THE SENSES OF INSECTS 103 



the sub-a?sophageal ganglion (i.e. the supplementary brain; 

 are united into a single mass situated in the fore-body. 

 Such fusion of nerve-centres may be said to result in con- 

 centration of individuality ; for where a nerve-centre is 

 present in each segment, each segment is to some extent 

 capable of independent life." 



From the central system, nerves radiate to all parts of the 

 body. These may be either sensory or motor, the former 

 transmitting impressions (or calls) inward from a sense- 

 organ, the latter conveying impulsive stimuli (or answers) 

 outwards to the muscles, glands, &c. The most wonderful 

 of all the special sense-organs — indeed, the most wonderful 

 of all the structures of the insect — are the compound eyes. 

 Their anatomy varies greatly in detail, but our present pur- 

 pose will be served by the following brief description. If 

 we examine the surface (or cornea) of these eyes with the 

 aid of a microscope, we see that it is divided into a multi- 

 tude of minute areas, commonly six-sided, known as facets. 

 The number of facets varies with the size of the eve. 

 Thus, the eye of the little " silver-fish " has as few as 

 twelve ; while it has been calculated that in the eye of 

 the cockroach there are 1800, of the house-fly 4000, of the 

 goat moth 7000, and of the swallow-tail butterfly 17,000. 

 Some dragon-flies have 20,000, and some hawk-moths as 

 many as 27,000. Now each of these facets is the surface 

 of a lens, and each lens is mounted upon a minute inverted 

 cone, termed a crystalline cone, which in its turn connects 

 at its tapering end with a nerve-rod. A crystalline cone 

 consists of a group of sensitive visual cells in touch with 

 the nerve-fibres, and each cone is optically separated from 

 its neighbours by means of dark pigment. The whole 

 eye is intimately connected with the brain, of which, 

 indeed, it may almost be said to form an offshoot. 



Insects never have more than one pair of compound 



