Apical antennal sensilla in nymphs of Libellula depressa (Odonata : Libellulidae)

In an ultrastructural study of the apical antenna of the last nymphal stage s of Libellula depressa (Odonata: Libellulidae), we found long sensilla tri chodea, 2 sensory pegs, and a coeloconic sensillum on the last article of t he flagellum (the distal part of the antenna). The long sensilla trichodea. are mechanoreceptors, almost identical to the long filiform hairs of some terrestrial insects and the first sensilla. of this kind to be described in aquatic insects. Particular attention was given to the complex coeloconic sensillum, a compound sensillurn innervated by 2 groups of 3 neurons wrappe d in a dendritic sheath. A cuticular sleeve envelops the distal portion of the outer dendritic segment. The cuticle of the coeloconic sensillurn shows wide channels and is contiguous to the underlying granular and fibrillar l ayer. Similar structures on the antennae of the adults of other dragonflies were identified as chemoreceptors in previous studies. We hypothesize that this larval coeloconic sensillurn might likewise have a chemosensory funct ion, responding to molecules that diffuse through the cuticle and the under lying granular and fibrillar layer, as no clear pore or pore-tubule system is visible. Alternative functions are also explored on the basis of morphol ogical details.