"But," you may object, "there are no scorpions in England: how then can the Devil's Coach-horse be benefited by imitating an animal which he has never seen, and of whose very existence he has not been able to read in pretty picture books?" Your objection has some force—though not so much as you imagine. It is quite true that there are no scorpions in England; but then, there are Devil's Coach-horses in many other countries, and the habit of tail-cocking need not necessarily have been acquired in these islands of Britain. That is not all, however: it suffices the beetle if the tactics it adopts happen to frighten and repel its enemies, no matter why. Now, in the first place, many of our migratory birds go in winter to Southern Europe and Africa—especially the insect-eaters, which can find no food in frozen weather. The hard-billed seed-eaters and fruit-eaters remain with us, but the soft-billed kinds retire to warmer climates, where food is plentiful. Of course, however, it is just these insect-eating birds that the Devil's Coach-horse has most to fear from. The birds must be quite familiar with the habits and manners of scorpions in their southern homes; and they are not likely to inquire closely whether the dangerous beast they know on the Mediterranean has, or has not been scheduled in Britain. We all of us dislike and distrust any insect that resembles a bee or wasp, and that buzzes or hums in a hostile manner: we give all such creatures a wide berth, wherever found, on the bare off-chance that they may turn out to be venomous—be hornets or so forth. Just in the same way, a bird, when it sees an unknown black beastie cock up its tail and assume a threatening attitude, is not likely to inquire too curiously whether or not it is really a scorpion: the bare suspicion of a sting is quite enough to warn it off from interfering with any doubtful customer. Moreover, in the second place, even those birds or men who have never seen a scorpion at all are yet sure to be alarmed when an insect sticks up its forked tail menacingly, and shows fight, instead of skulking or flying away. As a general rule, if any animal makes signs of resistance, we take it for granted he has adequate arms or weapons to resist with: and so this mere dumb-show of being a sort of scorpion proves quite sufficient to protect the Devil's Coach-horse from the majority of his enemies.
I ought to add that while our beetle thus frightens larger enemies, he is actively and offensively objectionable to small ones. The main use of his tail, indeed, is for folding away his wings, much as the earwig folds hers by aid of her pincers. But the Devil's Coach-horse makes it serve a double purpose. For he has a couple of yellow scent-glands in his tail, which secrete an unpleasant and acrid aromatic substance. These scent-glands are protruded in No. 4: you can just see them at the tip of the tail; and if the annoyance to which the beetle is subjected seems to call for their intervention, a drop of the volatile body they distil is set free, and is at once discharged in the face of the enemy. Such a manœuvre is in essence like that of the skunk: it is defence by means of a nasty odour, and it occurs not only in the Coach-horse's case, but also among a number of beetles and other insects.
The odd little creatures known as Bombardier Beetles are still quainter in their habits: they carry the last-mentioned mode of defence to an even greater pitch of perfection. For, like miniature artillery-men, they actually fire off a regular volley of explosive gas in the faces of their pursuers. The gas is secreted as a liquid; but it is very volatile, and it vaporizes at once on contact with the air, so as to form a small, white cloud of pungent smoke, resembling in its effects nitric acid. Our native English species of Bombardier roams about in large flocks or regiments: and when one member of a clan is disturbed, all the other beetles of the company let off their artillery at once, so that the scattered volley has something the appearance of platoon firing. The chief enemy of the Bombardiers is a much larger and very handsome carnivorous beetle known as Calosoma. When this insect tiger hunts down a single Bombardier, and has almost caught him, the fugitive waits till his pursuer is quite close, and then salutes him with a discharge of fire-arms: the pungent gas gets into the Calosoma's eyes and mouth and distracts him for a moment; and the Bombardier escapes in the midst of the confusion thus caused, under cover of the cloud he himself has exploded. That is the most highly evolved mode of defence of which I know among the British insects.
There are few creatures, again, which one would so little suspect of any attempt to bully and bluff others as the soft-bodied caterpillars. They are as a rule so plump and squashy and defenceless: a mere peck from a bird's beak is enough to kill them, for when once their tight, thin skin is broken, were it but with a pin-prick, all the flabby contents burst out at once in the messiest fashion. Yet even caterpillars, strange to say, have their tricks of terrifying. They pretend to be dangerous characters. I will set out with some of the simplest and least developed cases, and then pass on to a more complex and wily class of deceivers.
