PLIGHT OF INSECTS. 243 



ance, and all the developments of their internal mechanism, 

 are expressly directed. Wings are added to the frame only 

 in the last stage of its completion; after it has disencum- 

 bered itself of every ponderous material that could be spared, 

 after it has been condensed into a small compass, and after 

 it has been perforated in all directions by air-tubes, giving 

 lightness and buoyancy to every part. Curiously folded up 

 in the pupa, the wings there attain their full dimensions, 

 ready to expand whenever the bandages that surround them 

 are removed. No sooner is the insect emancipated form its 

 confinement, than- these organs, which are composed of du- 

 plicatures of a dense but exceedingly fine membrane, iden- 

 tical in its composition with the general integuments, begin 

 to separate from the sides of the body, and to unfold all 

 their parts. Their moisture rapidly evgiporates, leaving the 

 delicate film dry and firm, so as to be ready for immediate 

 action. The fibres, or nervures, as they are called, form a 

 delicate net-work, for the support of this fine membrane, 

 like the frame of the arms of a windmill, which supports 

 the canvass spread over them. The microscope shows that 

 these fibres are tubular, and contain air; a structure the most 

 effectual for conjoining lightness with strength; and many 

 entomologists are of opinion that the insect has the power, 

 during the act of flying, of directing air into the nervures, 

 so as to dilate them to the utmost, and render them quite 

 tense and rigid. 



In the great majority of insects, the wings are four in 

 number; of which the first pair are, as we have seen, affixed 

 to the mesothorax, and the second to the mctalhorax. 

 These two segments of the thorax, composing what has been 

 termed the alitrunk, constitute the most solid portion of 

 the skeleton, and are frequently strengthened hy ridges, 

 and other mechanical contrivances for support. The su- 

 perior extremities of these supports, which have been com- 

 pared to the clavicles, or furcular bones of birds, are always 

 curved inwards. This part of the trunk requires to be al- 

 ternately dilated and contracted during flight; and, hence, 



