OF MUSCULAR MOTION. 455 



and then again narrowing by degrees, contributes very considerably to 

 facilitate its swimming. The legs also, particularly the posterior ones, 

 are flat, compressed, and either upon one or upon both edges thickly 

 furnished with long setae or bristles. The first joint of the tarsus has 

 a very free motion, and can so place itself that either the sharp edge 

 or broad flat surface of the entire foot is brought forward and opposed 

 to the pressure of the water. In the flrst case the motion finds little 

 resistance, and easily cuts through the water, and in the last the 

 pressure of the water acts as a resisting medium against the broad flat 

 surface of the foot, which is increased to about double its width by 

 means of the long fringes, and thus the beetle is enabled to advance. In 

 addition to the repulsion which the rowing of the insect occasions we 

 may also add the pressure exercised by the water itself, occasioned by 

 the specific gravity of the insect. Were the beetle placed horizontally 

 in the water it would thereby be raised upwards, but its posture is 

 not horizontal, its axis forming an acute angle with the surface, and 

 indeed the head is the deepest situated. By means of this position its 

 swimming is much facilitated, as the pressure of the water from beneath, 

 acting against an oblique surface, pushes both sidewards and upwards. 

 The rowing of the beetle therefore has only to overcome that portion of 

 the pressure which urges upwar, and then, without further exertion 

 on its part, the beetle swims forward. If, therefore, it applies more 

 power than is requisite for its swimming direct, it necessarily descends 

 obliquely, and we consequently always observe it to dive in this direction, 

 and never perpendicularly. But its own muscular activity is, however, 

 the chief cause of its motion in water. This muscular motion is exer- 

 cised principally by the posterior legs, which bend forwards as far as 

 possible, when the narrow edge is directed anteriorly. In their dis- 

 tending motion all the joints bend, but particularly those of the foot, 

 so that their broad surface is opposed to the pressure of the water ; at 

 the same time, but probably merely mechanically, by this pressure the 

 stiff marginal fringe is expanded, so that by lying closely contiguous 

 it forms as it were the face of the oar which the insect uses in its 

 posterior legs. The violent extension of the leg to where it meets its 

 opponent of the opposite side, behind the body,, then propels it, and a 

 repeated rowing continues the commenced motion. 



The genera Nolonecla, Naucoris, and Sigara swim in the same 

 manner, but with this essential difference, that in them the ventral 

 surface is directed upwards, and the keel-shaped back is directed 

 downwards. 



