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

With regard to this insect I have been rather fortunate, having found many specimens of the nests, and bred from them several hundred insects.

Although plentiful enough in certain places, the BURNET ICHNEUMON, as I shall venture to call this species, is very local, and while abounding in one place, may never be seen in another spot at the distance of a very few hundred yards. I give it the popular name of Burnet Ichneumon for the same reason—comparing great things with small—that Caius Martius bore the title of Coriolanus, and Publius Cornelius Scipio was termed Africanus, namely, that it destroys so many Burnet Moths.

In its perfect state the Ichneumon looks like a rather small gnat, and would probably be mistaken for that insect by a non-entomological observer. When examined through an ordinary magnifying glass, it is seen to possess a wondrous beauty which no one could ever suspect when looking at it with the unaided eye. The body and head are of a pale yellow color, except the prominent compound eyes, which are dark blackish-brown. The head is round and rather small, but the thorax is of enormous size, quite as proportionately large as the chest of a man would be did it project some eighteen inches in front and reach to his heels.

In singular contrast to the huge thorax is the very tiny abdomen, which is of a retort shape, curved, and fixed in the upper surface of the thorax by its smaller end. Indeed, the abdomen bears the same relation to the thorax that the "tick" in the capital letter Q does to the whole of the letter. The limbs are long, and, when the size of the insect is considered, are singularly powerful, especially the last pair of legs. We think the legs of the kangaroo are enormously large in proportion to the size of its body, but they must be doubled in length as well as in thickness to equal those of the Burnet Ichneumon. The fore-limbs are not so very large, but they are long and possessed of great clasping power, aided by the hooked feet.

What, then, is the use of such powerful limbs? The habits of the insect supply the answer.

As is the case with many ichneumon flies, this insect—which, by the way, is not a fly, but a near relation to the bee and ant—deposits its eggs upon caterpillars, boring holes in their skin with its pointed ovipositor, which is the analogue of the bee's sting, and inserting its eggs in the perforations. As may naturally be imagined, the caterpillar has a very strong objection to this proceeding, and when the ichneumon settles upon it, and begins to use her weapon, twists and wriggles about like a captured eel.

Now the strong limbs of the ichneumon come into play. Minute as is the insect when compared to the caterpillar, bearing about the same relationship that a rabbit bears to an elephant, the legs are so long that they can include a considerable portion of the skin in their embrace, and so strong that they can retain their hold in spite of the contortions with which the caterpillar tries to rid itself of its persecutor. Retaining her place, therefore, the ichneumon deposits a great number of eggs in the poor caterpillar, and then goes to find another victim.

I am not sure whether or not the ichneumon makes a separate wound for every egg. If so, the feelings of the caterpillar are not to be envied, for I have found nearly a hundred and fifty ichneumon larvæ in the body of a single caterpillar. No wonder that the persecuted being endeavors to fling off the creature that is inflicting so many wounds. The numerous short and bristle-like hairs with which the legs are thickly clad are doubtless useful in retaining the hold of the insect.

The long and slender many-jointed antennæ are also covered with a thick down, which has an iridescent effect when the light plays on those organs, and during the life of the insect has a most beautiful effect, owing to the restless, quivering movements which characterize all the antennæ of ichneumons, and which at once serve to distinguish those insects at a glance.

The chief beauty, however, of the insect lies in the wings. To the naked eye they are simply colorless, transparent appendages, with a little black spot on the outer edge of the upper pair. But when placed under a magnifier, and with the light properly directed upon them, they blaze out in iridescent glory that almost fatigues the eye with its resplendence. One of these insects is now under the microscope before me, a low power of only thirty-six diameters being used, so that each wing appears to be about three inches in length; and in order to give an idea of the extraordinary coloring of these apparently transparent organs, I will describe as far as I can the appearance of the right-hand upper wing.

The material of which it is made is a translucent membrane, appearing single with this low power, but shown by a higher power to be double. The wing is traversed by numerous nervures to support it, as the tracery of a Gothic window supports the glass, and which divide it into numerous compartments, technically called cells, each of which is known by name to entomologists. The whole of the membrane is covered with very minute hairs, dotted at regular intervals, like the holes in perforated zinc, and as each of these hairs is in fact a minute prism, they break up the light into the well-known prismatic colors.

