Popular Science Monthly/Volume 45/July 1894/Homes of Social Insects
|HOMES OF SOCIAL INSECTS.|
IN no branch of insect work are more admirable means employed to bring about the desired ends, or is greater diversity of method found, than in that of insect architecture. The beauty of the buildings in many cases is incomparable, and generally speaking the abodes attain a magnitude colossal as compared with that of their creators. It may be exception will be taken to the use of the word architecture to designate this portion of the insect economy, and perhaps the term can hardly be applied in fairness to homes which are mere tunnels and galleries bored in the earth or in wood. But who would deny it to the exquisite pensile nests of the English wasps, or those of many a foreign relative, to the geometric precision exhibited within the hive of the honey bee, or to the edifices of some ants, as will be presently discovered?
Among the communities which combine their operations, there are those of which the object is simply the protection of the individuals composing them. To these societies belong the caterpillars of certain species of moths. The homes formed by these larvae, though they are not elaborate, are interesting in several minute circumstances. But they fall short in every respect of the attractive nests fabricated by companies of insects in their perfect state, in view not only of self-preservation, but of the nurture and education of their young as well.
The nests of an extraordinary tree ant, Œcophylla smaragdina, are cunningly wrought with leaves, united together with web (see Fig. 1). One was observed in New South Wales in the expedition under Captain Cook. The leaves utilized were as broad as one's hand, and were bent and glued to each other at their tips. How the insects manage to bring the leaves into the required position was never ascertained, but thousands were seen uniting their strength to hold them down, while other busy multitudes were employed within in applying the gluten that was to prevent them returning back. The observers, to satisfy themselves that the foliage was indeed incurvated and held in this form by the efforts of the ants, disturbed the builders at their work, and as soon as they were driven away the leaves sprang up, with a force much greater than it would have been deemed possible for such laborers to overcome by any combination of strength. The more compact and elegant dwelling of Œ. virecens is made of leaves, cut and masticated until they become a coarse pulp. Its diameter is about six inches; it is suspended among thickest foliage, and sustained not only by the branches on which it hangs, but by the leaves, which are worked into the composition, and in many parts project from its outer wall. It may be at once distinguished from the nest of Crematogaster by its smoothness and regularity of surface. A species of this genus was discovered in Africa by Foxcroft, who observed that whenever the ants were molested, they rushed out of their house
in such numbers that their pattering upon the papery covering deluded him into thinking rain was falling on the leaves above.
In the forests of Cayenne, the nests of Formica bispinosa are remarkably like a sponge or an overgrown fungus. The down or cottony matter enveloping the seeds in the pods of the Bombax ceiba is used for their construction, vegetable fibers that are too short to convert into fabrics, but which the ants contrive to felt and weave into a compact and uniform mass, so dexterously that all trace of the individuality of the threads is lost. The material much resembles amadou, and like that substance is valuable for stopping violent discharges of blood. In size the nests generally have a diameter of eight or nine inches. The ant itself is little and dark, and noted for two long, sharp spines on its thorax, one on either side; hence its scientific name of bispinosa, from the Latin, meaning two-spined. Popularly it has been called the fungus ant.
The true social wasps, which are arranged in one large family, the Vespidæ, form communities whose architectural labors will not suffer on comparison even with those of the inhabitants of the beehive. In fact, for daintiness and delicacy the nests of many of the Vespidæ constitute the most beautiful examples of insect architecture.
Not the least extraordinary of the wasps are the Icarias, a genus that extends through most of the warmer regions of the Fig. 2.—Nest of Icaria variegata. world, specimens having been taken in Africa, India, China, and Australia, and in many parts of the Asiatic Archipelago. Like the Polistes, their nests are attached to leaves, stalks, or branches by a single foot-stalk, composed of the same papery material as the cells. Though slender, it is hard, tough, and solid, and the strength with which it is fastened to the tree or plant is surprising, enabling it to uphold considerable weight. At the end of the petiole usually a single cell, its mouth directed downward, is fixed; the rest of the nest consists of a double series of lateral cells until the group is complete. Those nearest to the footstalk are the largest and most perfect, since they are finished first; toward the other extremity the cells gradually diminish in size, and at that point they are only just begun. As a whole they are well-defined hexagons; their color is often a rather dark yellowish brown, preventing them from being conspicuous in spite of their curious projection. The cell masses are small, so that the societies must be restricted. Possibly each group is the work of a single female, who confines herself to raising her own progeny which escape as soon as they are hatched. The nests are frequently numerous in the same spot, and each society may set up a number of separate homes in the vicinity of one another. Perhaps in this genus, as among the Polistes, workers are wanting (see Fig. 2).
