28 



THE STUDY OF INSECTS 



well seen if the figure showing the internal structure of the leg of a may- 

 beetle (Fig. 53) be compared with one of our own limbs, either arm or leg. 



The body of an insect is built on the same 



plan as arc its legs. The outside of the body 



is more or less firm, and this firm outer wall 



supports the muscles and other organs, thus 



The skeleton is therefore, in general outline, a 



Fig. 55- — A Larva. 



Fig. 56. — An 



earwig. 



serving as a skeleton 

 hollow cylinder. 



The outer body-wall is composed of three layers: (1) an outer pro- 

 tective layer, the cuticula; (2) a middle layer, the hypodermis; 

 and (3) an inner delicate thin layer, the basement membrane 

 (Fig. 54). The cuticula is a tough sheet of chitin often hard- 

 ened by the deposition in it of other substances. 



The cuticula is not hardened throughout but remains soft 

 and flexible at certain transverse indentations running around 

 the body. These soft places in the cuticula mark the divi- 

 sions between the ring-like portions of the body called seg- 

 ments. 



In this way provision is made for the various motions of 

 the body. The ring-like nature of the segments of the body 

 is best seen in larvae (Fig. 55), and in the abdomen of an 

 adult insect (Fig. 56). The movements of the legs, antennae, and certain 

 other appendages are provided for in the same way; each one is a cylin- 

 der made up of several segments, and be- 

 tween these segments the wall of the 

 cylinder remains flexible. 



When a single segment of the body is 

 examined, its cuticula is not found to be 

 side-view of locust with wings a continuous ring, but is seen to be made 

 up of several portions more or less mov- 

 able upon each other. Such a hardened portion of the cuticula is termed 

 a sclerite. 



The sclerites constitute the greater part of the cuticula, the soft 

 membranous portions separating them being in 

 most cases narrow. Usually these narrow por- 

 tions are mere lines; they are then called su- 

 tures. 



Frequently the sutures become entirely ef- 

 faced. We are therefore often unable to distin- 

 guish certain sclerites in one species of insect 

 which we know to exist in another. In such cases 

 the effaced sutures are said to be obsolete. 



If the central portion or thorax of an adult 

 insect be examined, numerous sclerites and su- 

 tures can be observed (Fig. 57). 



The subject of external anatomy of insects 

 consists very largely in a study of the sclerites of 

 which the different segments of the body and of 

 its appendages are composed. This part of the 

 subject is quite difficult, and will not be discussed 

 here. It is treated, however, in the discussion 

 of the characters used in the classification of the Coleoptera given on 



Fig. 58. — Wasp, with head, 

 thorax, and abdomen separated. 



