xi PHYLUM ARTHROPOD A (525 



paddles ; in some the first pair are prehensile, and develop a sub- 

 chelate extremity ; in others, again, the legs, or the first pair of 

 them, are stout and adapted for burrowing. In addition to the 

 legs the meso- and metathorax may each bear a pair of wings. 

 The wings are thin transparent expansions of the integument of the 

 body, supported by a system of branching ribs or nervures consisting 

 of chitinous material with branches of the tracheae, nerves, and 

 tubular diverticula of the body-cavity. In most Lepidoptera the 

 wings are opaque, owing to their being covered with numerous 

 overlapping microscopic scales, to which the various colours of the 

 wing are due. In some insects e.g., Beetles and Orthoptera the 

 posterior wings alone are delicate and membranous, the anterior 

 pair being converted into hard or tough cases the elytra which 

 when folded up cover over and protect the delicate posterior wings. 

 In some Beetles the elytra are permanently united together along 

 the back of the Insect. In some Insects (Bugs) the anterior wings 

 are chitinous at the bases only. In the Diptera the anterior 

 wings alone are developed, the posterior being represented by 

 vestiges the halteres or balancers. In the Strepsiptera, or Bee- 

 parasites, an aberrant group of Neuroptera, on the other hand, 

 it is the anterior pair that are vestigial. In some Insects 

 (Spring-tails, Lice, Fleas) wings are entirely absent in all stages. 

 In others again they are present in one sex usually the male 

 and absent or vestigial in the other. In the Aptera there is no 

 vestige whatever of wings at any stage, and this, taken in connection 

 with the simplicity of the structure in other respects, seems to 

 indicate that in these Insects we have to do with the descendants 

 of a primitive group in which wings had not yet become developed. 



The segments of the abdomen are mostly devoid of paired 

 appendages in the adult condition (except in the Thysanura), 

 though vestiges of them may be present in the young at an early 

 stage. Each segment is enclosed in dorsal tergal and ventral 

 sternal plates, which usually remain separate laterally, but may be 

 united. At the extremity of the abdomen there are frequently 

 appendages which are perhaps of the nature of limbs, having the 

 function of stings, ovipositors, and genital processes. 



Haemoccele. The cavity intervening in an Insect between the 

 body-w T all and the various internal organs does not correspond, 

 as already explained (p. 578), to the ccelome of other groups, but is 

 found, when we study its mode of development, to be a hoeynocode 

 an extended part of the blood- vascular system. The coelome is 

 apparently represented only by the lumen of the reproductive organs. 



A fat-body is always present, either in the larval condition or 

 throughout life. It consists of a mass of polygonal cells bounding 

 the hsemoccele externally. When young the cells are nucleated 

 and possess a protoplasmic body. At a later stage a fluid loaded 

 with minute granules takes the place of the protoplasm, and 



