Exploring a key synapomorphy: correlations between structure and function in the sternum V glands of Trichoptera and Lepidoptera (Insecta)

Abstract

The sternum V glands are a key synapomorphy that unites Trichoptera with Lepidoptera, but their functional aspects have not been analysed from an evolutionary perspective. We examine phylogenetic trends and correlations between chemical and morphological features of these glands. The most likely ancestral gland compounds are heptan-2-ol, 4-hepten-2-one and -ol, nonan-2-one, and 6-nonen-2-one and -ol, making pheromone production a plausible ancestral function. The most widespread gland compounds (heptan-2-one and -ol and nonan-2-one and -ol) are not known from Apataniidae + Limnephilidae (Trichoptera), which in turn uniquely produce a number of methylated 3-ketones and their corresponding alcohols, probably functioning as pheromones. We propose a functional connection between perforated patches on sternum IV in females and a scaly/dome-covered area around the gland openings, as well as between perforated patches and lack of Trichoptera-type opening muscles. We also propose a functional connection between the shape of the gland reservoirs and the presence of gland reservoir musculature. The perforated patches were significantly correlated with several gland compounds that had double bonds between carbon atoms: the double bonds may lower the viscosity of the compounds, facilitating secretion through the tiny pores of the perforated patches. The production of defensive substances in Pycnopsyche (Trichoptera: Limnephilidae) is probably connected to the presence of large, compartmentalized gland reservoirs. Large glands in male Hydropsyche (Trichoptera: Hydropsychidae) are probably linked to male aggregation pheromone production. The relative sizes of sternum V gland reservoirs and fenestral gland reservoirs in female philopotamids (Trichoptera) suggest a complementary function of the two structures. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106, 561–579.