How Insects Walk
( Originally Published Ealry 1900's )
BEFORE man's vision was intensified and instructed by the revealing power of the microscope, he might be compared to a child who had, since its earliest memory, been confined in a high tower with a distant view of the green fields. Although told that the green-colored patches in the distance were fields, yet, in truth, the child would have little or no conception of what a field really was. Then, if at some future time, when he had reached a thoughtful age, his liberty was given him, and he went towards those fields, viewed so often from his distant tower, he would at once be overwhelmed by the details of the individual plant leaves and flower blooms that composed those sheets of evergreen he knew so well. It would then naturally follow that a careful analysis of details would have to be made, so that a new and more truthful conception of what constitutes a field should be formed.
So it was with man when the microscope first assisted him to unravel the mysteries of things organic and inorganic that surround him. And so it is to-day with the tyro who views through the microscope even the most common and familiar objects with which he feels himself well acquainted. He might, with apparent truth, reason that, by enlarging an object and revealing the details of its structure, he would better understand it, but that would be correct only in a limited degree. It would be quite true that, after those details had been analyzed and their relative values considered, he would know more about it, yet it should be remembered that, at first, the fresh details disclosed by the microscope add to the complexity rather than remove it. Let me give a practical example.
Everybody, doubtless, is familiar with the common blue-bottle-fly, and has observed its six black legs, and, probably, concluded that these terminate in six feet.
Now, if a detached leg of the blow-fly is seen by a per-son possessing normal vision, he would have little difficulty in arriving at the conclusion that he was looking at the leg of a fly; and if it should be viewed through a reading-glass, it would probably be even more easily distinguished as such. But now let us place under a micro-scope a small portion of this insect's leg, which roughly measures three-eighths of an inch in length; say we magnify the twenty-fifth part of an inch of the foot end, and see what is then revealed. I wonder how many of my readers would recognize these objects as tiny portions from the foot end of the legs of the familiar blow-fly--yet such is the fact. It will be seen that various details have come into view, which, previously, were quite unknown to us. What are these curious structures? And what is their function? are questions that naturally follow.
In the first place, it is obvious that each foot of the blow-fly is provided with a pair of stout claws. These are of service to the insect when walking over rough surfaces or penetrating tiny crevices quite invisible to human sight. For example, when it climbs a perpendicular wall, or walks upside-down upon a ceiling, the apparently smooth surface has sufficient roughness for these tiny claws to get a grip upon it. However, it occasionally occurs that the blow-fly has other situations to meet, where it cannot use these hooked claws to any advantage. For instance, in its casual roaming, it may alight upon the smooth surface of the butcher's scales and dishes; or, occasionally it appears to confuse the transparent window panes of the shop with the surrounding atmosphere. In the latter case, we see it butting at the glass, with alternate rambles up and down the pane, as if it were in a great state of perplexity over the inexplicable discovery that a portion of the atmosphere had suddenly be-come tough and hard to penetrate.
The point we need to particularly observe, however, is that the fly walks up the perpendicular, smooth glass with perfect ease, although there is here no rough surface on which its claws can find a hold. This brings us to the next point in the anatomy of the foot of this in-sect, namely, the two pads seen between the claws. These pads are brought into play on such smooth surfaces as that of glass and similar materials, the claws being then thrown back, one to each side, beneath the foot and out of the way; just as, when the claws are in use, the pads are lifted up between them, also to be out of the way.
These pads or flaps act by adhesion, and for a long time were thought to hold, like suckers, by forming a vacuum; but when some flies were put into a vessel from which the air was withdrawn, and the flies still adhered, it was conclusively shown that a mistake had been made; and some other explanation was necessary. Since then, it has been observed that the under side of these foot flaps are studded with tiny hairs which exude a sticky fluid, and it has, therefore, been surmised that this secretion both expels the air from beneath the pads and at the same time gums the pads to the smooth surface. In this way it is explained that, when we find a dead fly still adhering to a window pane or frame, it secreted this sticky fluid while its strength was failing, and later be-came too feeble to remove its feet again, Of course, a hold of this kind would stand a considerable pull, and so it bears the weight of the fly when upside-down. When the active fly would lift its foot, it loosens the pad by rolling it off the surface from opposite sides, just as we might readily open an envelope by pulling up the flap by the point before it has properly stuck, although it would be extremely difficult to open by pulling directly from above.
Although various views in explanation of the exact use of these adhesive organs which are familiar on many other flies besides the blow-fly, and also on bees, butter-flies and other insects, have been put forward from time to time, yet that given here is perhaps the most probable, and the one most largely accepted among zoologists.
The remaining structures worthy of notice in the blow-fly's foot are the stout bristles which clothe its joints, and, also, the longer and more delicate ones termed " touch-bristles " seen nearest the pads, which are probably associated with the sense of touch, and serve to direct the insect to suitable footholds. The coarser bristles may serve a variety of purposes in the economy of the insect, acting as brushes, combs, etc., as may be observed by watching a fly arranging its toilet.
