232 ARTHROPODA [CH. 



shields are very much reduced, and thus it comes about that the 

 bases of each pair of legs, instead of being separated by the width 

 of the body, are close together. Another peculiarity in this sub- 

 division is that each tergum corresponds with two segments and 

 that each apparent segment bears two pairs of legs and has the 

 internal organs also duplicated. This double arrangement however 

 only begins at the fifth segment behind the head. 



The appendages on the head are: (i) short, usually club- 

 shaped antennae; (ii) mandibles; (iii) a single pair of maxillae 

 fused into a lobed plate. 



Both the first two segments behind the head bear but one pair 

 of legs, the third has no legs but carries the openings of the 

 generative ducts. Although oviducts and vasa deferentia are paired 

 the ovary and the testis are single. Each consists of a large sac 

 lying beneath the alimentary canal on the walls of which the genital 

 cells are developed. The development shows that the sac is a 

 remnant of the coelom. The fourth free segment has one pair of 

 legs, the remainder two pairs. 



The female lulus lays her eggs, some 60 to 100, in an earthen 

 receptacle she has prepared beneath the surface of the ground. 



Sub-class G. INSECTA. 



The immense group of Insects far outnumbers in species any 

 other group of animals and in all probability exceeds in number all 

 the species of the rest of the animal world. New insects are con- 

 stantly being discovered and although some quarter of a million 

 have already been named and to some extent described, it is 

 believed that at least as many more remain unrecorded. 



In spite of these numbers Insects are as a whole a uniform 

 group and show less diversity in size and structure than many of 

 the smaller groups, as for instance the Crustacea or the Mollusca. 

 Probably their great number and small range of structural variation 

 is not unconnected with the fact that they have found a new 

 medium in which to pass some part, at any rate, of their life. The 

 other group of animals which have taken to flying and lead an 

 aerial life the birds show a somewhat similar range of species 

 accompanied by a uniformity of structure which in their case is 

 even more marked. 



Insects may be characterised by their body being divided into 



three distinct regions, the head, the thorax and 



feature* tne abdomen (Fig. 97). The head bears one pair 



of antennae and three pairs of gnathites. The 



thorax consists of three enlarged segments, each of which bears 



