30 SILKWORMS. 



is composed of many hundreds, and hence the name " compound 

 eyes," by which they are known. If we apply to them notions 

 derived from the knowledge of our own eyes, and imagine that 

 they are anything like what we are ourselves accustomed to, we 

 shall be grievously in error. If they were constructed at all like 

 ours we should be able, to some extent, to argue from our own 

 experience, as to what is the nature of insect vision. But, un- 

 fortunately, this is not the case, and it is very difficult for us, 

 who have never had any experience in the management of com- 

 pound eyes, to understand what sort of vision they can produce. 

 We are accustomed to think of an eye as a delicate, somewhat 

 yielding, globular object, snugly quartered in a hollow fitted up 

 for its reception with paddings of flesh, and capable by means of 

 elastic bands of flesh attached to its sides, of pretty free move- 

 ment, noiselessly and without friction, in the well lubricated 

 cavity. Nothing, however, could be farther than this from 

 presenting a true picture of an insect's eyes. They are not sunk 

 in sockets as ours are, nor are they movable ; there are no 

 eyelids, nor any other kind of covering, by which they can be 

 withdrawn from the action of the light, so that their possessor 

 cannot perform such a very simple and familiar operation as 

 " shutting its eyes." Neither is their surface soft and moist, as 

 with our own ; they are hard and dry, and fully exposed to the 

 glare of the light, and unquestionably, their surface is not acutely 

 sensitive to the touch, so that no such exquisite torture as we 

 are ourselves sometimes subjected to, can be produced by the 

 contact of foreign bodies. 



And yet these eyes are exceedingly complicated structures. 

 Each of the hexagonal facets, as already mentioned, is the outer 

 surface of a single organ of vision. It consists of a piece of 

 transparent, horny substance, beneath which, and pointing 

 towards the centre of the spherical eye-mass, is a curious, 

 elongated body called a crystalline cone and spindle, which is 

 itself surrounded by a dark-coloured material. Thus we have 

 hundreds of these structures lying side by side, all placed per- 

 pendicularly to the surface, and all tending to converge towards 

 the centre of the spherical mass, where is placed a large expansion 

 of the optic nerve, one of which proceeds on each side from 

 the insect's brain. 



An insect's compound eye is a very beautiful object, when 

 examined under the microscope with a low power. If it be 

 viewed as an opaque object, with a bright light directed upon 

 it by a " bull's-eye," all the facets show up wonderfully distinctly, 

 and present a most attractive appearance. But the clear outer 



