GENERAL LAWS OF ANIMAL STRUCTURE. 



273 



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pendages (Fig. 177). In the abdomen the rings are 

 perfect and movable, although the appendages are 

 wanting; but they are present in the 

 larval or caterpillar state. The thorax 

 is a consolidation of three rings, each 

 with its pair of appendages greatly en- 

 larged for walking (legs). The head 

 consists of three or four consolidated 

 segments with appendages much modi- 

 fied — the first pair into antennas, the 

 second pair into mandibles, and the 

 third pair into maxillae and maxillary 

 appendages or feelers, and the fourth 

 pair into labium and labial append- 

 ages. 



Origin of Insects' Wings.— The 

 wings of insects are not homologous 

 with legs or other appendages. There 

 is some doubt as to their origin, but it 

 is most probable that they are modifi- 

 cations of the trachi-branchiae of the 

 larvae of aquatic forms. If so, insects 

 sprang from aquatic species. 



That insects are higher than crusta- 

 ceans is shown by the distinctness of the 

 head. The ring-series in crustaceans 

 are grouped into two regions, the ab- 

 domen and the cephalothorax, the head 

 being undistinguishable from the tho- 

 rax. The same is true of spiders and 

 scorpions. But in insects we have three 

 distinct groups — the head, the thorax, 

 and the abdomen. A distinct movable 

 head is always a sign of the dominance 

 of head functions. 



