GENERAL CHARACTERISTICS OF THE SENSORY SYSTEM 7 



smallest are smaller than many protozoa (Folsom and Wardlc, 1943). 

 The range extends from about 166 mm. (the Venezuelan grasshopper 

 Tropidacris latreillei and some East Indian walking sticks which arc 

 even larger) down to a fraction of a millimetre (some springtails, 

 ceratopogonine midges, and beetles of the family Trichopterygidae). 



An exoskeleton and small size are therefore two of the outstanding 

 characteristics of insects. Clearly they must impose upon the nervous 

 system certain restrictions which will be reflected in behaviour. For 

 example, smallness reduces the distance over which conduction of 

 impulses is required. This would imply, other things being equal, more 

 rapid response and movement. At the same time, however, size 

 limitations compel a reduction in the number of neurons possible in 

 the system. A reduction in number of units implies a reduction in the 

 informational capacity of the system. Reduction is carried further by 

 the development of so-called giant fibres. Thus, in the abdominal 

 nerve cord of the cockroach Periplaneta americana the giant fibres 

 occupy about 12 per cent of its cross-sectional area (Roeder, 1948). 

 The largest of these fibres measure 30 microns in diameter, exceeding 

 in this respect the largest (alpha) fibres in the mammalian system. 



Roeder (1959) argues persuasively that the relative merits of a 

 nervous system composed of a few large units versus one consisting of 

 many small units can be appreciated if one concluded that detail of 

 information has been sacrificed for speed. It is noteworthy that large 

 insects tend to react more slowly than smaller ones and that one at 

 least, the giant Australian cockroach {Macropanesthia rhinocerus 

 Suass.), lacks giant fibres (Day, 1950). A large fibre cannot carry as 

 much information from one point to another as can a number of 

 smaller fibres because of the on-off or all-or-none nature of the nerve 

 impulse, but it can transmit its information more rapidly. The giant 

 fibres are the internuncial units in an alarm reaction. In the detection 

 of, and escape from, predators, speed has greater survival value than 

 detailed information. From the point of view of a predator also, speed 

 is important, since attack must be as rapid as the startle response of a 

 prey. Here, however, the information required is of a much more 

 complex nature, but it, too, must be handled by a small nervous system 

 with relatively (as compared with vertebrates) few units. 



Another example of the parsimony of neuronal elements is seen in 

 the motor system of insects. As Hoyle (1957) has pointed out, there 

 are functionally important muscles which are microscopically small 

 and yet move joints with precision and delicacy. In contrast with 

 vertebrate muscles, which are innervated by hundreds of nerve fibres 



B 



