WINGS OF INSECTS 



387 



" nervures," which include air-tubes, nerves, and vessel- 

 Uke continuations of the body cavity. Most insects have 

 two pairs, but many sluggish females and parasites, like lice 

 and fleas, have lost them. On the other hand, there is no 

 reason to believe that the very simplest wingless insects, 

 known as Collembola and Thysanura, ever had wings. 



There are many interesting differences in regard to wings in the 

 various orders of Insects. Thus in beetles the front pair form wing- 

 covers or elytra ; in the little bee parasites — Strepsiptera — they are 

 twisted rudiments ; in flies the posterior 

 pair are small knobbed stalks (halteres 

 or balancers) ; in bees the wings on 

 each side are hooked together. When 

 the insect is at rest, the wings are usually 

 folded neatly on the back ; but dragon- 

 flies and others keep them expanded ; 

 butterflies raise them like a single sail 

 on the back ; moths keep them flat. 

 Many wings bear small scales or hairs, 

 and are often brightly coloured. It is 

 well known that the colours also vary 

 with sex, chmate, and surroundings. 

 Most interesting are those cases in which 

 the colours of an insect harmonise exactly 

 with those of its habitat, or make it a 

 mimetic copy of some more successfully 

 protected neighbour. 



As to the origin of wings, it may be 

 mentioned that in many cases they are 

 of some use in respiration as well as in 

 locomotion, and the theory seems plau- 

 sible that wings were originally respira- 

 tory outgrowths, which by and by became 

 useful for aerial locomotion. New organs 

 seem often to have arisen by the pre- 

 dominance of some new function in 

 organs which had some prior signiflcance. 



Moreover, we can fancy that an increase in respiratory efficiency 

 brought about by the outgrowths in qffestion would quicken the 

 whole life, and would tend to raise insects into the air, just as terrestrial 

 insects can be made to frisk and jump when placed in a vessel with 

 relatively more oxygen than there is in the atmosphere. Finally, we 

 must note that the aquatic larvae of some insects, e.g. may-flies, have 

 a series of respiratory outgrowths from the sides of the abdomen, the 

 so-called " tracheal gills," which in origin and appearance are like 

 young wings (Fig. 219). 



Insects excel in locomotion. " They walk, run, and 

 jump with the quadrupeds ; they fly with the birds ; they 



Fig. 219. — Young may-fly 

 or ephemerid. — After Eaton. 



Showing tracheal gills, and Avings 

 appearing in front of them. 



