WINGS. 



309 



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 order 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. Pro- 

 fessor Eimer maintains that the arrange- 

 ment 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 ex- 

 actly with those of its habitat, or make 

 it a mimetic copy of some more success- 

 fully protected neighbour. 



As to the origin of wings, this at least 

 should be remembered, 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 accom- 

 plishment which the original insects did 

 not possess, the theory seems plausible 

 that wings were originally respiratoiy 

 outgrowths, which ,by-and-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 out- 

 growths in question would quicken the whole life, and would tend to 

 raise insects into the air, just as terrestrial insects can he. 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 out- 

 growths from the sides of the abdomen, the so-called "tracheal gills," 

 which in origin and appearance are like young wings. 



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

 with the quadrupeds ; they fly with the birds ; they glide 



Fig. ioi. — Young May 

 fly or Epheraerid. (After 

 Eaton.) 



Showing tracheal gills, .ind 

 wings appearing in front of 

 them. 



