THE NERVOUS SYSTEM 123 



Temperature and humidity senses 



All insects react to changes in temperature and humidity, but the 

 sense organs responsible are not well defined. The temperature sense 

 is spread all over the surface of the body; but in blood-sucking in- 

 sects the antennae are particularly sensitive and are used for locating 

 warm-blooded hosts. This response seems always to be to the tem- 

 perature of the air and not to radiant heat. The sensilla concerned 

 are probably the fine hairs which are abundant on the antennae. 



Fine tufts of hair connected with sense cells serve as organs for 

 detecting changes in atmospheric humidity in the louse Pediculus 

 and in the larva of Musca. But in most insects it has not proved 

 possible to distinguish between thin-walled chemoreceptors and the 

 organs sensitive to humidity changes. 



Vision 



We have already seen how varied are the visual responses of insects. 

 The receptor organs which serve these responses range from small 

 groups of light-sensitive cells beneath lens-like thickenings of the 

 cuticle, to the compound eyes of the highest forms, each with many 

 thousands of retinal elements and capable of tolerably good image 

 reception. 



The stemmata, or simple eyes of larvae, which occur in the absence 

 of compound eyes, are responsible for the orientating responses to 

 light in maggots and caterpillars. In their highest development, which 

 is attained in the predaceous larvae of Cicindelids and other beetles, 

 they serve to locate the prey; and although it is doubtful if they are 

 capable of image perception, they can certainly perceive the move- 

 ment of objects within the field of view. The ocelli, or simple eyes of 

 adult insects, on the other hand, are optically unsuited to even the 

 crudest type of image formation ; for not only are the retinal elements 

 few in number, but any image that is formed by the lens must fall far 

 beyond the level of what elements exist. In adult insects it is the 

 compound eyes which are responsible for the movements towards 

 light: such movements do not occur if the compound eyes are 

 covered, and only the ocelli remain. 



What, then, is the function of ocelli ? So far, the only suggestion 

 of the many that have been put forward, which has any experimental 



