To this class belong all the diverse instincts by which insects protect themselves against attack during the pupa stage. Even the way in which the caterpillars of many diurnal butterflies hang themselves up in pupation is not by any means a very simple instinctive action. The caterpillar first spins, in a suitable place, a small round disk of silk threads, to which it then attaches the posterior end of its body, so securely that it cannot be easily torn away. More complicated still is the securing of the pupa when it does not hang freely, but is to remain pressed against a wall or a tree, as is the case in the Papilionidæ and the Pieridæ. In this case the caterpillar must, in addition to the usual cradle, spin a thread of silk, in an ingenious way, diagonally across the thorax, so that it may cross about the middle of the wing rudiments, and not be too loose, lest the pupa fall out, yet not too tight, lest the thread cut too deeply into the wing rudiments and hinder their development. When one remembers that it is the caterpillar that does all this, before it has taken the form of the pupa, and that it must all be adapted to the pupa's form, we are amazed at the extraordinary exactness with which instinct prescribes all the individual movements which make the whole of the complex performance effective. And yet, as each caterpillar only accomplishes this performance once in its life, it could at no time in the development of the species have become a habit in the case of any individual caterpillar, and it cannot therefore be an 'inherited habit.'
But however diverse are the methods of securing the safety of the pupæ in the different families of butterflies, they must all be referred back to a single root, if the butterfly pedigree can be traced back to a single ancestral group. The caterpillar of the Sphingidæ does not creep up walls and trees when it is ready to enter on the pupa stage, as so many of the caterpillars of the diurnal butterflies do, but instead its instinct compels it to run about on the ground until it has found a spot which seems to it suited for boring into the earth, or, to speak less metaphorically, until it comes to a place which, from its nature, acts as a liberating stimulus to the instinct to burrow. Then it penetrates more or less deeply, according to the species, and makes a small chamber, which it lines with silken threads to prevent it collapsing; this done, it moults, and enters on the pupa stage. The exactness with which the individual movements are prescribed by instinct is seen in the way in which the size of the chamber is regulated so as to be exactly as large as is necessary to give the pupa room enough without leaving any superfluous free space. This is not so simple as it seems, and is not directly conditioned by the size of the animal, for the caterpillar is longer and altogether of greater volume than the pupa. The same thing is seen in the stag-beetle (Lucanus cervus), the largest of our indigenous beetles, which gets its name from the powerful antler-like jaws which distinguish the male. It also undergoes its pupal metamorphosis in the earth, and makes a large hard ball of clay, hollow inside, and as smooth as if polished, and its cavity is exactly the size of the future pupa, or to speak more precisely, of the fully-formed beetle. For, as Rösel von Rosenhof in his day 'observed with amazement,' the balls in which the males lie have a much longer cavity than those built by the females, and for this reason, that when the fully-formed beetle emerges from the pupa it must, if it is a male, have room to stretch out its horns, which have till then lain upon the breast. 'For the beetles do not leave their dwelling-place until all their parts are sufficiently strong and properly hardened, and till the season has arrived in which they are wont to fly about.' The male larva thus makes a much longer pupa-house than the female larva, in anticipation, so to speak, of the enormous size of the jaws which will grow out later!
Here the instinct has two modes of expression, according as the bodily parts are male or female. Here we have to do with an action which is performed once in a lifetime, and thus the possibility of any other explanation of the origin of this instinct than through natural selection is excluded.
Not less significant is the case of the silk-cocoons. The cocoons spun by the silkworm are egg-shaped, and consist of a single thread many thousand yards in length, which is wound round the spinning caterpillar so that not a space is left uncovered. The web is firm, tough, and very difficult to tear; therefore we must grant that the pupa resting within will enjoy a very considerable degree of security against injury. But the moth must be able to get out, and that this may be possible the caterpillar is impelled by instinct to make its spinning movements such that the cocoon is eventually looser at the anterior end, so that the insect, when it is ready to emerge, can tear it asunder with its feet and make a way out for itself. For this very reason, because the silk must be torn and spoilt by the emerging insect, silk-breeders kill the pupating insect before it begins to make its way out.
But there are species whose cocoons are provided from the very start with an outlet, for the caterpillar spins the silk round itself in such a way that a round opening is left. But this opening would be not only a convenient door for the butterfly to emerge by, but an equally convenient entrance for all its enemies. It is, therefore, closed up. In the case of the 'emperor moth' (Saturnia carpini) this is effected by means of a circle of stiff bristles of silk on the inside (Fig. 33), the points of which bend outwards like those of a weir-basket (r); from the inside the emerging moth can easily push aside the bristles, while the threatening enemy from without is scared off by the stiff points of the bristles.
Fig. 33. Cocoon of the Emperor Moth
(Saturnia carpini), after Rösel. A, enclosed
pupa. B, emerging moth. r, hedge of bristles.
fl, wings.
Such a cocoon is comparable to a work of art in which every part harmonizes with the rest, and all together are adapted as well as possible to their purpose. And yet it is all accomplished without the caterpillar having the remotest conception of what it is aiming at when it winds the endless silken thread about itself in the artistic and precisely prescribed coils. Nor has it any time for trying experiments or for learning; it must make all the complex bendings and turnings of the head which spins the thread, and of the anterior part of the body which guides the thread, quite exactly and correctly the first time if a good cocoon is to be produced. Here every possibility of interpreting this instinct as 'an inherited habit' is excluded, for each caterpillar becomes a pupa only once; and it is just as impossible to suppose that it can be directed by intelligence, since it can neither know that it is about to become a pupa, nor that, in the pupa stage, it will be in danger from enemies which will attempt to force their way into the cocoon, nor that the hedge of bristles will protect it from such enemies. Our only clue to an interpretation is in the slow process by which minute useful variations in the primitive instinct of spinning are accumulated through selection; and it is wonderful to see how exactly these spinning powers are adapted to the particular life-conditions of individual species.
Thus there are several of the Saturnides whose enormous caterpillars live on large-leaved trees, and these make use of the large leaves to form a shelter for the pupa stage, spinning them together so that the cocoon is for the most part surrounded by leaf. But as the leaf might easily fall off with the weight of the pupa, they make the leaf-stalk fast to the twig from which it grows by binding the two firmly together with a broad, strong, closely-apposed silken band. Seitz relates of the largest of all these spinners, the Chinese Attacus atlas, that this silk sheath 'is continued to the nearest strong branch, so that it is impossible with the hand to detach the leaves that conceal an Atlas-pupa from the tree.' To be sure, this pupa weighs about eleven grammes!
Since instincts vary, as well as the visible parts of an animal, a fulcrum is afforded by means of which selection can bring about all these very special adaptations to given conditions, since it always preserves for breeding the best suited variations of an already existing instinct. Any other interpretation is once more excluded.