18 THE PHYSIOLOGY OF INSECT SENSES 



with which they are usually associated originate from the same parent 

 epidermal cell. The combination of the neuron (or neurons), the 

 immediate cuticular area, when present, and its generative cells is 

 termed a sensillum. A sensillum is thus a sense organ ; the neuron is 

 the receptor. With the possible exception of the photoreceptors, all 

 sensilla are believed to be homologous and to have been derived from 

 setae. Classically, because of the ease of examining the cuticular 

 portion, sensilla were classified on the basis of external form. They 



Fig. 1 1 . Sensillum placodeum 

 from the antenna of the 

 honeybee. N, bipolar 

 neuron; A, axon; D, 

 dendrites; P, plate; F, 

 attachment of fibres. (Re- 

 drawn from Snodgrass, 

 1935.) 



include setiform varieties (sensilla trichodea), bristles (sensilla 

 chaetica), scales (sensilla squamiformia), pegs and cones (sensilla 

 basiconica), pegs, cones, or bristles sunk in shallow depressions 

 (sensilla coelonconica) or in deep pits (sensilla ampullacea) (Schenk, 

 1903). 



Other sensilla whose external cuticular structures are not seta-like 

 are undoubtedly derived from setae (Snodgrass, 1935; Lees, 1942). 

 The campaniform organs (sensilla campaniformia, sense pores, etc.) 

 are visible externally as minute pits in the cuticle, but in section are 

 seen to possess a bell-shaped cuticular cap (Figs. 10, 30). Each is 

 innervated by a single neuron. Their structure will be discussed in 



