

Reduviidae and some other groups of cimicomorphan Heteroptera possess a hairy attachment structure on the apex of the tibia called “fossula spongiosa”. The fossula spongiosa was never studied comparatively across Reduviidae, its fine structure and mode of function is not well documented, and attachment structures in immature stages are virtually unknown. Here, a sample of 171 species of Reduviidae representing 22 subfamilies is examined for presence–absence of the fossula spongiosa on the three pairs of legs. Representatives of 11 of the 22 subfamilies are shown to possess a fossula spongiosa. The fine structure of the fossula spongiosa is examined for a more limited sample of Reduviidae and several Pachynomidae and Nabidae. In addition, scanning micrographs for the fossula spongiosa in other Cimicomorpha are given, among them Anthocoridae, Cimicidae, Microphysidae (first record of a fossula spongiosa), and Thaumastocoridae. The fossula spongiosa in Reduviidae consists of tenent hairs (acanthae) with spatulate or tapering apices interspersed with sensory setae, both of which are embedded in a thick and flexible cuticle, underlain by a hemolymph cavity separated almost entirely from the interior of the remaining tibia by a cuticular invagination. Judging from comparison with non-reduviid Cimicomorpha, this separation of the fossula spongiosa cavity from the tibial interior may be unique to Reduviidae. A simple experiment using live specimens of Platymeris biguttata (Reduviinae) revealed a liquid on the tip of the tenent hairs that might be involved in the attachment of the fossula spongiosa by adhesion mechanisms. The nymphs of Reduviidae whose adults have a fossula spongiosa are here documented for the first time to possess pads of ventrally barbed setae instead of tenent hairs and their tibia lacks the internal cuticular invagination. The nymphal attachment structures seem to rely on increase of friction rather than the adhesion mechanism proposed to be present in the adult. Combined with the tenent setae on the third tarsomere known in some Emesinae and here documented for Saicinae, three types of hairy attachment structures occur on the legs of Reduviidae: tenent hairs (acanthae), which form the fossula spongiosa in many Reduviidae, barbed setae that substitute the fossula in the immatures, and tenent setae on the tarsus which are restricted to only a few taxa.