HOW INSECTS FLY. IX 



doubtless, by other agents still more subtle, with which we have 

 at present scarcely any acquaintance." 



How Insects Fly. Who of us, as remarked by an eminent orni- 

 thologist, can even now explain the long sustained, peculiar 

 flight of the hawk, or turkey buzzard, as it sails in the air with- 

 out changing the position of its wings ? and, we would add, the 

 somewhat similar flight of a butterfly ? It is the poetry of mo- 

 tion, and a marvellous exhibition of grace and ease, combined 

 with a wonderful underlying strength and lightness of the parts 

 concerned in flight. 



Before we give a partial account of the results obtained by 

 the delicate experiments of Professor Marey on the flight of 

 birds and insects, our readers should be reminded of the great 

 differences between an insect and a bird, remembering that the 

 former, is, in brief, a chitinous sac, so to speak, or rather a 

 series of three such spherical or elliptical sacs (the head, thorax 

 and abdomen) ; the outer walls of the body forming a solid but 

 light crust, to which are attached broad, membranous wings, 

 the wing being a sort of membranous bag stretched over a 

 framework of hollow tubes (the tracheae), so disposed as to give 

 the greatest lightness and strength to the wing. The wings are 

 moved by powerful muscles of flight, filling up the cavity of the 

 thorax, just as the muscles are the largest about the tnorax of 

 a bird. Moreover in .the bodies of insects that fly (such as the 

 bee, cockchafer, and dragon fly), as distinguished from those 

 that creep exclusively, the air tubes (tracheae) which ramify 

 into every part of the body, are dilated here and there, espec- 

 ially in the base of the abdomen, into large sacs, which are filled 

 with air when the insect is about to take flight, so that the 

 specific gravity of the body is greatly diminished. Indeed, 

 these air sacs, dilatable at will by the insect, may be compared 

 to the swimming bladder of fishes, which enables them to rise 

 and fall at will to different levels in the sea, thus effecting an 

 immense saving of the labor of swimming. In the birds, as 



