420 
Fig. 220. F 
MOTION. 
Areas of wings within the parallels, showing the ratio of increase with their distances from 
the axis of the body. 
1.228 1.07743 0.40631 
0.26 1.28° 
uv 
eee 
<7" 
en ae eS a coe 
‘ 
A 
> se . 
— 
—— 
e<J 
- 
me, 
~s < 
eee ee oO ee Te eee a | 
4 3 2 1 
Distances of parallels from the axis of the body in 0.4 of an inch. ( Morpho automedon, ) 
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 Chabrier sur le vole des insects, c. i, p. 424. 
im Pe, 
0 
physes of the wings, and being fixed at their 
extremities upon the two axillary first pieces, the 
latter, with the wings, form a /ever g the 
order, aud when the internal borders of the — 
two axillary pieces are lowered the wings are 
raised, and vice versi; 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 are of 180°,so as to — 
meet each other at each elevation and depres- 
sion, In some other orders the are of 7 
tion appears to be much less. Amongst the 
Coleoptera, in some of which the elytra assis 
the under wing in flight, according to Chabrier, 
the latter describe an are four times as great as — 
the former.t In estimating the number of — 
* See Straus-Diirckheim, loco cit. 212. ‘ 
+ Dans les hannetons chaque aile, en volant 
paroit décrire un arc de plus de 200° cent, tandis 
que celui tracé dans le méme temps les é 
est peut-étre au dessous de 509 cent. See Chabr 
sur le vole des insects, p. 31. 
