effect of the stroke. In the back stroke, the 
thigh (d, fig. 229) moves first, describing an 
are of a circle about their iliac extremity, from 
_ dtod’ and d” successively. During this move- 
ment the legs e and tarsi 7 are flexed pas- 
_ sively, and carried forwards to e’, f’,s0 as to 
act with little effect on the water. The two 
thighs d” d” begin to approach their greatest 
- flexion forwards, the legs e’ ¢’ and the phalanges 
_ f@ f* extend in succession, so that the water 
“opposes resistance to only one of them ata 
1e. The tarsi (/”) having been completely 
tended outwards, the two legs are suddenly 
ied backwards, pressing on the water, with 
entire plane of the tarsi, and as the silky 
fringes which cover the phalanges are extended 
al the same instant, their surface is considerably 
‘augmented. The other parts of the two legs press- 
ing upon the tarsi, as in the walk or the leap, 
“projects the body forward in the direction of 
its axis. In walking, the two members of the 
me pair act alternately, in order to serve, 
ch in its turn, as a support to the centre 
gravity. In swimming, on the contrary, the 
dy being supported by the water, the two 
‘members move simultaneously, in order to give 
the greater impulse, and it is in this respect 
_ that swimming differs essentially from walking, 
d more nearly approaches leaping. The 
dle legs of the Dytiscus act in a manner 
“Similar to the posterior, but being shorter 
and weaker, contribute little towards accele- 
‘ating the movements of the animal; the an- 
terior pair appear to be used chiefly for the 
of altering the direction of its motion. 
€ motions of insects in the water may be 
‘thus explained : let a b (fig. 230,) be the axis 
of the body passing through the centre of gravity 
0; and let co be the excess of the specific gra- 
vity of the water over that of the insect, acting 
a vertical direction upwards; and do the re- 
ce of the water to the insect moving in a 
ection oblique to the axis of the body; this 
Fig. 230. 
atter force is decomposed into two forces, one 
in d g parallel, and the other in de or go 
perpendicular to the body, the former of which 
is lost, and the latter forces it obliquely down- 
MOTION. 
435 
wards and backwards; this force being com- 
bined with the force of the water c o produces 
a resultant in A 0, opposite to d o, which is 
the force by which the insect ascends passively. 
On the other hand, when the insect moves ho- 
rizontally with its axis inclined in a 6, and 
its centre of gravity in 0, (fig. 231,) let its 
Fig. 231. 
J 
A figure from Straus-Diirckheim, to illustrate the 
movements of insects swimming horizontally. 
velocity be represented by h 0, so that the force 
in this direction may be the resultant of the 
forces of the locomotive organs, of the water 
co, and of the resistance which it opposes to 
the motion of the body in the direction of d 0, 
opposite to h o. The resistance of the liquid 
in d 0, acting obliquely upon the plane of the 
body, a b, (which is known from the velocity 
of the body and the inclination of its axis to 
the horizontal plane,) may he decomposed into 
two forces, one in dg oreo parallel, and the 
other in d e or g o perpendicular to the body; 
the former is ineffective, and the latter tends 
to force the body backwards and downwards. 
The force g 0, being combined with that of 
the water c 0, produces the resultant i o, 
which, with the force of the oars, must give 
the two components of h o. Completing the 
parallelogram of forces, of which 4 o is the 
diagonal, we find that the legs must generate 
a force represented in magnitude and direction 
by f 0, and it is in the direction of this com- 
ponent, or, more correctly, in a line parallel 
to it, that the centre of force of the feet ought 
to act, which is in fact the case. 
The Hydrophilus has nearly the same form 
as the Dytiscus, but is not quite so well or- 
ganized for swimming. The Gyrinus also, as 
well as the Hydrophilus, swims in conformity 
with the same principles as the Dytiscus. The 
Nepa, being ill organized for swimming, usually 
walks in the water. The Hydrometra being so 
light, and having a little globule of air attached 
to its feet, has the power of swimming on the 
water without sinking. The Notonecta, in which 
the centre of gravity lies above the centre of 
2Fr2 
