THE ISSECT: ITS EXTERSAL STBUCTTRE 



«rf^ 



process of fusion, partly bv a sort of tekset^iing or the gradual diiftiiig 

 c^ one segment within another untfl it is parthr or entiiehr coneeaited. 



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

 tion from the outside living layer of ceDs. the hypodetmis. Thk secre- 

 tion gradually hardens on exposure 

 to the air. providing the support iV 



necessary for the scrft parts within. 

 Chemically it conasts of chitin 

 (CisHjsXjOit). which remains thin 

 and flexible at the movable joints 

 and wing articulations, but eke- 

 where becomes thicker and usually 

 darker in color. Here and there 

 over its surface are imiHcssed lines 

 like scratches, very definite and 

 fixed in position in most insects, 

 and these are termed sutures, and 

 are c^ great use as landmarks in 

 description. These sutures have 

 such an arrangement that in an 

 ordinary segment erf the body its 

 upper surface has often been re- 

 garded as a plate or selerite. the notum; cme at each sider the idearon; 

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

 dividing them. 



In the head the sutures ^r^ fevr in nimb»?r. mi ozly 3. few pLites or 

 sclerrtes are seneraHv in evi.ie!i >?. In. riie 'b 



iatjjL 



Br 



Fk. 11. — Froait ^itw oi famd <tf & da^ 

 hopfi^ Covins a faorpognathoss kead; ««£. 

 aMtemaan c e~. eonpoimd c^nr: eft, efccek: <i, 



lafaram; wad. mandibfe; ibx. p^^axBair 

 pal^Mzs: 'J. o«?eQi: ». vHtex. 



nvs "iii-rv inf. more 



o«s. wnve 





'^rcasHMialhr the weakly chitin- 

 Lzed areas are quite large (que^i 

 white ant) and elastic. Usualhr 

 he elasticity of these places, as 

 :o>r example, the pordcMis co»- 

 rLei.Ting the segments, is rathex* 

 -li^ht- Spines, hairs, scales or 

 other structures are often ]xesent 

 on the diitin. sometimes entirely 

 concealing its surface and its 

 sutures. 

 The heads erf different ii^ects vary much in form and in the locatMm 



of the mouth I Fig. 11). In some eases thk is on the undetsde i see Fig. 



10), while in others (F%. 12) it is practically on the froot. Heack with 



■-j sfaowiag a 



pro^nAtboos lead. 



