Fig. 31—Section of the subterranean nest of the common wasp, Vespa germanica, in position. (After Janet.) a, One of the chambers of an ant's nest, Lasius flavus, placed above the wasps' nest; b, root to which the first attachment of the nest was made; c, secondary attachments; d, the first-made attachment; e, a flint within the envelopes of the nest; f, the chief suspensory pillar of the second layer of comb; g, lateral galleries; h, one of the secondary pillars of suspension between two layers of comb; i, the layers of wasp-paper forming the envelope of nest; j, vacant space round the nest; k, flints that fell to the bottom during the work of excavation; l, numerous larvae of a fly, Pegomyia inanis (?) placed vertically in ground beneath the nest; m1 to m7, the layers of comb, in m2 the cells are indicated, in m8 (above the main figure) the arrangement of the three cells forming the commencement of the new layer of comb, m7, is shown; n, gallery of access from surface; o, burrow of a mole; p, interval of 90 mm. between top of nest and surface; q, height of the nest, 163 mm.
Although the nests of the social wasps are very elaborate constructions, yet they serve the purposes of the Insects for only a single season. This is certainly the case in our own country. Here each nest is commenced by a single female or queen; she at first performs unaided all the duties for the inauguration of the colony; she lays the foundation of the cells, deposits the eggs in them, feeds the young, and thus rears a brood of workers that at once assist her, and for the future relieve her of a considerable portion of her former occupations; the nest is by them added to and increased, till the cold weather of the autumn is at hand; at this time many males and females are produced; the cold weather either destroys the inhabitants of the nest, or reduces their vitality so that it is impossible for them to pursue successfully the avocations necessary for their subsistence, and they succumb to adversity. The young females, however, hibernate, and each one that lives through the winter is the potential founder of a new nest in the way we have already described. It might be supposed that in tropical countries where no cold season occurs the phenomena would be different, that the colonies would be permanent, and that the nests would be inhabited until they were worn out. De Saussure, however, informs us that this is not the case, but that in the tropics also the colonies die off annually. "The nests are abandoned," he says, "without it being possible to discover the reason, for apparently neither diminution of temperature nor scarcity of food cause them (the Insects) to suffer. One is tempted to suppose that the death of the Insects is the result of a physiological necessity."
Nests of Social Wasps.—In Europe wasps' nests disappear very soon after they are deserted. As it would appear from de Saussure's conclusions that in the tropics as well as in the temperate regions the rule is that the colonies endure only a portion of one year, and that a new nest is commenced by a single founder once in twelve months, it is a somewhat remarkable fact that some tropical wasp-nests are much more durable than the lives of the inhabitants require, so that solidly constructed nests are often found hanging to the trees long after they have been deserted, and are sometimes overgrown with moss. Cuming has recorded the fact that he found in South America an old wasp-nest that had been taken possession of by swallows. We do not assign, however, much importance to the views of de Saussure, because we may anticipate that enquiry will reveal much variety in the habits of tropical and sub-tropical wasps. It is known that species exist that store up honey, after the fashion of bees, and von Ihering has recently shown[[41]] that in Brazil, species of several genera form new colonies by swarming, after the manner of bees. So that it is possible that certain colonies may remain for a long period in the same nest.
Much more variety exists in wasps' nests than would be supposed probable; those formed by some of the tropical species of Vespidae are enveloped in so solid and beautifully constructed an envelope of papier-maché, that they resist with complete success the torrential rains of the tropics; while some of those found in our own country are made of extremely soft and delicate paper, which is probably chiefly glandular products. Our British Vespidae number only eight species, all belonging to the one genus Vespa, and yet they exhibit three different modes of nidification. Vespa vulgaris, V. germanica and V. rufa form subterranean nests, while V. arborea, V. sylvestris and V. norvegica suspend their habitations from the branches of trees, bushes, or strong annual plants. Vespa crabro, the hornet, usually adopts an intermediate course, forming its nest above ground, but in a spot where it is protected and concealed. The favourite habitat of this formidable Insect is the interior of an old tree, but the hornet will sometimes avail itself of the protection of a thatched roof. Both it and other arboreal species are said, however, to occasionally make subterranean nests. It is ascertained that V. austriaca, the eighth species, is an inquiline.
Fig. 32—Nest of (?) Polybia sp. The envelope partly cut open; o, entrance. (After de Saussure.)
De Saussure,[[42]] the monographer of the social wasps, classifies them according to the architecture of their nests. He establishes three groups: (1) Stelocyttares, in which the layers of comb are not connected with the envelope, but are supported by pillars made by the wasps (Fig. 31); (2) Poecilocyttares, an unsatisfactory group of which the chief characteristics appear to be that the nest is always covered by an envelope, and the comb is supported by an object such as the branch of a tree, round, or on, which the envelope is placed (Fig. 32); (3) Phragmocyttares, in which the layers of comb are supported, in part or entirely, by the envelope of the nest, communication being effected by a hole in each layer of the comb (Fig. 33). de Saussure's classification is far from satisfactory. There are many social wasps that construct nests destitute of any proper envelope; as an example of this, we may mention the species of the abundant genus Polistes; these Insects make hexagonal cells, of paper-like material, forming an irregular comb, or mass, attached to bushes by a stalk near its centre; these nests are placed so that the mouths of the open cells look downwards. The species of Ischnogaster (Fig. 34) make layers of comb, connected by a pedicel, but without any envelope; these Insects form a section of Stelocyttares called Gymnodomes.
Most of the nests of the Poecilocyttares have only a single layer of comb. The wasps of the genera Synoeca and Polybia have the habit of spreading a layer of cells on a leaf, or on the bark of a tree, and of covering this with an envelope that is pierced by a single orifice only, but that does not rest on the cells, and so allows circulation of the Insects between the cells and the envelope. This appears to be the arrangement in a nest of Synoeca cyanea preserved in the British Museum; in this construction a large layer of cells is moulded on the branch of a tree, whose contour, for a length of two or three feet, it consequently follows; while outside the mass there is placed a continuous envelope, leaving a considerable distance between it and the cells.
It would be impossible in the space at our disposal to give a satisfactory account of all the forms of wasp-nests, and we must therefore refer the student to de Saussure's work, confining ourselves to a brief notice of some specially interesting forms. The habitation of the Brazilian Polybia (Myrapetra) scutellaris is a very solid, closed structure, covered externally with rough knobs or angular projections. Although of very large size—it may be upwards of two feet in length—it is suspended from a branch, and has but one orifice; the arrangement of the combs in the interior is that of the Phragmocyttares, they being firmly attached to the outer envelope, and so placed as to form a curved surface, the convexity of which is downwards: the number of wasps in a well-developed nest of this kind must be very great. This species is said to be a honey-gathering wasp.