THE ORIGIN OF INSECTS 33 



ticks, and mites ; the latter, the water-fleas, barnacles, 

 true wood-lice, and animals of the crab and lobster kind. 

 Sir Ray Lankester tells us that the Arachnida have two 

 rings or segments, in front of the mouth, added from the 

 body to the primitive head ; whilst the Crustacea (also 

 the two remaining Gnathopod classes) have three such 

 additional, or " pre-oral," head-segments. The probable 

 derivation of the two remaining classes — viz. the centipedes 

 or Chilopoda (so-called because the lower lip is formed 

 by a pair of feet) and the insects or Hexapoda ("six- 

 footed") — is still debated; but strong evidence exists in 

 support of the view that they branched off from the Crus- 

 taceans. At least we are justified in asserting that insects 

 have descended from lowly Gnathopod ancestors. We 

 have already seen that the dominant characteristic of 

 these creatures is the modification of certain limbs to play 

 the part of jaws, and that the number of these foot-jaws 

 varies. The higher Crustaceans have a group of them 

 (as many as six pairs) devoted to the service of the mouth, 

 while centipedes and insects have only three pairs. More- 

 over, the total number of legs is subject to great variation 

 in the different classes. Peripatus, millipedes, and centi- 

 pedes have very numerous legs, and so have the primitive 

 Arachnids and Crustaceans. But among the higher re- 

 presentatives of the latter classes, and in all insects, many 

 pairs of legs are suppressed ; so that while a crab has five 

 pairs of walking legs, and a spider has four pairs, an insect 

 has only three pairs. Furthermore, with this reduction of 

 legs, a more or less definite grouping and fusion of the 

 segments is noticeable ; until, in the case of insects, we 

 find a constant division of the body into three areas — viz. 

 the head, the thorax, and the abdomen. As to the pre- 

 cise number of primitive body rings which go to make up 

 a typical insect, naturalists are by no means agreed ; 



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