The last of the five points to which I called your attention was the eye-spots.  In some cases, spots may serve for concealment, by resembling the marks on dead leaves.  In one species, which feeds on the hippophae, or sea buckthorn, a gray-green plant, the caterpillar also is a similar gray-green, and has, when full grown, a single red spot on each side—­which, as Weissmann suggests, at first sight much resembles in color and size one of the berries of the hippophae.  This might, at first, be supposed to constitute a danger, and therefore to be a disadvantage; but the seeds, though present, are not ripe, and consequently are not touched by birds.  Again, in another caterpillar, there is an eye-spot on each segment, which mimics the flower of the plant on which it feeds.  White spots, in some cases, also resemble the spots of light which penetrate foliage.  In other instances, however, and at any rate in our Elephant Hawk-moth, the eye-spots certainly render the insect more conspicuous.

Now in some cases, this is an advantage, rather than a drawback.  Suppose that from the nature of its food, from its being covered with hair, or from any other cause, a small green caterpillar were very bitter, or disagreeable or dangerous as food, still, in the number of small green caterpillars which birds love, it would be continually swallowed by mistake.  If, on the other hand, it had a conspicuous and peculiar color, its evil taste would serve to protect it, because the birds would soon recognize and avoid it, as has been proved experimentally.  I have already alluded to a case of this among the Hawk-moths, in a species which, feeding on euphorbia, with its bitter milky juice, is very distasteful to birds, and is thus actually protected by its bold and striking colors.  The spots on our Elephant Hawk-moth caterpillar do not admit of this explanation, because the insect is quite good to eat—­I mean, for birds.  We must, therefore, if possible, account for these spots in some other way.  There can, I think, be little doubt that Weissmann is right when he suggests that the eye-spots actually protect the caterpillar, by frightening its foes.

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