Innervation and fine structure of antennal sensilla of Simuliidae larvae (Diptera: Culicomorpha)

Abstract

Light, scanning electron, and transmission electron microscopy were used to investigate fine structure and innervation of antennal sensilla of Simuliidae larvae with particular attention to a bacteria-covered structure superficially similar to chloride-absorbing cells of other aquatic insects.Eight sensilla of five morphological types innervated by a total of 22 neurones are described. A single uniporous cone sensillum, with five distal dendrites in a common dendritic sheath is on the antennal apex. Two multiporous peg sensilla, each with two distal dendrites in separate, but fused dendritic sheaths, are at the base of the distal antennal article. Near the base of the proximal antennal article on the membranous antennal base is a single trichoid sensillum, probably with a single distal dendrite. Ventral to the base of the antennal articles is the bacteria-covered structure, shown here to be a multiporous sensillum, with five distal dendrites invested in a fused dendritic sheath, which has a foamy appearance and is fenestrated. A pore tubule system is described for this sensillum, the first time for sensilla of any aquatic insect. The sensillum is internally surrounded by a heavily cuticularized capsule which is supported laterally by buttresses and is penetrated by a canal, through which the dendrites enter.Because of a peculiar feature of moulting in Simuliidae larvae, the sensillar sinus of this sensillum is open to the moulting space between cuticle and epidermis. The cellular portion of that sinus is closed off.Probable gram-negative bacteria on this sensillum appear to prefer sensillar exudate. Comparison of chloride concentration in this sensillum, with that of another multiporous sensillum, using energy-dispersive X-ray spectrometry, showed no significant differences, indicating no enhanced chloride absorbing function for the former sensillum.The bacteria-covered sensillum is always closely accompanied by a trichoid sensillum innervated by a single distal dendrite with a tubular body.Two small uniporous peg sensilla located anteroventrally on the membranous antennal base have three distal dendrites each. In the large ciliary sinuses these dendrites become highly lamellated, bound with pleated septate junctions, and associated with concentrations of mitochondria. It is suggested that these lamellae have a secretory–absorbtive function.On the basis of similarity in structure and number, tentative homologies with antennal sensilla of other Culicomorpha larvae are suggested.