IV. INSECTA: HEXAPODA. 461 



dency to become compound. The mouth parts (fig. 482) are a 

 pair of mandibles and two pairs of maxillae, both united in the 

 median line. Besides, the first pair of legs (fig. 482, 5), with their 

 fused bases, extend forward beneath the head and form the poison 

 claws. Their terminal joints are sharp and contain the ducts of 

 poison glands. 



The spiracles (at least a pair to every other somite except those 

 of the head) are lateral in position in the soft integument between 

 the dorsal and ventral plates (fig. 481). The heart is elongate, with 

 chambers in each somite (fig. 66); there are two large Malpighian 

 tubes, and the nervous system is elongate, with ganglia in each 

 .somite. The gonads are dorsal to the intestine and are unpaired, 

 while the single duct opens ventrally in the preanal somite. 



The LITHOBIID.E, with 15 leg-bearing somites, have certain dorsal plates 

 enlarged and overlapping the succeeding somites ; Lithobius,* common 

 under stones, etc. SCOLOPENDRID.E, centipedes; at least 17 legs and 5 

 ocelli ; Scolopendra* in warmer regions (fig. 483). GEOPHILHLE, not less 

 than 30 pairs of legs, spiracles 2 less than legs. Geophilus* SCUTIGE- 

 RiDyE, legs very long, 15 leg-bearing segments, but only 8 dorsal plates. 

 jSoutigera.* 



Sub Class II. Hexapoda. 



The Hexapoda is by far the largest division of the Arthropods, 

 since it contains at least ten times as many known species as all 

 the rest. The 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 con- 

 ceivable 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 of structure, regional divi- 

 sions, and number of appendages under the most diverse conditions, 

 so that the difference between the most extreme forms is far less 

 than that in Crustacea or Arachnida. But while hexapods thus 

 lose in morphological interest, they gain in their life relations, in 

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

 ing habits, and in their intellectual and social relations. From the 

 evolutionary standpoint they show marked adaptations to environ- 

 ment, and the large number of species is only possible by taking 

 advantage of every opportunity in nature. 



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 con- 



