Morphology and ultrastructure of sensilla on the proboscis of the adult spruce budworm, Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae)

Styloconic and basiconic sensilla are the only types of chemosensilla located on the proboscis of the adult spruce budworm, Choristoneura fumiferana. The styloconic sensilla (Ss's) are arranged on each galea in two rows, an inner and an outer one, each with about 35 sensilla. Basiconic sensilla (Sb's) are interspersed in an irregular row between the Ss's of the inner row. They are also found in the proximal part of the food channel. Each Ss consists of a single sensory peg inserted at the top of a style bearing 6-9 ridges that extend from the base of the style to the sensory peg socket, at which point the ridges protrude to form sharp points. The peg has a terminal pore about 0.08 µm in diameter, which is permeable to cobalt ions and contains a sievelike structure. The sensillum houses four dendrites; the ultrastructure of three is consistent with a chemosensory function and that of one is consistent with a mechanosensory function. Each Sb has a peg inserted in a socket directly on the surface of the proboscis. Its terminal pore has a diameter, sieve-like contents, and permeability to cobalt ions similar to those of the Ss peg. There are 4 or 5 innervating neurons; the dendrite of one is structurally typical of a mechanosensory neuron and the remainder are typical of a chemosensory neuron. Both types of sensilla show an ultrastructure consistent with a bimodal chemo-mechanosensory function.