save labor. The irregular form of the cavities is a characteristic so universal in ant-nests that it would seem to be preferred to a monotonous regularity. It may be important, in fact, in enabling the ants to orient themselves readily. The nest entrance is sometimes peculiarly modified to suit the needs of the various species. It may be left permanently

open and guarded by workers or soldiers, or it may be closed at night; it may be enlarged or constricted for the purpose of regulating the ventilation of the cavities and preventing the inroads of enemies, it may be adroitly concealed or exposed to view and surrounded by conspicuous earth-works.

Even in this prevailing and opportunistic irregularity, however, there are singular differences of degree. The more primitive ants, like the Ponerinæ, build with a certain irregularity devoid of character. The Doryline may hardly be said to build nests at all, but merely to bivouac in some convenient cavity under a stone or log, or they may temporarily occupy the nests of other ants or dig irregular runways beneath the surface of the soil. The higher ants, however, which form

stationary and populous formicaries, devote a great deal of attention to architecture and work according to a more or less definite plan, which they skilfully modify to suit the conditions of a specific environ

ment.

The nests of nearly all ants are the result of two different activities, excavation and construction. Both of these may be simultaneously pursued by the workers, or either may predominate to the complete exclusion of the other, so that some nests are entirely excavated in soil or wood, whereas others are entirely constructed of soil, paper or silk. As the nests of the latter type resemble those of the social wasps, one might be led to suppose that they represent the original ancestral form and that the excavated are degenerate types, but the prevalence of earthen nests among ants of the most diverse genera in all parts of the world, as well as the occurrence of similar nests among the solitary bees, wasps and Mutillids, would seem to indicate that even the most

FIG. 107.

ancient ants practiced both methods of nesting. In other words, the variable architecture of ants may be an inheritance from presocial ancestors and may have been well-established before these insects came to live in communities.

The methods employed by worker ants in making their nests are

easily observed, and have been described in detail by Huber (1810) and Forel (1874). According to Forel, "They use their mandibles in two ways. When closed these organs form a kind of trowel, convex in front and above, concave beneath and behind, and pointed at the tip. This trowel is used for raking up the soft earth and also for moulding and compressing their constructions and thus rendering them. more solid and continuous. This is accomplished by pushing the anterior portion of the closed mandibles forward or upward. In the second place, the mandibles, when open, constitute a veritable pair of tongues with toothed edges, at least in all of the workers of our native ants that do any excavating. They thus serve not only for transporting but also for moulding or comminuting the earth." The forelegs are used for scratching up the soil, in moulding pellets and patting them down after they have been placed in position by the mandibles, and are of so much assistance in this work that when they are cut off the insects are unable to excavate or build without great difficulty and soon abandon their work altogether.

Ants dislike to excavate in soil that is too dry and friable. When compelled to do this in artificial nests they will sometimes moisten it with water brought from a distance, as Miss Fielde (1901) has observed. She says that the workers of Aphanogaster picea, "like the Termites, are able to carry water for domestic uses. They probably lap the water into the pouch above the lower lip [the hypopharyngeal pocket] and eject it at its destination. A hundred or two of ants that I brought in and left in a heap of dry earth upon a Lubbock nest, during the ensuing night took water from the surrounding moat, moistened a full pint of earth, built therein a proper nest, and were busy depositing their larvæ in its recesses when I saw them on the following morning."

As even the most extensively excavated nests represent little labor compared with the nests of social wasps and bees, ants are able to leave their homes and make new ones without serious inconvenience. Such changes are often necessitated by the habit of nesting in situations exposed to great and sudden changes in temperature and moisture or to the inroads of more aggressive ants and larger terrestrial animals. Barring the intervention of such unusual conditions, however, most ants cling to their nests tenaciously and with every evidence of a keen sense of proprietorship, although there are a few species, besides the nomadic Dorylinæ, that seem to delight in an occasional change of residence. Wasmann has shown that Formica sanguinea often has summer and winter residences analogous to the city and country homes of wealthy people. The ants migrate from one to the

other during March and April and again during late summer or early autumn (September). The summer nests are built in open, sunny places where food is abundant and the conditions most favorable to rearing the brood, whereas the winter nests are built under stumps and

rocks usually in protected spots in the woods, and are used as hibernacula, or, very rarely, for protection from excessive heat during the

summer.

The migration of ants from one nest to another is determined upon and initiated by a few workers which are either more sensitive to adverse conditions or of a more alert and venturesome disposition than the majority of their fellows. These workers, after selecting a site, begin to deport their brood, queens, males, fellow workers and even their myrmecophiles. The deported workers are at first too strongly attached to their old quarters to remain in the new ones and therefore keep returning and carrying back the brood. The enterprising workers, however, obstinately persist in their endeavors to move the colony till their intentions are grasped and become contagious. The indecision or indifference of many of the workers may last for days or even for weeks, during all which time files of ants move back and forth between the two nests carrying their larvæ and pupæ in both directions. But

more and more workers keep joining the ranks of the radicals till the conservative individuals constitute such a helpless minority that they

FIG. 109.

Large nest of Formica exsectoides, at Scotch Plains, N. J. Height 1 meter, basal diameter 3.25 m.,

circumference 10.21 m.

(Original.)

are compelled to abandon the old nest and join the majority. I once observed a colony of agricultural ants (Pononomyrmex molefaciens)