398 Mr Ewing on the Structure of the Eyes of Insects. 



ing light : they are of a hard substance, and change not on 

 being taken from the insect. 



Conglomerate Eyes. 



They are exactly the same as simple eyes, being double 

 convex lenses, each of them capable of refracting light ; only 

 they are more numerous than simple eyes, and are collected 

 into patches, and have a common retina. 



Compound Eyes. 



Under this head are included the whole of those eyes which 

 we find provided with a lace-like covering. I am satisfied, 

 however, that, under this division of the subject there are 

 different modifications, which I shall notice separately. 



Beetles'' Eyes. 



It appears to me that those eyes differ not from simple eyes 

 of the conglomerate kind. The form indeed varies, but the 

 structure is the same. It is spherical, and composed of a num- 

 ber of hexagonal apertures, filled with lenses, each of which 

 possesses the same properties as simple eyes ; and having a 

 common retina, which is connected to the external covering, 

 so as to exhibit the appearance that two watch glasses would 

 have if they were cemented together. 



Butterfly's Eye. 



This eye differs from all the insects eyes I have examined. 

 It consists of a ball which is orbicular, and of a dark purple 

 colour towards the external lace-like covering ; and on the 

 other side, where the optic nerve enters, it is white and less 

 convex. This ball occupies a circular cavity formed by the 

 external covering and the retina, and is surrounded by a very 

 clear gummy liquor into which it appears to move. My way 

 of ascertaining this, was by fixing the live insect in the pliers 

 under the microscope, and, putting a mark on the centre of the 

 external covering, I turned the insect backwards and for- 

 wards ; and I observed, that when the lace-like covering moved 

 round, the dark spots in the eye were stationary, and could 

 be moved from one edge of the eye to the other. Now, this 



