62 Annals Entomological Society of America [Vol. V,. 
effected by the up and down movement of this membrane, the: 
propulsion being determined by the flexibility of the posterior 
border, it will be seen that while this device provides beauti- 
fully for the forward progression of the insect, it does not 
account for such backward movements as have been noticed. 
This problem has been in mind for several years and I have 
presented it on a number of occasions to my classes in Ento- 
mology and it has provoked a good deal of discussion, and it 
appears to me that it is possible to offer an explanation which 
may be considered somewhat of a solution of the problem. 
This solution has been suggested and contributed to by a 
number of students in these discussions and it may be difficult 
to credit the explanation to any original source. 
The explanation of these movements seems most readily 
accounted for on the basis of an adaptation in the wing which 
provides for a forward and backward movement so that the 
angle of the wing with reference to the axis of the body repre- 
sents different degrees ranging from a right angle to an angle 
of 30 to 45 degrees for the anterior quadrant. It will be seen 
that when rotating forward in this manner, the rigid portion of 
the anterior part of the wing is shifted so that the flexible apical 
and posterior margins have a different extent and must present. 
a varying pressure upon the air. It appears quite certain that 
this rotation would allow for varying degrees of the forward 
and backward pressure, or to state it in another way that the 
direction of force of each wing would form an angle to the 
median axis of the body, and that at the point where these 
would neutralize each other, the effect would be to produce a 
stationary condition of the insect whereby it would hover at 
a fixed point, and that a slight further rotation forward would 
serve to push the insect in a backward direction. 
That this mechanism actually exists in many insects may 
be determined by the movement of the wing forward or back- 
ward in a horizontal plane, and it is easily noted in the position 
in which wings are fixed at death in many insects. Compari- 
son among different groups of insects will show that the extent. 
of rotation differs greatly in different groups and this would 
agree thoroughly with the fact that the ability to hover or to 
retreat in flight is very differently developed in different insects. 
Furthermore, it appears that the ability for this kind of flight 
depends in some degree upon the shape and especially upon 
