Morphology of the abdominal segmental glands and spinning behaviour of Stenus larvae (Coleoptera, Staphylinidae)

Abstract

We focus on the morphology of the "segmental glands" and their openings in third instar Stenus larvae. The location of the openings was similar in both studied species, with paired rosette-like structures present on the head, all thoracic segments and abdominal segments 1-9. No such openings could be found on the antennae, the maxillary palps, the urogomphi, and the legs as suggested in some older publications. We presume that the glands up to abdominal segment 7 are "adhesive" glands. They are compound glandular units consisting of a secretory syncytium with a common reservoir and a canal cell. The common reservoir is connected through a single efferent duct with the opening of the gland. Glands of abdominal segments 8 and 9 show differences in their length, number of reservoirs, the orientation of the efferent canal, the inner structures of the gland openings towards the exterior and the shape and content of the secretion vesicles indicating that they are silk glands for cocoon building. The spinning behaviour has been observed during the building of the hatching and pupation retreats. The larva first attaches to the substrate with its pygopod, secretes silk droplets from silk gland openings and pulls out a silk filament from the tip of its urogomphi. Whereas L1 and L2 instars produce an open single-layered net, L3 build a closed bi-layered cocoon.

Keywords: Adhesion; Beetle larva; Cocoon-spinning; Gland; Secretion; Silk; Stenus bimaculatus; Stenus guttula.