ANATOMY. 



four, six, or seven different plates. Above, in the centre, is the third 

 DORSAL PLATE (metanotum) ; opposite to it on the breast, the simple 

 or divided third BREAST PLATE (mefasternum) ; between the two, the 

 SIDE PLATES (pleurce}, and AUXILIARY SIDE PLATES (parapleurce), 

 sometimes separated, or either united together, or with the pectoral 

 plates. 



This is the result of the investigations there instituted upon the 

 thorax : it now remains for us to inspect the cavities formed by these 

 plates, from the interior; perhaps, also, from this point of view we 

 may discover some peculiarities. 



164. 



INTERNAL SKELETON OF THE HEAD. 



In the Hemiptera and Diptera, the head is a mere horny bladder 

 without any internal processes or bones for the insertion of muscles. 

 The same is the case in the head of the Lepidoptera, but the occipital 

 aperture is divided by a transverse bar into two holes, the under one 

 of which is the smallest, and admits only the nervous cord through it ; 

 through the upper one pass the pharynx, vessels, tracheae, and muscles. 

 These parts are not found in the Hymenoptcra, but, on the contrary, a 

 broad ridge springing upwards from the lower margin of the occipital 

 aperture, which is prolonged towards the frons in two points, and 

 divides the upper portion of the head from the under. The Libdlula; 

 among the Neuroplern exhibit the former division of the occipital 

 aperture into an upper and under one; they have also several ledges 

 in the head, which spring from the anterior margins of the eyes, and 

 divide the large eyes from the brain, and this again from the frons. 

 In the Orthoptera, we again find the separation of the aperture into an 

 upper and under one. On each side, contiguous to that cavity, there 

 springs a process ; both unite in an arch, forming a narrow cover, 

 which is attached in fi-ont to the frons by means of two other pro- 

 cesses. I call this cover the tentorium, because, as in the higher 

 animals, for example, Fells, beneath it lies the cerebellum of insects, 

 or the second ganglion of the nervous system, from which the 

 mandibular and labial nerves originate. Over it runs the pharynx, 

 and above it lies the first ganglion or the cerebrum. In the cavity of 

 the head of beetles we do not find the tentorium in the shape just 

 described, but as two high ledges originating from the throat and the 



