84 ON THE ANATOMY OF THE FLY. 



gradually separated from the snpra-cesophngeal ganglia, by the 

 development of the cephalo- thoracic nerve trunk from the 

 crura of the latter. It still supplies the twelve posterior 

 segments of the imago, although the number of ganglia is 

 reduced to four pairs, either by the suppression, or more pro- 

 bably by the coalescence, of several. 



The nerve centres of the larva are surrounded by the imaginal 

 discs, from which the head and part of the thorax of the 

 imago is developed. 



Section XII. The Compound Eyes. 

 PLATE VII., FIGS. 57, AND PLATE VIII., Fios. 1 & 2, 



The compound eyes consist of a compound cornea, beneath 

 which are the rods and cones. These are connected with the 

 nerve centre through the medium of nerve cells and fibres. 



The compound cornea is covered by a layer of protoderm, 

 which retains the form of the lenses beneath, when these 

 are dissolved out with a solution of caustic potash. The 

 lenses (Plate VIII., fig. 1), are bi-convex, consist of a trans- 

 parent substance, and are united to each other by a flat 

 frame-work of the same material. In fact, the compound 

 cornea ought properly to be considered as a single piece, sculp- 

 tured, as it were, into numerous lenses The diameter of each 

 lens is about l-1000th of an inch. The inter-lenticular spaces 

 are generally hexagonal frames, but some near the edge of the 

 eye are almost square. 



