ELECTROPHYSIOLOGICAL INVESTIGATION OF 

 INSECT OLFACTION 



Dietrich Schneider 



Deutsche Forschungsanstalt fiir Psychiatrie, Max-Planck-Institut, Ableilung 

 fiir vergleichende Neurophysiologie, Munich, Germany 



THE SENSE ORGAN 

 Insects possess well-developed chemical senses. But unlike the verte- 

 brates, their sense organs for these chemical modalities are located at the 

 body surface uncovered by mucous and readily accessible to the surround- 

 ing medium. During development of the insect cuticle two daughter epi- 

 dermal cells differentiate to govern the formation of a specialized area of 

 the body surface which will serve as a contact locus for certain sensory 

 nerve fibres. One of these two cells — the trichogenic cell — builds a hair-, 

 peg-, cup- or plate-like cuticular piece. The other — the tormogenic cell — 

 builds a ring or socket surrounding this area. Sensory nerve cells such as 

 the formative cells of the specialized part of cuticle are also derived from 

 epidermal cells by differential cell division. This developmental plan is 

 similar for all sense organs of the insect cuticle as well as for non-sensory 

 hairs, scales, and glands (Henke, 1953). Each of these morphologically 

 differentiated pieces of insect cuticle with its formative cells, sensory-nerve 

 cells and accessory cells is called a Sensillum (Fig. 1). 



To each sensillum belong from one to many nerve cells. The distal 

 process of the nerve fibre grows " through " the trichogenic cell body to 

 make contact with the cuticle. Tn unequivocally identified gustatory 

 chemoreceptors, the endings of each nerve cell are in direct contact with 

 the surrounding medium through an opening at the tip of the hair (Dethier, 

 1955). Peg-shaped sensilla basiconica, probably serving the olfactory 

 modality, have many cuticular pores where the dentritic processes of the 

 nerve cell are in contact with the outer medium (Slifer, Prestage and Beams, 

 1957). 



A single sensillum does not necessarily serve only one sensory modality, 

 as was shown in the fly. Here a single hair is supplied by the fibres of 

 several nerve cells, one of which ends at the hair base in the socket area 

 and responds to mechanical deflections of the lever-hke hair. The others — 

 usually two — are chemoreceptive. One responds to " acceptable " sub- 

 stances such as sugar; the other responds to "unacceptable" substances 



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