106 A BOOK OF INSECTS 



in the antennae. In the case of the hornet, it lias been 

 estimated that there are 13,000 sensory " pits " on each 

 antenna; in the blow-fly 17,000; while in the male cock- 

 chafer the number is nearly 40,000. 



Our knowledge of the senses of insects depends almost 

 entirely upon experimental evidence, although in the case 

 of specialised eyes and ears something may be inferred 

 from the structure of the organs. The simple eyes of an 

 insect are of very short focus, and there is no power of 

 adjustment. Thus, if the image on the retina is to be 

 " sharp," as a photographer would say, the object must 

 be at a definite distance from the lens ; and as the lens is 

 usually strongly convex, this distance must be small. The 

 simple eyes, in fact, are exceedingly short-sighted. Those 

 of a caterpillar, for example, are of such a focus that their 

 owner can see the surface upon which it crawls, and the 

 food which it is eating; but objects a few inches away 

 probably appear dim and unsubstantial. Indeed, in the 

 case of most insects which possess them, simple eyes are 

 probably more serviceable in distinguishing light from 

 darkness than in forming images. 



The compound eyes of adult insects have for many 

 years formed a subject for debate among naturalists. It 

 is admitted that they are scarcely if at all inferior to the 

 eyes of a vertebrate animal in the delicacy and intricacy 

 of their structure ; yet their physiology has proved a 

 vexed question. After much discussion and endless ex- 

 perimenting, the theory put forward early in the last 

 century by Johannes Muller is still generally accepted. 

 This supposes that an exceedingly fine ray of light passes 

 through each lens, or facet, and down the channel formed 

 by its corresponding cone and rod — all oblique rays being 

 absorbed by the surrounding dark pigment. Tims, while 

 each element of the compound eye is responsible for only 



