26 AN INTRODUCTION TO ENTOMOLOGY. 



This internal skeleton becomes much more highly developed in 

 adult insects than it is in larvae. Special names have been applied to 

 the parts of it in the head and the three thoracic segments. Thus 

 the internal skeleton of the head is termed the endocrdnium or tento- 

 rium; and the principal parts of it in the thoracic segments, those 

 which project from the sternal wall, are distinguished as the ante- 

 furca, the vicdifurca, and the postfiirca. These are usually bifurcated ; 

 they support the nervous cord and give attachment to muscles. 



The Minute Structure of the Body-wall. Under the head of ex- 

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

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

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

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

 section with high powers of the microscope. 



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

 with the proper reagents, so 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 hypodermis ; and third, a basal membrane. 



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

 nous layer is composed of many thin 

 .~c plates superimposed. It really con- 

 sists of an excretion of the inter- 

 mediate cellular layer. It is not 

 composed of cells, but sometimes 

 FIG. 37.-Section of body-wail. it i s marked by lines correspond- 



ing to the outlines of the subjacent cells of the hypodermis. The 

 hypodermis is composed of distinct nucleated cells ; as it gives 

 o;- : gin to the other parts of the skin, it is often termed the matrix. 

 \ '\e basal membrane is a thin sheet of homogeneous tissue. 



The Muscular System. The relative positions of the muscles 

 and the skeleton in insects are very different from what they are 

 in Man. With the Vertebrates, the bones constitute a central axis, 

 outside of which the muscles are arranged. But in Insects, the skele- 

 ton of the body, and of any of its appendages as well, is a hollow 

 cylinder, to the ent^Lsurface of which the muscles are attached. 

 This is illustrated 4|^Hg- 3$, which represents the muscles in the 

 leg of a beetle. 



If the body of an insect (preferably of a larva) be opened by a 

 longitudinal slit, and the alimentary canal removed from the centre, 



