CHAPTER II. 



SPECIAL SENSES OF INSECTS. 



Sight. 



Both simple and compound eyes are found on most 

 insects; but some have either kind alone, and a few have 

 no eyes at all. The most primitive living insects to-day 

 have eyes, and the larvae of the complex insects have 

 simple eyes. Hence, if the highest individual in any 

 given class repeats the life history of the individuals below 

 it, it is only fair to suppose that the possession of eyes is 

 inherent in all insects ; and, where there is a lack of eyes, 

 it must be attributed to disease or some other environ- 

 mental conditions bringing about non-development or 

 disuse, and therefore degeneracy of the part not used. 



The compound eye is not an aggregation of simple 

 eyes; the two sorts of eyes differ in structure, and there is 

 good evidence that they are not derived from the same 

 line of body differentiation. The compound eyes of 

 insects are two in number, and are usually situated on the 

 upper head areas, a little to one side of the middle line. 

 They are usually conspicuous; in the dragon fly and the 

 house fly they are large in proportion to the rest of the 

 head. The dermestidse, the pests of the insect collector, 

 count the head of a dragon fly a rich treat and will eat out 

 the soft internal parts, leaving the corneal facets quite 

 clear. Many insects, as the butterflies, the dragon flies 

 and the house flies, have an enormous number of facets, 



7 



