188 



ANIMAL LOCOMOTION. 



distance from the posterior, its middle being depressed, and 

 its sides brought nearer to each other. Thus its bending in 

 one direction produces a diminution of its curve in the direc- 

 tion normally opposed to it ; and by the alternations of this 

 motion, assisted by other means, the body is alternately com- 

 pressed and dilated, and the wings are raised and depressed 

 by turns." ^ 



In the libellulce or dragon-flies, the muscles are inserted 

 into the roots of the wings as in the bat and bird, the only 

 difference being that in the latter the muscles creep along the 

 wings to their extremities. 



In all the wings which I have examined, whether in the 

 insect, bat, or bird, the wings are recovered, flexed, or 

 drawn towards the body by the action of elastic ligaments, 

 these structures, by their mere contraction, causing the 

 wings, when fully extended and presenting their maximum 

 of surface, to resume their position of rest, and plane of 

 least resistance. The principal effort required in flight 

 would therefore seem to be made during extension and 

 the down stroke. The elastic ligaments are variously formed, 

 and the amount of contraction which they undergo is in 

 all cases accurately adapted to the size and form of the 

 wings, and the rapidity with which they are worked — the 

 contraction being greatest in the short- winged and heavy- 

 bodied insects and birds, and least in the light-bodied and 

 ample-winged ones, particularly in such as skim or glide. 

 The mechanical action of the elastic ligaments, I need scarcely 

 remark, insures a certain period of repose to the wings 

 at each stroke, and this is a point of some importance, as 

 showing that the lengthened and laborious flights of insects 

 and birds are not without their stated intervals of rest. 



Speed attained by Insects. — Many instances might be quoted 

 of the marvellous powers of flight possessed by insects as a 

 class. The male of the silkworm-moth (Attacus Paphia) is 

 stated to travel more than 100 miles a day;^ and an anony- 

 mous writer in Nicholson's JournaP calculates that the com- 

 mon house-fly {Musca domestica), in ordinary flight, makes 600 



1 Chabrier, as rendered by E. F. Bennett, F.L.S., etc. 



2 Linn. Trans, vii. p. 40. 3 YqI. iii. p. 36. 



