26o ARTHROPODS. 



Wings are flattened hollow sacs, which grow out from the 

 two posterior rings of the thorax. They are moved by 

 muscles, and traversed by " veins '' or " nervures," which 

 include air-tubes, nerves, and vessel-like continuations of 

 the body-cavity. Most insects have two pairs, but many slug- 

 gish 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 CoUembola 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 rudi- 

 ments, 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. 

 Professor Eimer maintains that the arrangement of the 

 nervures and the colouring of butterfly wings are certain 

 marks of the progress and relationships of species. It is 

 well known that the colours also vary with sex, climate, 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, this at least should be remem- 

 bered, that in many cases they are of some use in respiration 

 as well as in locomotion. Seeing that the power of flight is 

 evidently an accomplishment which the original insects did 

 not possess, it seems to me very likely that wings were 

 originally respiratory outgrowths, which by-ajid-by became 

 useful for aerial locomotion. This view is consistent with 

 an idea, which grows in favour with evolutionists, that new 

 organs develop by the predominance of some new function 

 in organs which had some prior significance. Moreover, we 

 can fancy that an increase in respiratory efficiency brought 

 about by the outgrowths in question 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. 



