THE HORNY SKELETON. 233 



lower margin of the occipital aperture, between which lies the cere- 

 bellum, and it is covered only by the pharynx. Sometimes (Dyticus') 

 the pharynx rests upon a bar, connecting both ledges, and then the 

 cerebellum lies beneath it, and further forward, but the nervous cord 

 runs between the ledges. Contiguous to the occipital aperture two 

 small hooks spring from the ledge, which encompass the nervous cord, 

 and other longer fine branches of them project forwards towards the 

 front, which they do not reach, but bend upwards., and serve for the 

 insertion of small muscles, which retain the pharynx, running between 

 these branches. This frame-work is larger or smaller according to the 

 development of the cibarial apparatus, consequently most distinct in 

 the predaceous beetles with large oral organs. 



165. 



INTERNAL SKELETON OF THE THORAX. 



In the structure of the thorax, the Hemiptera, Orthoptera, Neu- 

 roptera, and Coleoptera accord better together,, from their prothorax 

 being more distinctly separated than in the other orders, in which the 

 entire thorax forms but one whole. This last structure is certainly the 

 most simple, and we will therefore commence with its inspection. 



Upon paying some attention in the examination of the thorax of a 

 fly, bee, or butterfly, the important preponderance of the mesothorax 

 cannot escape immediate observation. The central dorsal plate oc- 

 cupies the entire dorsal surface, whereas the anterior one forms but a 

 ring (collar), and the posterior one also is not much more developed, 

 and, indeed, in flies and butterflies is entirely covered by the scutel- 

 lum, (compare PI. XIV. No. 1. f. 2. and No. 2. f. 2.). 



The internal skeleton of this simple thorax is very unimportant in 

 the Diptera. Where we observe furrows on the exterior there are 

 internal ridges which correspond, and which surround the muscles 

 at their insertion, and separate them from each other. Audouin 

 calls those projecting ridges, which are also generally found where two 

 separate parts join together, Apodemes. APODEMATA, and those to 

 which muscles are attached Apodemala insertionis. The largest of all 

 these ridges is Kirby and Spence's METAPHRAGMA, a thin, perga- 

 mentaceous partition, which, descending from the superior margin of 

 the metathorax, arches itself convexly outwards towards the abdomen, 

 and thus separates the entire cavity of the thorax from that of the 