Upon the outer edge of the wing is a triangular black spot, which is not transparent, and serves as a foil to show off the lovely colors by which it is surrounded. The whole upper part of the wing is pale yellow, passing, by the gentlest imaginable transition, through delicate rays into lively pink, of the character termed "rose-carmine." Toward the lower edge of the pink a slight infusion of blue steals in, being first purple and then changing to azure. Here the colors are abruptly cut by a nervure, and one of the large cells next comes into view. This cell is wonderfully beautiful, for the colors are no longer subdued, as is the case with the upper part of the wing, but are startling from their extreme brilliancy. The circumference of the cell is emerald, inclosing three distinct centres of coloring, which seem to divide the cell into three parts. The upper division consists of a large emerald patch, changing in the centre by imperceptible gradations to golden green. Immediately below the green comes a patch of fiery ruby, edged on one side with azure and on the other with golden yellow. The third, or lowermost division, is chiefly blue, edged on one side with ruby and on the other with golden yellow. Thus we have in this one cell three centres of color, each centre being one of the three primary colors, and changing by degrees to the secondaries and tertiaries. The next cell is colored in a similar manner, except that the colors which form the centre of the divisions in the last-mentioned cell form their circumferences in this case; and the base of the wings fades off into delicate shades of pink and rays of golden yellow like the tip.

Now it must be borne in mind that the microscope has nothing to do with the production of these colors, but is limited to their exhibition. These wondrous colors already exist, although they are on so small a scale that the unassisted eye fails to separate them, and so they are blended together and appear to be colorless. I mention these apparent platitudes because, while exhibiting the microscope, I have found many persons falling into the error of supposing that the wondrous beauties which they see are due either to the excellence of the instrument or the skill of the operator.

After the Burnet Ichneumon has laid the eggs she leaves them to be hatched in the animal, which is generally, but not always, the caterpillar of the Burnet Moth (Anthrocera filipendulæ), itself a pensile insect. This is not always the case, as one of my group of Burnet Ichneumons proceeded from the body of a caterpillar belonging to the geometridæ. It was too much shriveled for identification, but it was about as large as the larva of the swallow-tailed moth.

In the body of the caterpillar they live until the larval stage is nearly completed, and then they burst on all sides through the skin of their victim, proceed to a small twig, and there weave a number of cocoons. These cocoons are about the eighth of an inch in length, cylindrical in shape, set closely side by side and fastened firmly together, so as to form a flattish mass extremely variable in shape and size, the latter depending on the number of cocoons. One of these masses now before me consists of one hundred and seventeen cocoons, and its shape is that of a segment of a circle, fixed to the twig by the flat side.

The ends of the cocoons are both closed, but when the young ichneumon is hatched it makes its exit by cutting a circular flap from one end of the cocoon, pushing the flap outward and then creeping into the air. The insects are quite indifferent as to the end of the cocoon through which they escape, and in the example before me nearly two thirds of the creatures have escaped out of one end and the remaining third out of the other.

The texture of these cocoons is very firm and stiff, and the silken material is so closely fitted together as to be completely water-proof. The microscope shows that the exterior of the cocoon is composed of white silken fibres matted tightly together, and rather rough, while the inside of the circular flap shows that the interior of each cocoon is smooth, hard, and of a pale yellow hue.

The longest and largest cells occupy the centre of the mass, while those at either end are shorter, smaller, and fewer, being about one fifth of the entire number. Knowing the customs of most hymenopterous insects, we may conclude that the females occupy the centre and the males the extremities.

THERE is a very remarkable pensile cocoon constructed by the larva of another hymenopterous insect belonging to the same family as the Burnet Ichneumon, and placed in the genus Cryptus.

The insects of this genus seem to construct a strange variety of cocoons, some being white, some yellow, and some banded and mottled with black. The most remarkable forms, however, are those in which the cocoon is attached to a thread some inches in length, the other end of which is fastened to a bough or a leaf.

Réaumur, who discovered these curious objects, found that when the cocoon was detached from the branch and laid on the table it sprang to a distance of several inches, probably because the inclosed insect was able to bend itself and then suddenly straighten the body.

Réaumur believes that the Ichneumons which make these pensile cocoons are parasitic on the processionary caterpillars, because he found them plentiful near the nests of these insects.

How the cocoons are made and suspended is quite a mystery. Mr. Westwood offers a suggestion that, before changing into the pupal state, the insect spins its thread to the required length, and, while still suspended at the end of its rope, spins the cocoon, which thus becomes fastened to the thread. In a future page will be described a cocoon woven on a similar plan, but made by the caterpillar of one of the moths.

Mr. Westwood mentions that when examining the cocoon of the Cryptus, he found that it was composed of three distinct layers, that on the exterior being composed of loose silk, which could be wound off like that of the common silkworm, but that the two interior layers were very shining, smooth, and of a gummy membranous texture, thus agreeing with the cocoons of the Burnet Ichneumon.