INSECTS IN GENERAL 5 



insects suggest that their functions may be almost 

 equally varied. Insects may have two kinds of eyes 

 simple and compound. Each simple eye, or ocellus, 

 is a small polished lens set above a cup-shaped retina, 

 which is connected by nerves with the brain. Certain 

 young insects (e.g., a caterpillar) have little groups of 

 these eyes on each side of the head ; but in the adult, 

 if they are present at all, the ocelli usually occupy a 

 central position. A bee, a wasp, or a dragon-fly, has 

 three of these eyes arranged in a triangle upon its 

 brow. So far as we are able to judge, the ocelli are 

 exceedingly " short-sighted," and are probably more 

 serviceable in distinguishing light from darkness than 

 in forming images. The compound eyes, of which 

 most adult insects have a pair situated one on each 

 side of the head, are very marvellous organs. The 

 surface layer, or cornea, is composed of numerous 

 facets, or separate lenses, connected with the brain by 

 an extremely complex nervous mechanism. It is 

 possible by the aid of the microscope to photograph 

 an object, such as a postage stamp or the flame of a 

 candle, through the isolated cornea of an insect's eye ; 

 and when this is done, we obtain a multiplex picture 

 of the object one image, in fact, for every facet in 

 the cornea. This interesting experiment led certain 

 of the early naturalists to conclude that the compound 

 eyes present a multitude of separate images to the 

 insect's brain. In the case of some hawk-moths the 

 number would be something like 27,000 for each eye! 

 Of what use this amazing portrait gallery could be to 

 the creature concerned, no one attempted to explain. 

 We need not pass in review all the learned disputa- 

 tions to which the problem of insect vision has given 

 rise. Suffice it to say that the theory of a multiplex 



