296 MOTIONS OF INSECTS. 
strokes, between which an interval takes place. The legs 
are not employed in producing this progressive motion, 
for they are then applied close to the sides of the trunk, 
in astate of perfect inaction. But it is effected by a strong 
ejaculation of water from the anus. When I treat upon the 
respiration of insects, I shall explain to you the apparatus 
by which these animals separate the air from the water 
for that purpose; in the present case it is subsidiary to 
their motions, since it is by drawing in and then expelling 
_the water that they are enabled to swim. To see this, 
you have only to put one of these larvee into a plate with 
a little water. You will find that, while the animal moves 
forward, a current of water is produced by this pumping, 
in a contrary direction. As the larva, between every 
stroke of its internal piston, has to draw in a fresh supply 
of water, an interval must of course take place between 
the strokes. Sometimes it will lift its anus out of the 
water, when a long thread of water, if I may so speak, 
issues from it*. 
II. I am next to say something upon the motions 
of insects in their pupa state. This is usually to our little 
favourites a state of perfect repose; but, as I long since 
observed>, there are several that, even when become 
pups, are as active and feed as rapaciously as they 
do when they are either larvee or perfect insects. The 
Dermaptera, Orthoptera, Hemiptera, many of the Neu- 
roptera, and the majority of the Aptera, are of this de- 
scription. With respect to their motions, we may there- 
» De Geer, ii. 675— Compare Reaum, vi. 393— 
» Vou. 1. 4th Ed. 66. 
