INTRODUCTION. '5 



antennae of many beetles, which consist of thin 

 chitinous lamellae, may be hygrometric, indicating the 

 state of the atmosphere to the insect. I have little 

 doubt, however, in other insects, as in the fly, especially 

 when they are thick and club shaped, that they are 

 olfactory or rather partly olfactory organs."* Some 

 years ago I examined a great number of wasps, with a 

 view to satisfy myself on this point; I used the bleaching 

 process recommended by Dr. Hicks, but was unable to 

 come to any conclusion on this interesting but puzzling 

 point. 



Conspicuous on tlie head of every insect are its two 

 large compound eyes of various colours — emerald, blue, 

 chesnut, orange, or as beads of burnished gold ; besides 

 these are generally to be seen two or three simple eyes, 

 called ocelli, which are placed on the top of the head 

 between the two large compound eyes — these require 

 the aid of a lens to render them 

 visible. The compound eyes are 

 made up of an immense number 

 of hexagonal or six-sided facets, 

 which in some insects, as in the 

 dragon-flies, can be distinguished 

 by the naked eye ; each facet is ^^"^°"^^ ^'^ '' ^"^ ''''^'' 

 in itself a perfect eye, having a cornea, a lens, a pig- 

 ment-coating, and a nervous filament; the eyes are 

 immovable, and as the head is limited as to motion, it 

 might be supposed that an insect was not particularly 

 sharp sighted, but everyone who has tried to get at the 

 blind side of a common house fly, knows how quickly 



* The Anatomy of the Blow-tiy, p. 32. 



