The present study investigates the antennae and brain of the immature stages of the caddisfly Hydropsyche pellucidula (Andersen, and Klubnes 1834) using scanning and transmission electron microscopy as well as fluorescence microscopy. The larval antenna is unsegmented and bears two long, articulated trichoid sensilla and two large, non-articulated basiconic sensilla, all with an internal structure typical of mechanoreceptors. No changes were detected between the larval stages examined, but larval sensilla differ completely from the adult sensilla previously described in the same species, which include several chemoreceptors (e.g. trichoid, pseudoplacoid, pseudoplacoid, chaetoid, coronary, and styloconic sensilla). A preliminary account of larval brain anatomy reveals a small central body without columnar elements, mushroom bodies without calyces, and no antennal lobes in any larval stage. This condition differs markedly from the pupal brain, which—similar to the adult brain—shows a central body with a vague fan-shaped structure, mushroom bodies with small calyces, and well-structured antennal lobes containing few but relatively large glomeruli. Such dramatic change is similar to what occurs in other closely related holometabolous insects, such as Lepidoptera, but it is more pronounced, probably because of the ecological differences between larvae and adults, as also observed in other aquatic insects such as mosquitoes. The results of this research shed light on overlooked aspects of caddisfly biology. Moreover, they may enhance our understanding of the evolution of insect olfaction, since caddisflies are among the most important orders of aquatic insects and the closest relatives of Lepidoptera, a key model system in insect chemical ecology.
