640 ZOOLOGY sect. 



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 entirely 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. 



Haemocaele. — The cavity intervening in an Insect between the 

 body- wall and the various internal organs does not correspond, 

 as already explained (p. 631), to the coelome of other groups ; but is 

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

 — 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 haemoccele 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 

 crystals containing uric acid are formed. These crystals afterwards 

 become absorbed ; their appearance and subsequent absorption 

 would seem to point to the probability that the fat-body 

 is concerned in separating out nitrogenous waste matters, which 

 subsequently reach the exterior through the Malpighian tubes. 

 Its chief function is to serve as a reserve-store of nutrient material. 



Digestive system. — Some Insects do not feed in the adult 

 condition, and when this is the case the mouth may be absent, 



