338 
Proceedings of the Royal Society 
and backwards during flexion. The wing during its vibration 
descends farther below the body than it rises above it. This is 
necessary for elevating purposes. 
The distal portion of the posterior margin of the wing of the 
insect is twisted in a downward and forward direction at the end 
of the down stroke, whereas, at the end of the up stroke it is 
twisted downwards and backwards. The proximal portion of the 
posterior margin always assumes a reverse position to that occupied 
by the distal portion, so that the posterior and anterior margins of 
the wing are not in the same plane, and in certain situations the 
two margins appear to cross each other. What is here said of 
the insect’s wing applies equally to the wings of the bat and 
bird. 
The wing during its vibrations twists and untwists , so that it acts 
as a reversing reciprocating screw. The wing is consequently a 
screw structurally and functionally . 
The blur or impression produced on the eye by the rapidly 
oscillating wing is twisted upon itself \ and resembles the blade of an 
ordinary screw propeller. 
The twisted configuration of the wing and its screwing action 
are due to the presence of figure- of- 8 looped curves on its anterior and 
posterior margins ; these curves, when the wing is vibrating, re- 
versing and reciprocating in such a manner as to make the wing 
change form in all its parts. The curves in question are produced to 
a great extent by vital movements, independently alike of the elas- 
ticity of the wing and the reaction of the air. They can, however, 
be produced by the latter agencies likewise. The change and 
reversal of the curves occurring on the anterior and posterior 
margins cause the different portions of the wing to strike at various 
angles during the down and up strokes. 
The angles which the different parts of the wing make with the 
horizon are greatest towards the root, and least towards the tip of 
the wing. The angles are, in fact, adjusted to the speed at which 
the different portions of the wing travel — a large angle with a low 
speed giving the same amount of buoying and propelling power as 
a small angle with a high speed. 
The speed attained by the tip of the wing is always very much 
higher than that attained by those portions nearer the root— the 
