Morphology, function and evolution of the sternum V glands in Amphiesmenoptera

Abstract

I investigated the paired sternum V glands in thirty-eight trichopteran families and all lepidopteran families possessing the gland or associated structures. Using my morphological data and literature data on sternum V gland secretions, I examined phylogenetic trends in morphology and gland products and reconstructed ancestral states. I investigated correlations between gland products, between morphological traits and between chemistry and morphology. The gland is present in twenty-five trichopteran families. It is generally present in Annulipalpia, except Dipseudopsidae, and in Spicipalpia. It is widespread in Plenitentoria, but is often absent in Brevitentoria, especially in males. In Lepidoptera, I present the first report on the reduced, but functional glands in Neopseustidae and Nepticulidae. The gland is typically an invagination from sternum V with a duct leading to a reservoir surrounded by secretory tissue. An opening muscle inserts just inside the opening. I found two non-homologous opening-muscle types, one in Lepidoptera and some Trichoptera, another in the remaining Trichoptera. Muscle fibres often surround the reservoir, sometimes also the secretory tissue. Exceptions are found in Psychomyiidae (no opening muscle), female Philopotamidae (fenestra with separate glandular complex), Agathiphagidae (several unique features), Neopseustidae and Nepticulidae (gland present without gland opening). Using variations in gland structure, I identified phylogenetically useful characters from the superorder to the species level. The fenestrae in female Philopotamidae, Eriocraniidae, Neopseustidae and Nepticulidae are perforated, and perforated patches are present in female Psychomyiidae. The perforated patches are associated with a reservoir, secretory tissue and a distinctive ‘sunburst’ musculature in both Trichoptera and Lepidoptera. The probable ancestral gland compounds are heptan-2-ol, 4-hepten-2-one, 4-hepten-2-ol, nonan-2-one, 6- nonen-2-one and 6-nonen-2-ol, making pheromone production a plausible ancestral function. The most widespread gland compounds are heptan-2-one, heptan-2-ol, nonan-2-one and nonan-2-ol, but these are absent from Apataniidae + Limnephilidae, which instead produce methylated 3-ketones and -ols, unique within Trichoptera. These compounds all probably function as pheromones. Both large and small glands in females can function in sex pheromone production, while large glands in male Hydropsyche (Hydropsychidae) are likely linked to male aggregation pheromone production. Relative sizes of regular gland reservoirs and fenestral gland reservoirs in female philopotamids suggest a complementary function.