Fig. 44.—Raptorial leg of Water Scorpion.
In Fig. 44 we have a representation of the raptorial (front) leg of this insect. This leg should be compared with the legs of other Insects—not in pictures only, but in actual specimens—that we may see how, while the general plan is preserved, different joints are modified to suit the special function of this limb—that of taking prey. (See also Fig. 43.)
The thigh (f) is the largest joint, for the obvious reason that it contains the muscles that work the tibia and tarsus, which together form a kind of knife-blade, shutting down into a groove in the thigh, which may very well be compared to the handle of a pocket-knife. This description, however, is not to be taken on trust. It is not enough to read about the groove: we must see it for ourselves, raise the ‘knife-blade’ from the groove, and press it down again, and pass the needle along the groove. If we examine the interior of the groove, we shall find that there is a projection along the bottom, so that a cross section would appear like this—
. The inner portion of the tibia is also grooved; so that when once the prey is seized by this powerful limb, it has no chance of escape. The tarsus (tar) is not clearly distinguished from the tibia (tib) in the engraving, but it may be distinctly seen as a claw-like appendage in a living or dead specimen.
The breathing-tube remains to be examined. It may be detached from the body for more convenient manipulation. When this is done, the tube will be seen to consist of two pieces, each grooved on the inner side and set with hairs, which, as they interlock, prevent the entrance of water.
A somewhat similar arrangement occurs in the proboscis or tongue of butterflies. There is perhaps a closer parallel in the antennae of the masked crab, which, under certain conditions, form a kind of breathing-tube, each antenna being joined to its fellow by the hairs with which it is set.
There are two other Water Bugs which lend themselves to our purpose very well. Each is popularly called Water Boatman, though that name is better confined to Notonecta, because the insects of this genus ‘row themselves about on their backs with their long feathered legs.’ In the United States they are called Water Cicadae, from the shrill noise they make, probably by rubbing the fore-legs together.
Fig. 45.—Water Boatman.