Homes Without Hands: Being a Description of the Habitations of Animals, Classed According to Their Principle of Construction - Page 109

These examples entirely destroy both theories.

In the first instance we have nests of which the cells are perfectly hexagonal throughout, although some of them are only attached by one side, and are not pressed upon at either of the five remaining sides. We find that the external angles are as sharp, and their internal measurement as true, as those which occupy the very centre of the bee-comb; so that pressure is clearly not the cause of the hexagon. That excavation is not the cause is also evident, from the fact that the external cells can not have been excavated, and yet are hexagonal.

These examples, therefore, show that the hexagonal form can exist without pressure. But, as if to show that pressure can exist without producing the hexagonal form, we have the nest of the Mischocyttarus, whose long, delicate, soft-walled cells are grouped round each other, and yet retain their conical form, so that at any part of them a transverse section would show a circular edge.

The insect which makes this nest is rather long, measuring perhaps an inch in length. The color is pale yellow and the abdomen is much elongated, and attached to a slender foot-stalk or peduncle nearly as long as itself. Several of the cells have been occupied by larvæ which have begun to assume the pupal condition, as is shown by the white covers over their mouths.

One of the most remarkable of these branch-building insects is that which has been appropriately named the PROCESSIONARY MOTH (Cnethocampa processionea). This curious moth lays a number of eggs, mostly upon the oak, and as soon as they are hatched the little creatures begin to form their home.

Externally it is not unlike that of the brown-tailed moth, but it differs in one respect, namely, that it is not divided into separate chambers, and has only one aperture. When the larvæ sally out for the purpose of procuring food, they spin guide-lines, as is the case with many other caterpillars. But, instead of going out singly into the world, each to find its own food in its own way, they march out in regular order, like a military party on a foraging expedition.

A single caterpillar is always the leader, and often is followed by one or two others in Indian file. Presently, however, the caterpillars march two deep, and, if a large number should be on the move, the line is sometimes from five to six deep. They are all very close to each other, so that the procession flows on in one unbroken line, and, until the observer is close to it, he can not see that its component parts are moving at all.

On referring to the illustration, the reader will see that the artist has represented a nest of the Processionary Moth, part of which has been torn open so as to show the absence of partitions in the interior. A number of the caterpillars are also shown, but in the middle of the nest is one grub of very great size, being, in fact, six or seven times as large as the caterpillars. This creature has been introduced because it is generally to be found in the nest of the Processionary Moth, and because it is one of the most useful insects that a careful agriculturist can protect.

Processionary Moth and Calosoma.

It is the larva of a beautiful beetle, called scientifically Calosoma sycophanta, which is represented below in the act of ascending the tree. The beetle is a lovely blue-green, but the larva is as unsightly a being as can well be conceived, its body being fat, flattish, and scaly, and its color black. This creature feeds entirely upon the various caterpillars and other larvæ, even eating those of the destructive sawflies. At the end of the tail are two horny spines, and the head is furnished with a pair of curved, sharp, and powerful jaws, by means of which it seizes its prey.

Instinct teaches these grubs to find their prey, and it may easily be imagined that when they approach a nest of the Processionary Moth they are not slow to avail themselves of the opportunity. Indeed, so sure are they of discovering their prey, that Réaumur asserts that he never opened a nest of the Processionary Moth without finding one or more specimens of their rapacious enemy, as many as five or six having been seen in a single nest. They are most voracious creatures, as indeed is evident from their structure; and, as each grub will eat several large caterpillars in a day, the havoc which is made in the nest may easily be imagined. The caterpillars have no means of defense or escape. They can not leave their home, and they can not kill or expel the intruder. All that they can do is to go out and eat, and come back and be eaten, their numbers ever diminishing, like the companions of Ulysses in the Cyclops' cave.

But for the exertions of this most useful insect, the ravages of the Processionary Caterpillars would be greatly increased, for the creature does not only eat them while in the larval condition, but feeds upon them after they have become pupæ. Sometimes, however, this extreme voracity defeats its own purpose. It occasionally happens that a grub of the Calosoma habitually gorges itself to such an extent with Processionary Caterpillars that it becomes fat, unwieldy, and scarce able to move. If, when it is in this condition, leaner and hungrier grubs should come across it, they are too apt to seize upon it and devour it in sheer wantonness, even though the nest be full of their legitimate prey.

