26 /.V7W/ \TOMOLOGY. 



Th keleton becomes much more highly developed in 



Special names have been applied to 



t it in the head and the three thoracic segments. Thus 



the hit i :i of the head is termed the ciui^cranium or tcnto- 



;he principal parts of it in the thoracic segments, those 



which ; Tom the sternal wall, are distinguished as the ante- 



\ and the postfftrcii. These are usually bifurcated ; 



the\ : the nervous cord and give attachment to muse' 



Tk* MiXHte Structure of the Body-wall. Under the head ol 



ternal anatomy the body-wall has been studied from one point of 



Reference was there made to the hardening of it by chitine, 



and .'. study was made of the various sclerites. We have now 



tudy the more minute structure of the body-wall, as seen on 



>n with high powers of the microscope. 



If a very thin section of the body-wall be taken and then dyed 

 with the pr< . as to differentiate the various parts, it 



will be seen under high powers of the microscope to consist of 

 three principal layers; first, an outer chitinous layer, the cuticle, 

 which forms the parts already studied ; second, an intermediate 

 cellular layer, the Itypodcrmis ; and third, a basal membrane. 



The appearance of these layers is shown in Fig. 37. The chiti- 

 nous layer is composed of many thin 

 plates superimposed. It really con- 

 sists of an excretion of the inter- 

 mediate cellular layer. It is not 

 composed of cells, but sometimes 

 -.-Section of body-wall. j t j s marked by lines correspond- 



'ie outlines of the subjacent cells of the hypodcrmis. The 

 composed of distinct nucleated cells ; as it gives 

 parts of the skin, it is often termed the matrix. 

 il membrane is a thin sheet of homogeneous tissue. 



positions of the mu 



i in insects are very different from what they are 



in Man. With the \ . the bones constitute- a central axis, 



out- 1. Hut in Insects, the >kele- 



dy, and of any of its appendages as well, is a hollow 



of which the muscle^ are attached. 



This is illu^ .>. hich represents the muscles in the 



If t of an i: :ably of a larva) be opened by a 



longitudin.il : the alimentary canal removed from the centre, 



