Every one must have observed that these large caterpillars—as, for instance, that of the small Elephant Hawk-moth ([Fig. 10])—have a sort of uncanny poisonous appearance; that they suggest a small thick snake or other evil beast, and the so-called “eyes” do much to increase the deception. Moreover, the segment on which they are placed is swollen, and the insect, when in danger, has the habit of retracting its head and front segments, which gives it an additional resemblance to some small reptile. That small birds are, as a matter of fact, afraid of these caterpillars (which, however, I need not say, are in reality altogether harmless), Weissmann has proved by actual experiment. He put one of these caterpillars in a tray in which he was accustomed to place seed for birds. Soon a little flock of sparrows and other small birds assembled to feed as usual. One of them lit on the edge of this tray, and was just going to hop in, when she spied the caterpillar. Immediately she began bobbing her head up and down, but was afraid to go nearer. Another joined her, and then another, until at last there was a little company of ten or twelve birds, all looking on in astonishment, but not one ventured into the tray; while one bird, which lit in it unsuspectingly, beat a hasty retreat in evident alarm, as soon as she perceived the caterpillar. After watching for some time, Weissmann removed it, when the birds soon attacked the seeds. Other caterpillars also are probably protected by their curious resemblance to spotted snakes.
Fig. 10.—The Caterpillar of the Small Elephant Hawk-moth (Chærocampa porcellus).
Moreover, we may learn another very interesting lesson from these caterpillars. They leave the egg, as we have seen, a plain green, like so many other caterpillars, and gradually acquire a succession of markings, the utility of which I have just attempted to explain. The young larva, in fact, represents an old form, and the species, in the lapse of ages, has gone through the stage which each individual now passes through in a few weeks. Thus, the caterpillar of Chærocampa porcellus, a species very nearly allied to the Elephant Hawk-moth, passes through almost exactly the same stages as that species. But it leaves the egg with a subdorsal line, which the caterpillar of the Elephant Hawk-moth does not acquire until after its first moult. No one can doubt, however, that there was a time when the new-born caterpillars of the small Elephant Hawk-moth were plain green, like those of the large one. Again, if we compare the mature caterpillars of this group of Hawk-moths, we shall find there are some forms which never develop eye-spots, but which, even when full grown, correspond to the second stage of the Elephant Hawk-moth. Here, then, we seem to have species still in the stage which the Elephant Hawk-moth must have passed through long ago.
The genus Deilephila, of which we have three species—the Euphorbia Hawk-moth, the Galium Hawk-moth, and the Rayed Hawk-moth—is also very instructive. The caterpillar of the Euphorbia Hawk-moth begins life of a clear green color, without a trace of the subsequent markings. After the first moult, however, it has a number of black patches, a white line, and a series of white dots, and has, therefore, at one bound, acquired characters which in the Elephant Hawk-moth, as we have seen, were only very gradually assumed. In the third stage, the line has disappeared, leaving the white spots. In the fourth, the caterpillars have become very variable, but are generally much darker than before, and have a number of white dots under the spots. In the fifth stage, there is a second row of white spots under the first. The caterpillars not being good to eat, there is, as has been already pointed out, no need for, or attempt at, concealment. Now if we compare the mature caterpillars of other species of the genus, we shall find that they represent phases in the development of the Euphorbia Hawk-moth. The Sea Buckthorn Hawk-moth, for instance, even when full grown, is a plain green, with only a trace of the line, and corresponds, therefore, with a very early stage of the Euphorbia Hawk-moth; there is another species found in South Russia, which has the line, and represents the second stage of the Euphorbia Hawk-moth; another has the line and the row of spots, and represents, therefore, the third stage; lastly, there are some which have progressed further, and lost the longitudinal line, but they never acquire the second row of spots which characterizes the last stage of the Euphorbia Hawk-moth.
Thus, then, the individual life of certain caterpillars gives us a clue to the history of the species in past ages.
For such inquiries as this, the larvæ of Lepidoptera are particularly suitable, because they live an exposed life; because the different species, even of the same genus, often feed on different plants, and are therefore exposed to different conditions; and last, not least, because we know more about the larvæ of the butterflies and moths than about those of any other insects. The larvæ of ants all live in the dark; they are fed by the perfect ants, and being therefore all subject to very similar conditions, are all very much alike. It would puzzle even a good naturalist to determine the species of an ant larva, while, as we all know, the caterpillars of butterflies and moths are as easy to distinguish as the perfect insects; they differ from one another as much as, sometimes more than, the butterflies and moths themselves.
There are five principal types of coloring among caterpillars. Those which live inside wood, or leaves, or underground, are generally of a uniform pale hue; the small leaf-eating caterpillars are green, like the leaves on which they feed. The other three types may, to compare small things with great, be likened to the three types of coloring among cats. There are the ground cats, such as the lion or puma, which are brownish or sand color, like the open places they frequent. So also caterpillars which conceal themselves by day at the roots of their food-plant, tend, as we have seen, even if originally green, to assume the color of earth. Nor must I omit to mention the Geometridæ, to which I have already referred, and which, from their brown color, their peculiar attitudes, and the frequent presence of warts or protuberances, closely mimic bits of dry stick. That the caterpillars of these species were originally green, we may infer from the fact that some of them at least are still of that color when first born.
Then there are the spotted or eyed cats, such as the leopard, which live among trees; and their peculiar coloring renders them less conspicuous by simulating spots of light which penetrate through foliage. So also many caterpillars are marked with spots, eyes, or patches of color. Lastly, there are the jungle cats, of which the tiger is the typical species, and which have stripes, rendering them very difficult to see among the brown grass which they frequent. It may, perhaps, be said that this comparison fails, because the stripes of tigers are perpendicular, while those of caterpillars are either longitudinal or oblique. This, however, so far from constituting a real difference, confirms the explanation; because in each case the direction of the lines follows that of the foliage. The tiger, walking horizontally on the ground, has transverse bars; the caterpillar, clinging to the grass in a vertical position, has longitudinal lines; while those which live on large-veined leaves have oblique lines, like the oblique ribs of the leaves.