404 



ARTHROPODA 



number is so large that it cannot be given with accuracy; an estimate is 

 250,000. Since the tropics, which have not been exhaustively studied, 

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

 different species in the world. On the other hand, great uniformity of 

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

 and number of appendages, so that the difference 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 in their life relations,^ 

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

 habits, and in their intellectual and social relations. From the evolution- 

 ary standpoint they show marked adaptations to environment, and the 

 large number of species is only possible by taking advantage of every 

 opportunity in nature. 



FIG. 442. Schematic section of a hexapod through the thorax (orig.). ex, coxa; 

 d, digestive tract; f, femur; h, heart; n, notum; pi, pleuron; st, sternum; t, tibia; ta, 

 tarsus; tr, trochanter. 



Of systematic importance are the regional division of the body and 

 the number and character of the appendages. In the body three regions 

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

 and abdomen. The number of abdominal somites varies with the order 

 and even with the family, ranging between eleven (in some larvas and 

 embryos twelve) in the Orthoptera and five in many Diptera. Each 

 cuticular abdominal segment consists of two plates, tergite (dorsal) and 

 sternitc (ventral), united on the sides by a softer membrane which con- 

 tains the spiracles. Head and thorax, on the other hand, have a constant 

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

 plainly divided into three segments, pro-, me so- and metathorax, each com- 

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

 lateral plates, pleura; and an unpaired ventral sternum (fig. 442). For 

 simplicity one speaks of pronotum, mesosternum, etc., to indicate the por- 



