This curious apparatus is used for the purpose of securing prey.
I have kept many of these creatures, and watched their mode of feeding. As has already been mentioned, they have two modes of progression, i.e. walking by means of legs like those of ordinary insects; and driving themselves along by ejecting water from the tail, on the principle of the rocket. As far as I have seen, the latter mode is always used in taking prey. The Dragon-fly larva always lives at the bottom of the water, though it can force itself to the surface if needful. And, like the dreaded ground-shark, it seizes its prey from beneath.
Its favourite food is the larva of the whirlwig-beetle, a fat white grub, with a number of white, soft, feathery gills fringing its sides. In order to produce a current of air over these gills, the larva wriggles itself up to a height of several inches, and then sinks slowly down, with the white gills floating on either side.
Should a Dragon-fly larva be near, it sees the grub ascending, glides quietly under it without using its legs so as to cause alarm, waits for it to sink, darts out the mask, seizes it in the jaws, drags it to its mouth, and the grub is seen no more. So voracious are these larvæ, that, if only two are kept in the same vessel, one is sure to devour the other.
Another good example of the Lazy-tongs is the Proboscis of the common House-fly. We have all seen these insects alight near sugar, or any other tempting food, unfold the proboscis, pour a drop of liquid in the sugar, dissolve it, suck it up, and then shut up the proboscis as if by hinges.
Another labour-saving machine is the Apple-parer, a comparatively modern invention. The principle is, that a knife is pressed lightly by a spring against a revolving apple, and set at such an angle that nothing but the outside peel can be removed. Where large numbers of apples have to be pared, as in making preserves or in hotels, this is a most useful invention.
When I first saw it at work, the operation seemed familiar to me, but I could not at first remember the parallel. At last it flashed across me that a Squirrel eating a nut was the natural parallel of the Paring Machine.
After splitting the shell and extracting the kernel, the Squirrel takes the latter between its fore-paws, presses it against its upper incisor teeth, and makes it revolve rapidly. In a second or two the kernel is perfectly peeled, and is then eaten.
In this case the incisor teeth of the Squirrel take the part of the knife, the muscles of the leg that of the spring, and the sharp edges of the upper teeth that of the knife. The structure of the Rodent teeth has already been explained in page 233.