404 



ARTHROrODA 



number is so large that it cannot l.^e given with accnracv; an estimate is 

 JtO.ooo. Since tlie tropics, wliicli lia\"e not Keen exliausiixeh' studied, 

 are verv rich in insects, it is conceivable that there are at least a million 

 dilTerent species in the \\'orld. On the other hand, great unitormity of 

 structure exists, all adhering with great fulelity to plan, regional divisions, 

 and number of appendages, so that the dilTerence between the most extreme 

 forms is far less than that in Crustacea or Arachnida. But while hexa- 

 pods thus lose in morphological interest, they gain iir their life relations, 

 in the way that they are iirjurious or beneficial to man, in their breeding 

 haliits, and in their intellectual and social relations. From the e\'olution- 

 arv standpoint thev show marked adaptations to en\ironment, and the 

 large number of species is onl}' possil>le bv taking ad\'antage of every 

 opportunit}' in nature. 



Zrt>: 



Fic. 44:'. — Schematic section of a hexapod throuL;h tlie thom.x i^oris;.'). cv, coxa ; 

 d, digesti\-e tract; /", femur; /;, lieart; 11, notum; p! . pleuron; s!. stemuiu; /, tibia; /,;, 

 tarsus; (r, noclianter. 



Of systematic importance are the regional tlivision of the botlv and 

 the numl.ier aird character of the appeirdages. In the bod\- three regions 

 are distinguished, often separated by marked constrictions; head, thorax, 

 and abdomen. The numljer of abdontinal somites \-aries wth the order 

 and e\-en with the family, rangiitg between ele\-cir (in some lar\-;e and 

 emljryos twehe) in the Orlho]>tera and li\ e iit nrany Diptera. Kach 

 cuticular abdoniii^al segment consists of two ]ilates, /(7;t,';7c (tiorsal) and 

 sicrnilr (\-entral), um'ted on the sides by a softer nunnbrane which con- 

 tains the spiracles. Head and thorax, on tho other hand, ha\e a const;iiit 

 numljer of somites. (See, however, Hymenoptera.) The thorax is 

 plainly divided into three segments, /)n>-, incso- txnd nuia/Iionix, each com- 

 posed of three elements, an unpaired dorsal portion, nohivi; a pair of 

 lateral plates, pinini; and an unpaired ventral slrnnim (tig. 44^). I'or 

 simplicity one sp-;iks of pro/iolinii. incso.slcDiiini. etc., to indicate the por- 