The wasps hitherto considered are distinguished as manufacturers of paper, in general fine and thin and more or less brittle, the weakness of which they overcome by the superposition of a great number of leaves. There is a large class who, while they make many kinds of papyraceous tissues, are noted for a feature in common—the fabrication of a solid and tough paper, a veritable cardboard, composed of only one layer of material, at times very thick and resisting, at others slight and supple. Of this substance, after the manner of Vespa, the wasps usually build a papyraceous envelope or sac for the inclosure of their combs, and as in that genus, the covering follows closely the direction of the plan of the cells.
The genus Chartergus, one of the important groups of the cardboard makers, includes insects apparently similar which practice two strangely different forms of nidification. The nests of C. chartarius, the most common in collections, are of frequent occurrence in tropical America. Their cardboard is white, gray, or of a buff color, tending to yellow, very fine and of polished smoothness; at the same time it is strong and so solid as to be impervious to the weather. It can not be urged sufficiently, says Réaumur, that this kind of envelope is indeed of a veritable cardboard, as beautiful as any we know how to make. Réaumur once showed a piece to a cardboard manufacturer, and not the slightest suspicion of its real nature was suggested to his mind. He turned it over and over, he examined it thoroughly by the touch, he tore it, and after all declared it to be made by one of his own profession, mentioning manufacturers at Orleans as the probable producers. The nests may be conical or cylindrical, they may be straight, but more often are somewhat curved; some are almost globe-shaped, but these varieties are of little importance. The length of a well-sized nest is about a foot; the largest yet discovered was in Ceylon, and measured the astonishing size of six feet. The edifice is pendulous on trees and attached, as it were, to a suspensory ring, which embraces the branch and is tightly impasted round it, or, according to Westwood, may be large compared with the latter's circumference; but it is probably a mistake to say that the nest ever swings freely as on a pivot. The interior consists of circular concave horizontal platforms of cells, their mouths turned downward, each tier stretching right across like so many floors, and fastened along its entire edge to the walls. Communication is effected by a central opening through the bottom and through every tier. When the number of inhabitants becomes very great and a fresh series of cells is added, unlike the British wasps who add to their abodes by a preliminary increase of the envelope to admit of extension, of the tiers, the Chartergi go to work on precisely the opposite plan, first forming new cells and covering them afterward. Taking the bottom of the nest as a starting point, they set cells over its exterior surface, being careful to extend the circumference by a row or two to
augment the diameter in proportion to the length, so that the symmetry of the building may not be lost. The walls are then lengthened to include the fresh stage, and the end is closed with a new floor, in its turn to become the ceiling of the next tier of cells when further enlargement is desired. No trace of the addition is visible on the outside of the envelope, which would seem constructed at one stroke.
The other kind of nest of Chartergus is constructed on a straight and upright branch, having no lateral twigs. Its elegance can not be sufficiently admired. Composed of a few cells only, the combs are attached to the branch by means of petioles, or solid masses of wax, keeping the groups in a horizontal and parallel position. They stand one over the other, sometimes to the number of ten, separated by considerable intervals, and so admirably upheld by the petioles that the aid of all pillars or columns is dispensed with. The envelope is a spindle of a single leaf of ligneous paper, most artistic in appearance, being marked with transverse parallel tubings and goffered. The fibers of the tissue are arranged with surprising regularity; all the zones are united with consummate art, and meet in a long and plainly shown line; the paper may be also variegated with longitudinal bands of different colors. The vase is firmly fixed to its axis at points slightly above and below the uppermost and lowermost combs; at no part is it in continuity with the combs; there is plenty of space between the two fabrics for the wasps to pass up and down within their home with ease. Taking advantage of the wholly lateral position of the combs with respect to the axis, the wasps render their building less fragile than it would otherwise have been by placing the branch to one side of the spindle, and it saves time and trouble, without materially impairing the support, to leave the wood exposed at the posterior surface of the papery mass. The opening is small and situated at the lower end (see Fig. 3).
