ARTHROPODA. 293 



suggests that they are more closely related to the ancestral types from 

 which the insects have sprung than to the insects themselves (Fig. 134). 

 Class III. Myriapoda (Centipedes, etc.)- Tracheate arthropods with 

 a worm-like body. Segments numerous, and much alike, one (or, in 

 Diplopoda, two) pair of appendages to each segment. The head is dis- 

 tinct and bears antennae and mouth parts. The eyes are numerous and 

 simple (ocelli}. In fundamental structure and development the myriapods 



FIG. 135. 



FIG. 135. Centipede (Scolopendra heros). Photo by Folsom. Four-fifths natural size. 



Questions on the figure. What differentiation of segments is ap- 

 parent? Are there any fusions into body-regions? What is the law of 

 the occurrence of appendages? What diversity is there among them? 



resemble insects. There are two principal orders. One embraces the 

 centipedes which are carnivorous, have biting jaws, have one pair of 

 appendages to each segment, and are poisonous. The second includes the 

 millipedes which are vegetable feeders and possess mandibles suited to 

 chewing vegetable matter. They are wholly harmless. They have two 

 pairs of legs to each of the numerous segments except the first four. Both 

 centipedes and millipedes inhabit the land, and frequent dark places. Many 

 are nocturnal in habit (Fig. 135). 



Class IV. Hexapoda (Insects). Tracheate arthropods with three dis- 

 tinct body regions, head, thorax, and abdomen. The head has four seg- 

 ments with appendages, a pair of antennae and three pairs of mouth parts. 

 The thorax has three segments (pro-, meso- and meta-thorax), each of 

 which bears a pair of legs ; the meso-thorax and the meta-thorax may each 

 bear a pair of wings. The abdomen has a variable (often obscure) 

 number of segments. Its appendages are usually entirely wanting or much 

 reduced. A metamorphosis frequently occurs. The larval condition often 

 suggests the annelids and the myriapods in the similarity of its segments, 

 and in the numerous appendages. 



The student is referred to more comprehensive works for an exposi- 

 tion of the numerous orders of this enormous group of Hexapoda. Only 

 the more important are suggested below. 



Order Aptera (without wings). This order embraces a number of 



