ENTOMOLOGY IN OUTLINE — STRUCTURE OF INSECTS. 18 



THE STRUCTURE OF INSECTS. 



The external structure of insects is termed the exo-skeleton. In the 

 Vertebrata, the bony framework of the body is internal and supports 

 the muscular and nervous systems, which are attached to the outside 

 of the osseous or bony system. With the insects, the reverse is the 

 case. Here the skeleton is a hard, horny crust, composed largely of a 

 substance called chitine, and it is situated on the outer surface, all the 

 muscles and other organs of the body being attached to the inside, 

 instead of the outside, of the skeleton. It is really a many-jointed 

 tube, varying in size in different species, and composed of thirteen 

 rings, most of them so articulated as to be movable at the will of the 

 insect. Of these, the first composes the head, and is the most distinct. 

 Three are fused together in the thorax, and, in most species, appear as 

 one, there being no well-defined mark between the segments, which are 

 immovably joined to each other. In the last section of the body the 

 rings are loosely articulated and freely movable, being joined together 

 with a yielding membrane, allowing the insect the widest freedom of 

 movement. It must not be supposed, however, that these rings, or seg- 

 ments, are composed of one solid, unyielding piece, and that their only 

 motion is at the joints. Each ring is composed of many plates, more 

 or less movable, to accommodate the needs of the insect and to allow 

 the movem.ent of its various organs of locomotion, flight, etc. This 

 outer integument, or case, varies very greatly in density in different 

 insects; in some it is very thin, and easily crushed, while in others it is 

 excessively hard. In some of the beetles, for instance, the wing covers 

 are so hard that it is difficult to force a pin through them. 



An insect is divided into three sections: the head, the thorax, and the 

 abdomen. The upper portion, or back, is known as the dorsal surface, 

 or dorsum; the under side is the ventral surface, or venter; while the 

 sides are designated as the lateral surface, or pleurites. The upper and 

 under surfaces of the thorax are sometimes designated respectively as 

 the notum and sternum. 



There are very many minor subdivisions into which the different 

 sections of the body are separated, but it is not necessary in tins 

 treatise to name or enumerate them, nor for the young entomologist to 

 learn them. If our readers desire to go deeper into the subject, the 

 scientific text-books will give them an account of the more minute sub- 

 divisions and their various uses. It is enough for us to get a general 

 idea of our subject, and we shall therefore have to proceed at once to 

 investigate the three principal divisions, their attachments and various 

 organs. 



