Questions of molecular evolution of pheromone communication in caddisflies and lower moths (Insecta: Trichoptera,Lepidoptera)

Recent GS-MS and GC-EAD studies of pheromone production and perception in caddisflies and lower moths have shown that these insects use a rather limited selection of volatiles as attractants. Most of them are alcohols and ketones, although the diversity of chemicals produced by sternal glands of abdominal segment V is much wider, especially in the lower Trichoptera. Sternal pheromone glands produce only short-chain polymers in all Amphiesmenoptera. These glands are part of the ground-plan for the related orders, Trichoptera and Lepidoptera, occurring in both sexes and producing similar but not identical sets of components in males and females. The presence of pheromone volatiles is shown to be restricted to the gland segments (Fig. 1), although some other shortchain polymers do occur in the head of females of Molanna angustata (Molannidae). The pheromone blends of lower Trichoptera (Glossosomatidae, Rhyacophilidae, and Philopotamidae) are multi-component and resemble plant volatiles in composition. A hypothesis of the origin of pheromone communication is proposed postulating basic resemblance of early pheromones and plant volatiles in variety and chemical composition. These pheromones were detected by non-specialized receptors of the amphiesmenopteran ancestor and served as guides for insect aggregation on plants as well as on shores of reservoirs, marking the places suitable for a wide variety of species. The primary aggregation function of pheromones was changed in more advanced communication systems to the species-specific signaling with sex-related asymmetry of signals, although the aggregation significance persisted in some species. Pheromone communication has disappeared in some most advanced lineages (e.g., Leptoceridae) with parallel reduction of glands, secretion, and antennal receptors. The pheromone composition does not show gradual divergent evolution in related species; instead, abrupt transformation of pheromone blends with persistence of major components in remote families seems to be the typical case.