STATES OF INSECTS. 295 



a trachea, which, proceeding from the interior of the 

 trunk in a serpentine direction, follows all the ramifica- 

 tion of the nervure, though it does not fill it a . Though 

 Reaumur attributes the expansion of the wings chiefly to 

 an aqueous fluid, yet he suspects that the air on some 

 occasions contributed to it b . 



The wings of the other tribes of insects probably differ 

 from the Lepidoptera in the manner in which they are 

 folded. It should seem from Reaumur's description, that 

 those of some flies, instead of the straight transverse folds 

 of the former, have angular or zigzag folds c ; which 

 equally shorten the wing. Many Hymenoptera have 

 wings without any nervures except the marginal. We 

 may conjecture that these are more simply folded, so as 

 to render their expansion more easy ; but even in these 

 wings there are often tracheae, which appear as spurious 

 nervures, and help to effect the purpose we are consi- 

 dering. 



The operation of expanding their wings, in by far the 

 larger number of insects, takes place gradually as de- 

 scribed above ; and, according to their size, is ended in 

 five, ten, or fifteen minutes ; in some butterflies half an 



a Jurine Hymenopt. 16. 



b iv. 342. Herold also attributes the rapid expansion of the wing 

 to the flow of an aqueous fluid, which he calls blood? into the ner- 

 vures, the orifices of which open into the breast. EntwicJceliings. 

 der Schmetterl. 101. sect. 100. — M. Chabrier, in his admirable Essai 

 sur le Vol des Insectes {Mem. du Mus. 4ieme, ann. 325), having ob- 

 served a fluid in the interior of the nervures of the wings of insects, 

 thinks it probable that they can introduce it into them and withdraw 

 it at their pleasure : the object of which, he conjectures, is either to 

 strengthen them and facilitate their unfolding, or to vary the centre 

 of gravity in flight, and increase the intensity of the centrifugal force, 



c Ibid. 340. 



