8 THE PHYSIOLOGY OF INSECT SENSES 



under a complex central control, the insect muscle is supplied with a 

 very small number of motor fibres. Some muscles are supplied by four 

 or more axons; some are mono-axonic. More commonly, a muscle is 

 supplied by two axons. Thus, the entire system of nervous control 

 differs from that in the vertebrates (Hoyle, 1957). 



In the sensory system, too, there is a limitation on the number of 

 neurons. This is related not only to the small size of the body but also 

 to the fact of its being encased in a rigid non-living cuticle. Only at 

 certain points do the energies of the external environment filter 

 through to sense cells. As compared with the integument of an 

 echinoderm, which may have as many as 4,000 sense cells per square 

 millimetre of surface, and a mammal, whose skin receptors may run 

 into the millions, the integument of an insect is relatively barren and 

 insensitive. The fourth-stage larva of Rhodnius, for example, has only 

 about 420 receptors on the entire ventral surface of each abdominal 

 segment (Wigglesworth, 1953); the sensory complement of the entire 

 leg of a fly is less than 500 (Grabowski and Dethier, 1954; Dethier, 

 1955 b) ; the total number of stress receptors on the whole body of the 

 drone honeybee is only about 2,948 (Mclndoo, 1914 b, 1916). It is 

 only when one counts the cells in the two most highly developed 

 sensory areas of insects, the eye and the antennae, that the number of 

 receptors becomes large. And even then they fall short, by several 

 orders of magnitude, of equalling the number in vertebrates. The 

 maximum number of receptors in any compound eye is that found in 

 Odonata and is estimated to be approximately 210,000 (Snodgrass, 

 1935). The number is more usually a few hundred or thousand. The 

 antennal sense cells of the honeybee only number between 30,000 and 

 500,000 (Vogel, 1923 b; Snodgrass, 1956). 



Economy of cells is seen further in the remarkable organization of 

 the nervous system whereby stimulation of a single sense cell may be 

 adequate to set off a whole chain of behaviour. Stimulation of one 

 neuron in the labellar taste organs of flies initiates proboscis extension 

 and the initial steps in the feeding pattern (Grabowski and Dethier, 

 1954; Dethier, 1955 a; Arab, 1959). Roeder and Treat (1957) have 

 shown that the acoustic response in noctuid moths is mediated by 

 only two receptor cells. Similarly, stimulation of one neuron in tactile 

 receptors can initiate a whole series of behaviour patterns ranging 

 from simple withdrawal of an appendage to running. 



The ultimate in parsimony is achieved by all known receptors being 

 primary sense cells, that is to say, they are true neurons rather than 

 modified epithelial cells connected synaptically to a neuron (vom 



