Fig. 256.—Erebus strix.
The family of Geometrinæ, or Geometers, comprises moths of a middling size, and usually flying after sunset and during the night. [70] They frequent the alleys of damp woods, where they become the prey of the Libellulæ [71] and other carnivorous insects. Their bodies and abdomens are slender, their wings large, thin, fragile, often of a dark colour, with brilliant markings.
The caterpillars of the Geometrinæ are known by the name of loopers or geometers. We have previously described their singular organisation. They are continually spinning a silken thread, which keeps them attached to the plant on which they live. If you touch the leaf which supports them they immediately let themselves fall.
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| Fig. 257.—Looper hanging by its thread. | Fig. 258.—Seen at the side. | Fig. 259.—Front view. | Figs. 260 and 261.—Remounting its thread. | |||||
"Nevertheless, they do not generally fall to the ground," says Réaumur; "there is a cord ready to support them in the air ([Fig. 257]), and a cord which they can lengthen as they will; this cord is only a very thin thread, but has nevertheless strength enough to support the caterpillar (Figs. [258], [259]). All that there seems to fear is, that the thread may lengthen too quickly and the caterpillar fall, rather than descend gently, to the ground. But what we must first remark and admire is, that the caterpillar is mistress of its movements, and is not obliged to descend too quickly; it descends by stages; it stops in the air when it pleases. Generally it only descends at most about one foot at a time, and sometimes only half a foot or a few inches, after which it makes a pause more or less long, as it pleases." It is in this way that the caterpillars let themselves fall from the top of the highest trees: they remount again with no less ease.
Let us listen to Réaumur's description of the means employed by this caterpillar to ascend these heights. Figs. [260] and [261], drawn, as the three preceding ones, from the plates in Réaumur's Memoir, help us to follow the explanation given by the illustrious naturalist of the evolutions of our little acrobat:—"To remount," says Réaumur, "the caterpillar seizes the thread between its jaws, as high up as it can; as soon as it has done this it twists its head round, lays it over on one side, and continues to do so more and more every moment. Its head seems to descend below the last of the scaly legs which are on the same side as that to which it is inclined. The truth is, however, that it is not its head which descends, the part of the thread which it holds between its jaws is a fixed point for its head and for the rest of its body; it is that portion of the back corresponding with its scaly legs which the caterpillar twists upwards; the consequence is that the scaly legs and that part of the body to which they belong then ascend. When the last pair of legs is just over the jaws of the caterpillar, one leg, viz., that which is on the side towards which the head is inclined, seizes the thread and brings it over to the corresponding leg on the other side, which is advanced to receive it. If the head is then raised, which is done immediately, it is in order that it may seize the thread at a higher point than that at which it was caught at first; or, which is the same thing, the head, and consequently the whole body of the caterpillar, is found to have ascended to a height equal to the length of the thread which is between the place where its jaws seized it the first time and that where they seized it the second time. The first move in the upward direction is thus made, and the second soon follows.... If you were to seize the caterpillar on its arrival at the end of its upward journey, you would see a packet of threads huddled together between the four hindmost of the scaly legs. The greater the height ascended the greater is the size of this packet. All the turns of the thread which compose it are entangled. So the caterpillar does not consider it of any value. As soon as the insect can walk it gets rid of the mass, sets its legs free, and leaves it behind before one or at most two steps have been taken. The cord is wasted on each ascent, but the caterpillar can afford to lose as many as it likes, for it has in itself the source of the matter necessary for the composition of the thread, and it is a source from which that which was drawn off is being continually re-supplied. Moreover, spinning the thread costs the caterpillars but little; indeed, the loopers economise this thread so badly that most of them leave it behind them wherever they go."
Fig. 262.—Hybernia leucophearia, male.