To begin with, I must premise that two sets of caterpillars have two different ways of evading the unpleasant notice of birds and other insect-eaters. One way is that adopted by the common "woolly-bear," a great hairy caterpillar, frequent in gardens, and covered from head to tail with long needles or bristles. These prickly points make the creature into a sort of insect hedgehog; birds refuse to touch him, because the serried spikes, which to us are mere hairs, seem to them perfect spines or thorns, sticking into their tongues and throats, or clogging their gizzards. Protected caterpillars like the woolly-bears live quite openly, exposed on the leaves and branches of their food-plant; they are not afraid of being seen: nay, they rather court observation than shun it, because they know nobody will attack them. The porcupine has no need to run away like the rabbit. Similar tactics are also adopted by many nasty-tasting caterpillars, in whose bodies natural selection has developed bitter or unpleasant juices. These caterpillars are rejected by birds and lizards—the great enemies of the race—and therefore they find it worth while to clothe themselves in gaudy and conspicuous red or yellow bands, so as to advertise all comers of their inedible qualities. Whenever you see such brilliantly-attired grubs (like those of the Magpie Moth, so common on gooseberry-bushes—a striking creature tricked out in belts of black and orange), you may be sure of two things: first, they live openly and undisguisedly on the leaves of their food-plant, without any attempt at mean concealment; and second, they are nasty to the taste, and therefore rejected as food by insect-eating animals. Now and then a young and inexperienced bird may eat one, to be sure; but it never tries twice, and the solitary martyr is sacrificed for the good of the race. Their bright colours and gaudy bands are just advertisements, as it were, of their inedible qualities. For, of course, nasty taste would do a caterpillar no good if the bird had always to sample it before rejecting it; the broken skin alone would be enough to kill it. Hence almost all uneatable caterpillars have acquired bright colours by natural selection—that is to say, by the less bright being continuously devoured or killed; and birds on their side have learned to know (after one trial, or, perhaps, even before it by inherited instinct) that red or yellow bands and belts in caterpillars are the outward and visible sign of uneatableness.
The second group or set of caterpillars is edible and tasty: it, therefore, governs itself accordingly, and has recourse to the exactly opposite tactics. Caterpillars of this class are smooth and naked: they never have the brilliant "warning colours" of the nasty-tasted kinds: and they show a marked absence of the beautiful metallic sheen, the strange melting iridescent hues and spots which add beauty to the charms of so many among the uneatable species. Such fat and smooth-skinned edible caterpillars are, of course, very tempting juicy morsels to birds and other insect-eating animals. Their motions, like those of all grubs, are slow; and if they lived exposed on their food-plants, after the fashion of the protected hairy and bitter kinds, they would all he eaten up before they had time to turn into moths or butterflies. Here, therefore, natural selection has produced the contrary result from that which it produces among protected kinds. Caterpillars of this edible type which showed themselves too openly and imprudently have got picked off by birds, like sentries and pickets who make themselves too conspicuous to the enemy's sharpshooters. Only the most prudent, modest, and retiring grubs have survived to become moths or butterflies, and so be the parents of future generations, to whom they hand on their own peculiarities. In this way the edible caterpillars have acquired at last a fixed hereditary instinct of lurking under leaves, or in dark spots, and never showing themselves openly. The larvæ of the butterfly group as a whole thus fall into two great classes (as far as regards habits alone, I mean): the protected, which are either hairy or nasty, and which flaunt themselves openly; and the unprotected, which lurk and skulk, endeavouring to escape notice as sedulously as their rivals the protected endeavour to attract it.
Nor is that all. It would clearly be useless for a bright red or yellow caterpillar to hide under a green leaf, and then suppose by that simple device he was going to escape observation. Birds are always looking out for insects under leaves. The consequence is that skulking or lurking caterpillars are soon found out by sharp-eyed and hungry enemies, unless they closely resemble the foliage or stems upon which they lie. From generation to generation, accordingly, the less imitative insects get eaten, and the more imitative spared: so that nowadays, most unarmed caterpillars are green like the leaves or grey like the stems, and are even provided with markings of light and shade upon their skins which mimic the distribution of light and shade among the ribs and veins of the surrounding foliage. Such deceptive leaf-like caterpillars are always very difficult to find: so that careless observers as a rule know only those of the other type, the great hairy "woolly-bears" and the brilliant red and yellow-banded bitter kinds; they never observe the unobtrusive green and brown sorts, which harmonize so admirably with their native tree in colour and markings.
5.—CATERPILLAR OF THE BROAD-BORDERED BEE-HAWK TRYING TO LOOK ALARMING.
Many greenish caterpillars, however, when discovered and disturbed, fall back on their second line of defence: they endeavour to frighten their enemies by devices closely similar to those of the Devil's Coach-horse. The caterpillar of the Broad-bordered Bee-hawk, for example, forms a good instance of a very simple stage in the development of such brazen-faced "terrifying" tactics. This warlike grub is shown in No. 5, trying on its simple little attempt to make itself alarming. Though by no means an uncanny-looking or appalling insect, it will rear itself up on its haunches (so to speak) when attacked, raising the fore part of its body erect with a sudden jerk, and holding its head high, as if it meant to bite or sting, so as to give itself as formidable an aspect as possible. The mild ruse succeeds, too; for birds will eye the harmless creature askance when it attempts this evolution, putting their heads on one side, and ruffling their crests in evident terror. The attitude is all a simple piece of bluff, to be sure, but it pays; indeed, bluff in warfare is often more than half the battle. If you put on a bold face in a row, and seem able to take care of yourself, people are apt to think you have a knife up your sleeve, and therefore to refrain from unnecessarily annoying you.