XI 



PHYLUM ARTHROPODA 



591 



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 

 rudimentary. 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 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 re- 

 spects, seems to indicate 

 that in these Insects we 

 have to do with the de- 

 scendants of a primitive 

 group in which wings 

 had not yet become de- 

 veloped. 



The segments of the 

 abdomen ' are entirely 

 devoid of paired ap- 

 pendages 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. 



Haemocoele. The cavity intervening in an Insect between the 

 body wall and the various internal organs does not correspond 

 as already explained (p. 576) to the coelome of other groups ; but is 



FIG. 480. Mouth parts of the Lepidoptera. B, the 

 second maxillae. Lettering as in preceding figures. 

 pi. labial palp ; pm. palp of the anterior maxilla? ; 

 s<; sucking tube. (From Lang.) 



\7F.~RftTTV 