Knowing the habits of this grub, a French entomologist, M. Boisgerard, managed very ingeniously to avail himself of its devouring capacities. There is a well-known insect, Bombyx dispar, popularly called the Gipsy Moth, which is very common in France, though scarce in most parts of England. The larva of this moth is destructive to trees, feeding on their leaves, and then retreating to a cunning little hiding-place in some crevice of the bark. Finding that his trees were infested with these caterpillars, M. Boisgerard procured a number of female Calosomas, and placed them on the trees. They laid their eggs, and in due season the larvæ were hatched. In process of time the destructive grubs increased so much that they ate all the noxious caterpillars, and at the end of the third year the trees were cleared, and the Calosoma beetles had to go elsewhere for a living.

In England the Calosoma is exceedingly rare, all specimens hitherto captured having been apparently blown over the sea from the Continent or brought in ships. Toward the south of France it is plentiful enough, as is needed from the enormous multitudes of crop-destroying caterpillars on which it feeds. There is, however, a closely allied species, Calosoma inquisitor, which is not so scarce, and, although comparatively seldom seen, may be captured by those who know where to look for it. I have taken it in Bagley Wood, near Oxford.

THE reader may remember that two species of wasp, namely, Vespa vulgaris and Vespa germanica, will work harmoniously at the same nest. This curious sociability, which is contrary to the usual custom of nature, is shared by moths as well as wasps. When experimenting upon the nests of this species, M. Réaumur found that several distinct broods of caterpillars would spin a common web and live in peace together, just as if they had been the offspring of one mother. Mr. Rennie, however, carried the experiments still farther, and found that two different species would act in the same social manner.

"We ourselves ascertained during the present summer (1829) that this principle of sociality is not confined to the same species, nor even to the same genus. The experiment which we tried was to confine two broods of different species to the same branch, by placing it in a glass of water to prevent their escape. The caterpillars which we experimented upon were several broods of the brown-tailed moth (Porthesia auriflua) and the lackey (Clisiocampa neustria). These we found to work with as much industry and harmony in constructing their common tent as if they had been at liberty in their native trees; and when the lackeys encountered the brown-tails they manifested no alarm or uneasiness, but passed over the backs of one another as if they had made only a portion of the branch.

"In none of their operations did they seem to be subject to any discipline, each individual appearing to work in perfecting the structure from individual instinct, in the same manner as was remarked by M. Huber in the case of the hive-bees. In making such experiments, it is obvious that the species of caterpillars experimented with must feed upon the same sort of plant."

One remark ought to be made on this interesting narrative. The author lays some stress on the fact that the two insects belonged not only to different species, but to different genera. It must, however, be remembered that, although the distinction of insects into species is easy enough, their grouping into genera is quite arbitrary, depending entirely on the classifier. Linnæus, for example, divided all the butterflies into two genera, while the modern classification admits some thirty genera. While, therefore, we may lay every stress on the species, we need not trouble ourselves much about the genus.

The two moths mentioned in this history are very different in appearance, and the larvæ are still more unlike. They have, however, this point of similarity, that they construct large dwellings upon branches, spinning them of silk, and making them large enough to contain a whole brood at once. The Lackey Moths are so called on account of the bright colors of the caterpillars, which are striped and decorated like modern footmen. Some species, however, derive the name from a different source.

When the mother insect lays her eggs, she deposits them on a small branch or twig, disposing them in a ring that completely encircles the twig, as a bracelet surrounds a lady's wrist. When she has completed the circle, she covers the eggs with a kind of varnish, which soon hardens, and forms a perfect defense from the rain. The varnish is so hard, and binds the eggs so firmly together, that, if the twig be carefully severed, the whole mass of eggs can be slipped off entire. As this varnish produces the same effect on eggs as lacquer does upon polished metal, preserving the surface and defending it from moisture, the insect is called the Lacquer, a word which has been corrupted into Lackey.

In wet weather the Lackey Caterpillars prefer to remain in their silken home, leaving it only for the purpose of feeding. They never lose their way, because, like the larvæ of the little ermine moth, which has been already described, they continually spin a single silken thread as they go along, and are therefore provided with an infallible guide to the track. Before they change to the pupal state they leave the nest.

The larva of this species is a very prettily-marked creature, the body being striped with blue, and yellow, and white. The moth itself is yellow, with a slight tinge of orange, and across the upper pair of wings runs a dark band edged on either side by a paler streak. As there is another allied species, which lives on various sea-side plants, the present insect ought more properly to be called the Tree Lackey. The moth seems to be rather periodical and local; for, although specimens are found annually in most years, they swarm to such an extent in certain places that whole rows of fruit-trees are denuded of their leaves, and covered with the silken webs of the pretty but destructive caterpillars.

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