236 SYSTEMATIC ZOOLOGY. 



subclass, Myriapoda, but the differences between them 

 are too great for this. The Diplopods have but three seg- 

 ments in the head, and, after the first three, each segment 



FIG. 65. A Diplopod (Spirostrephori), showing the two legs to a segment- 



From Packard. 



of the body bears two pairs of legs, while the reproductive 

 organs open far forward. The thousand-legged worms 

 live in moist places, where they feed upon decaying vege- 

 table matter. They are harmless forms, but several 

 species secrete a strong-smelling substance, which pro- 

 tects them against their foes. 



SUBCLASS II. HEXAPODA (Insects). 



The group of Hexapoda contains more species than all 

 the rest of the animal kingdom together, a conservative 

 estimate placing the number of distinct forms at nearly a 

 million. Yet all of these agree in certain essential points. 

 Thus, in all, the body is divided into three regions, head, 

 thorax, and abdomen, and of these the thorax alone bears 

 organs of locomotion. Three pairs of legs are always 

 present (whence the name Hexapoda six-footed given 

 to the group). Of wings, which also are attached to the 

 thorax, there may be one or two pairs. The head bears 

 four pairs of appendages, one pair (the antennae) being 

 sensory; the others (mouth-parts) being used in eating. 

 The sexes are always separate, and the reproductive organs 

 open at the hinder end of the body just beneath the vent. 



In the head no evidence of segments is seen, except as 



