THE INSECT: ITS EXTERNAL STRUCTURE 



ce 



arrt. 



process of fusion, partly by a sort of telescoping or the gradual shifting 

 of one segment within another until it is partly or entirely concealed. 



The skeleton covering the body is generally considered to be a secre- 

 tion from the outside living layer of cells, the hypodermis. This secre- 

 tion gradually hardens on exposure 

 to the air, providing the support 

 necessary for the soft parts within. 

 Chemically it consists of chitin 

 (Ci 6 H 26 N2Oio), which remains thin 

 and flexible at the movable joints 

 and wing articulations, but else- 

 where becomes thicker and usually 

 darker in color. Here and there 

 over its surface are impressed lines 

 like scratches, very definite and 

 fixed in position in most insects, 

 and these are termed sutures, and 

 are of great use as landmarks in 



ITJX.p. 



lab.p. 



FIG. 11. Front view of head of a Grass- 

 hopper showing a hypognathous head ; ant, 

 antenna; c. e., compound eye; ch, cheek; cl, 

 clypeus;/r, frons; lab. p., labial palpus ;lbr., 

 labrum; md, mandible; mx. p., maxillary 

 palpus; o, ocelli; v, vertex. 



description. These sutures have 

 such an arrangement that in an 

 ordinary segment of the body its 

 upper surface has often been re- 

 garded as a plate or sclerite, the notum; one at each side, the pleuron; 

 and one beneath, the sternum. These plates may have sutures sub- 

 dividing them. 



In the head the sutures are few in number, and only a few plates or 

 sclerites are generally in evidence. In the thorax they are more numer- 

 ous, while in the abdomen often 

 only a dorsal and ventral sclerite 

 for each segment are found. 

 Occasionally the weakly chitin- 

 ized areas are quite large (queen 

 white ant) and elastic. Usually 

 the elasticity of these places, as 

 for example, the portions con- 

 necting the segments, is rather 

 slight. Spines, hairs, scales or 

 other structures are often present 

 " the chitin, sometimes entirely 

 (Original.) concealing its surface and its 



sutures. 



The heads of different insects vary much in form and in the location 

 of the mouth (Fig. 11). In some cases this is on the underside (see Fig. 

 10), while in others (Fig. 12) it is practically on the front. Heads with 



