THORAX, LEGS, AND WINGS. 
41 
their views are all more or less based upon theories 
which would take too much of our space to discuss. 
If we refer to Fig. i8, we shall see that the costal 
nervure (c) is a strong chitinous rod, running along the 
edge of the wing and tapering to its extremity. It is 
this nervure, Marey says, which, carried up and down 
by the elevating and depressing forces in the muscles 
actuating it, moves the wing. x\s an experiment, if 
the wing of an insect be taken off and exposed to a 
strong current of air, it is seen that the plane of the 
wing is inclined more and more as the current be- 
comes stronger. 
By referring to Fig. 19, a, it will be seen that when 
the current is in the direction shown by the arrow, the 
anterior nervure (c) resists, 
but the membranous por- 
tion bends, owing to its 
greater pliancy. If we 
blow on the opposite sur- 
face, we see this carried 
backwards, as shown in b. 
It is evident that in 
the movement which 
3 
Fig. 19. — Diagram illustrating 
Pressure of Air on Wings. 
takes place during flight, the resistance of the air will 
produce upon the plane of the wing the same effects 
as the currents of air just employed. The changes in 
the plane caused by the resistance of the air are just 
those which occur in flight. The descending wing 
presents its upper surface forwards, from the resistance 
of the air from below upwards ; and the ascending 
wing turns its upper surface backwards, because the 