Very extraordinary are some of the nests in the collection of the British Museum—the works of Myrapetra scutellaris (see
Fig. 4), a mere fanciful title. These huge erections are from Central America, and the native authorities say of one that it is not composed of wood fibers, but of the dung of the capincha, one of the aquatic cavies of the region. One's attention is instantly attracted to the fairly conical knobs or tubercles with which the surface is thickly beset, of various size, and most pointed where they are least exposed. Their disposition is in horizontal zones, seeming to correspond more or less with the comb tiers. While at the top of the nest they are comparatively few, gradually the numbers increase toward the lower end, and on the bottom they are so numerous that one's finger can scarcely be laid between them. Like the envelope, they are made up of several papery layers so closely blended as to be hardly distinguishable, forming a substance astonishingly thick, hard, and firm, in color of a dull dark brown, and of very coarse texture. Of what use they are it is difficult to decide; they may be simply
freaks of Nature. Although their tips are not acute, they may defend the abode against the attacks of tigers, jaguars, kuguars, and other mammalia partial to honey and the grubs of the hive. The nest always hangs low, seldom more than three or four feet from the ground, and protection would appear much needed. It seems hardly possible to deny that they are for the double purpose of concealing and of sheltering the entrances, which are invisible when the nest is looked at from above. Examination reveals them beneath a row of the projections, which overhang them and keep off the rains like the eaves of a house; the passages are also intricately twisted, so as to prevent the ingress of moths or other enemies of any size. It is strange that the interior surface of the nest is provided with tubercles, a circumstance that must put the insects to the trouble of gnawing them away each time they add a stage. Probably the same material is again employed in establishing fresh cells and in building the new platform.
A longitudinal section shows the peculiar disposition of the combs. Just as in the spherical nests of Polybia, the highest ones are perfect or almost perfect spheres; but this method of construction is soon found to be too laborious. A nearly globular mass of the brown paper-like substance exists at the top the nucleus, so to speak. The first combs closely surround this, so that they form the best parts of hollow spheres; then come great arcs of circles, followed in regular order by other tiers, their rotundity becoming gradually reduced until the curve of the lower ones is extremely shallow, exactly like the tiers of Tatua, except that they exhibit a trifling convexity on their lower surfaces. They are carried to the common wall and thereto affixed, small spaces being left open here and there between their edges and the envelope. The solid wall at the top is of great thickness (see Fig. 5).
In the nest in the British Museum already described, a quantity of brownish-red honey was found in the upper combs, but hard and dry. Even so long ago as the beginning of the century, Azara, a Spanish officer, who was sent out by his Government to Paraguay to make certain investigations in that country, mentions that a South American wasp which he calls chiguana has the strange habit of hoarding honey. The chiguana of Azara, it would seem, is identical with Polybia scutellaris. At the time of publication Azara's statement was not believed, so opposed was the habit that he claimed for this insect to the known actions of wasps. He and his men ate from the chiguana's store, and it proved deleterious. St. Hilaire, a subsequent traveler, speaks of two South American honey wasps. The honey of one was white and innocuous, that of the other was reddish brown and poisonous. The good honey was in an oval, light-colored nest of thin, papery material, totally different from the paper of Myrapetra, and was observed by Hilaire on a small bush near Uruguay, at a distance of only about a foot from the ground. This wasp has been described as lecheguana. Probably under the term lecheguana, or chiguana, as Azara has it, the inhabitants of America confound many wasps of similar kinds, and it is rather a generic title for all honey wasps than for one species in particular.
- Reprinted, with the kind permission of Messrs. Macmillan & Co., from the author's popular work, Romance of the Insect World.