There is another variety of the pad and claw arrangement. This example is that of an ichneumon fly — a parasitic fly whose business it is to seek out plump and healthy caterpillars and carefully deposit its eggs in their bodies, its offspring eventually feeding on the sub-stance of their caterpillar host.
Now, if the reader will think for one moment of this insect's occupation, and then glance at its strong comb-like claws, and then consider the plump and soft bodies of the caterpillars it has to deal with, no large effort of the imagination will be required to understand how the caterpillars are persuaded into a gentle submission to the egg-depositing business, when gripped with six feet like this.
We begin now to grasp the fact that the varied and wonderful structures and modifications revealed in in-sects' feet when viewed by the microscope have to do with the economy of the individual insect. This fact be-comes plainer still as we investigate. For example, take the head and one of the fore legs of the common flea. We see in this fore leg some extraordinary basal parts, considering it is only an insect's leg, and great muscular development—as one might expect, considering the flea's marvellous jumping capabilities—and some more of the spines and bristles so characteristic of the legs of insects. Also, we note that the feet terminate in two delicate claws, by means of which it retains its connection with the soft skin of its host.
It will be interesting to make a little comparison with the legs of this parasitic insect. Let us, for example, consider another familiar parasite, viz., the common sheeptick—which is largely in evidence at sheep-shearing. If we compare the feet of these two parasites, we see at once the delicacy of the one compared with the other. The legs and feet of the flea are adapted for a soft-bodied host, while those of the sheep-tick are specially formed for creeping about amongst the wool on the tougher skin of the sheep.
Although fully developed insects possess only six legs, yet, in their larval or caterpillar stage, we often find them with as many as sixteen. On careful observation, how-ever, we discover that the legs of caterpillars are of two distinct kinds. The first three pairs taper from the body, are jointed, and terminate in a claw, and these are the chief organs of locomotion. It often happens, though, that the caterpillar, while reaching out to the green leaves on which it feeds, has to loose its hold with these first three pairs of legs, and then the hinder pairs of " pro-legs " or clasping organs come into use. These pro-legs can be lengthened out and withdrawn again, much after the manner of the " horns " of snails, and each foot is terminated with a row of horny hooks on its inner edge, which penetrate tiny inequalities of surface in the branches and leaves and so give the caterpillar a firm grip. In this manner is explained that clinging or " creepy " sensation which a caterpillar gives to the hand when it crawls over it.
Only the first thee pairs of jointed legs are legs proper, however, and these remain throughout the metamorphoses of the insect, becoming the true jointed legs of the perfect insect. The fleshy prolegs, or clinging organs, being mere membraneous projections of the skin, disappear when the caterpillar makes its last moult, before becoming a pupa or chrysalis.
The principal swimming organs with nearly all adult aquatic insects are the hind legs, and another very beautiful and remarkable example may be instanced in the leg of the little whirlwig or whirligig beetles seen on almost any sunny day during the summer or winter in quiet nooks of rivers, carrying on their mazy turnings and whirlings on the surface of the water in most merry fashion. Of course, to carry out such manoeuvres as these sudden twistings and turnings, a special organ is needed, and in this case the hinder pairs of legs are the modified structures, the joints forming a series of plates which open out and form an expanding paddle, each joint being fringed with flat hairs, which probably help to present a greater surface of the water.
Another pond insect that presents a curious leg modification, but in this instance the fore leg, is the water-scorpion — a flattened, dingy-colored insect, of sluggish habits, which is as often as not mistaken for a bit of dead leaf ; and in this way it probably deceives its prey. Its method is to lie in wait until a worm or some aquatic larva comes near, and this it suddenly seizes with a fora leg. This fore leg acts clasp-knife fashion, the broadest joint being grooved and possessing strong muscles, by means of which the bladelike portion is pulled down upon it, the victim, of course, being gripped between them.
As a concluding example, we may leave aquatic insects and consider one of subterranean habits, namely, the mole-cricket. This insect excavates its burrows by means of its marvellous fore legs, which are wonderfully specialized for the work, bringing forcibly to mind the strong hands of the familiar mole; from this feature, indeed, the insect derives its popular name.
The fore legs, like the hands of the mole, possess extraordinary strength, and those joints of the feet which in insects are usually slender have developed here into four legs and strong projections used for digging out the soil, the foot being terminated with a small joint which bears two claws.
Another feature of interest is the curious oval opening seen at the back portion of this digging organ. This is one of the ears of this insect; the hearing organs of crickets and green grasshoppers, strange to pay, are found placed in this curious position below the knee.
On the opposite side of the fore leg, on the digging parts, are placed some other strong projections which point outwards and serve the insect to cut through any strong roots which it may happen to meet while making its excavations; these projections are brought in contact with similar prominences higher up the leg, the two sets forming together most efficient shears.
It is plain, then, that the wonders of anatomy exhibited by the microscope in the feet and legs of insects are not provided for mere ornamentation, but invariably serve some other and practical purpose in the economy of the insect that bears them.