420 



MOTION. 



Fig. 220. 



Areas of wings within the parallels, showing the ratio of increase with their distances from 



the axis of the body. 



0.26 



1.28 1.228 1.07743 0.40631 



43210 



Distances of parallels from the axis of the body in 0.4 of an inch. ( Morpho automedan.) 



ever, considerably in different orders of insects, 

 and they are here described rather in the 

 language of geometry than in that of entomo- 

 logy. The ratio of the area of the wings to 

 the weight of the insect varies in each order, 

 and approximates to a constant quantity only 

 in the same order. The wings of insects dimi- 

 nish in thickness from their base to their apex, 

 and from their anterior to their posterior 

 margin;* the strongest nervures traverse the 

 anterior margin of the wing, and confer 

 on that portion the greatest resisting power. 

 The posterior margin being weaker is inclined 

 upwards and backwards in reference to the 

 direction of its stroke. The plane of the 

 wing, as Straus has correctly remarked, is 

 therefore inclined at a different angle to the 

 horizon at every moment of its descent. By 

 the composition of forces the obliquity of the 

 wings backwards and downwards gives to the 

 centre of gravity an impulse upwards and for- 

 wards. The radial and cubital nervures in 

 insects supply the place of the bones of the 

 arm in birds ; and though different in structure, 

 they have the same mechanical effect. The 

 anterior nervures being articulated to the apo- 



* See ChabriersuT le vole cles insocts, c. i. p. 424. 



physes of the wings, and being fixed at their 

 extremities upon the two axillary first pieces, the 

 latter, with the wings, form a lever of the first 

 order, and when the internal borders of the 

 two axillary pieces are lowered the wings are 

 raised, and vice versa ; or as the axillaries are 

 articulated upon the border of the clypeus, 

 the movements of elevation and depression in 

 these produce the contrary movements in the 

 wings.* The wings of insects oscillate during 

 flight through arcs of various lengths, which 

 depend on the distances of the centres of the 

 wings from their axes of motion, and other dy- 

 namic conditions. In the Lepidoptera they 

 appear to describe an arc of 180, so as to 

 meet each other at each elevation and depres- 

 sion. In some other orders the arc of oscilla- 

 tion appears to be much less. Amongst the 

 Coleoptera, in some of which the elytra assist 

 the under wing in flight, according to Chabrier, 

 the latter describe an arc four times as great as 

 the former.f In estimating the number of 



* See Straus-Diirckheim, loco cit. 212. 



t Dans les hannetons chaque aile, en volant 

 paroit decrire un arc de plus de 200 cent, tandis 

 que celui trace dans le memo temps par les elytres 

 est peut-etre au dessous de 50 cent. See Chabrier, 

 sur le vole des insects, p. 31. 



