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THE YOUNG NATURALIST. 



small Hymenopterous insects, though I am not aware that their 

 powers of flight are at all impaired thereby. 



The number of wings varies in different orders, there never being 

 more than four (with one exception, referred to later on) or less than 

 two, when present at all ; but numbers of insects never acquire these 

 organs, their peculiar habits of life giving them no use for such 

 appendages. 



The development of the wings is a curious and interesting feature. 

 In some species they are present in both larval and pupal stages, but 

 are rudimentary and totally unfit for flight. Among insects passing 

 through complete metamorphoses the rudiments of the wings are 

 formed immediately after the larva has thrown off its last skin, some- 

 times appearing as lumps on those segments corresponding to the 

 thorax of the imago, and in the pupal stage, are plainly visible through 

 the external swathing of the insect. 



Presently the day comes when the last great change is about to 

 take place, when the insect bursts from its living tomb, to unfold its 

 wet and shrivelled wings to the view, and enter upon the final phases 

 of its chequered career. Truly it presents but a sorry forecast of 

 the splendid robes with which it will shortly become decked ; the 

 membranes are covered with folds and creases, the nervures are also 

 contracted to their utmost, and the whole structure of the wing is soft, 

 moist and pliable. It is at this period of the creature's existence 

 that the nervures are called upon to fulfil a most important function, 

 which their peculiar formation fully qualifies them to perform. They 

 are straightened out laterally by the combined effects of two causes, 

 air and fluid. The enclosed tracheae serve for the passage of the 

 former, while the latter is pumped through the unoccupied space 

 between the boundary walls and the tracheae. The ready extension 

 of the nervures will be sufficiently plain from the consideration of 

 their structure, which admits of expansion much in the same manner 

 as a concertina filled with air. As the nervures, together with the 

 lesser branches, increase in length, so the membrane stretches in all 

 directions, the process drawing it out perfectly flat and smooth, and 

 removing all creases and convolutions from its surface. 



That the nervures are injected by fluid for the purpose of develop- 

 ing the wings, the discovery before referred to, of the circulation of 

 the blood through these tubes, goes far to prove, but that this liquid 

 undergoes a change when brought into more intimate communication 

 with the air is evident. When it has accomplished its purpose, 

 coagulation sets in, and it becomes a colourless, and jelly-like substance, 



