Old World tropics. Gesomyrmer was supposed to be an extinct genus till Ern. André (1892c) described a species (G. chaperi) from Borneo. In the same paper and from the same locality he described the type of another interesting Camponotine genus, Dimorphomyrmex janeti. This has polymorphic workers with large reniform eyes and 8-jointed antennæ. Some years later (1905e) Emery found a species (D. theryi, Fig. 98) of this same genus in the Baltic amber. Rhopalomyrmex (Fig. 91) resembles the neotropical Myrmelachista. It has 10-jointed antennæ, with 4-jointed clubs. Only a few species of the recent genera Lasius, Formica and Camponotus have been described from the Baltic amber. The workers of one' of the Camponoti, C. constrictus (Fig. 102), are peculiar in possessing ocelli and in having a thorax like Formica. Of this latter genus Mayr described only a single species, F. flori, which is very closely related to the existing F. fusca.

FIG. 102.

L

Worker of Camponotus constrictus, with ocelli and sellate thorax, from the Baltic Amber. (Mayr.)

Our knowledge of the fossil ants of North America is insignificant. Scudder (1890) described Lasius terreus and a Myrmica sp. from the Green River Oligocene, Camponotus vetus and Liometopum pingue from the White River Oligocene and Formica arcana, Dolichoderus obliteratus and Aphanogaster longava from the Quesnel formation, but neither the descriptions nor the figures make it at all certain that these ants are assigned to their proper genera. He also described and figured (p. 606, pl. III, fig. 32) the wing of an ant as that of a Braconid, Calyptites antediluvianus. Cockerell (1906) has described a Ponera hendersoni from the Florissant shales but the size of the specimen shows that it cannot be a true Ponera. My own studies on the Florissant ants are not yet completed.

Very few ants are known from the Quaternary, or Pleistocene. Some Camponotinæ and Dolichoderinæ are recorded by Handlirsch as having been found in the interglacial deposits of Re, Italy by Benassi (1896) and a number of unidentified species are enumerated from the copal, an amber-like fossil resin found in several tropical countries (Africa, Brazil, New Zealand, etc.). One of the earliest accounts of copal ants is that of Blochs (1776) who describes and figures specimens of what he calls Formica saccharivora, salomonis, nigra and Formica sp. In a fine series of copal specimens from Zanzibar in the American Museum of Natural History, I find well-preserved specimens belong

ing to the following genera: Camponotus, Polyrhachis, Myrmicaria, Cremastogaster, Pheidole, Cataulacus, Atopomyrmex, Ponera and Anomma, and to species very closely related to living forms of the same territory if not identical with them. In a specimen of copal

from Demerara in the same collection there is a worker Azteca.

In reviewing the Tertiary and Quaternary ants one is impressed with two facts that have not been emphasized in the preceding pages. One of these is the close similarity of some of the ants of the Baltic amber to species now living in the same region. So intimate is this similarity that it may, in a few cases at least amount to identity, e. g., in Ponera atavia, Lasius schiefferdeckeri and Formica flori which neither Mayr nor myself have been able to distinguish by any satisfactory characters from the living Ponera coarctata, Lasius niger and Formica fusca! Such cases bring home to us very forcibly the enormous age and stability of species which the student, dealing exclusively with living forms, would be inclined to regard as of very recent origin.

The second fact is one to which attention seems not to have been called by previous authors, namely, the absence of polymorphism in the workers of the Tertiary ants. There are, indeed, differences in stature between workers of the same species, but I have seen no specimens with sufficient differences in the size and shape of the head to indicate the existence of soldiers and workers proper. This is the more noticeable, because there are recorded from the amber several genera whose living species have polymorphic workers, such as Anomma, Aëromyrma, Oligomyrmex, Camponotus and Dimorphomyrmex. The known specimens of Aëromyrma and Oligomyrmex are all males and females, so that nothing is known concerning the workers, which may have been monomorphic. To the former genus belongs also, according to Emery, the Pheidologeton antiquus described by Mayr from a female specimen. The occurrence of Anomma in the amber is very doubtful. There remain then only the genera Camponotus and Dimorphomyrmex in which we might expect to find polymorphic workers. I have examined a number of specimens of the three species of Camponotus (mengei, igneus and constrictus) described by Mayr, but all of them have the form of the minor workers of our existing Camponoti. Dimorphomyrmex theryi was based on a single specimen, but several others which I have seen are monomorphic and in this respect unlike the living type of the genus from Borneo. It may be objected, of course, that no conclusions as to the presence or absence of polymorphism in the workers can be drawn from the amber material, both because it is too meager and because the soldiers do not forage like the workers and would not therefore be caught in the liquid resin.

This is certainly true of some genera, but not of Camponotus, to judge from our modern species. The fact remains that no polymorphic workers have been seen in the amber, that the great majority of the species certainly had only monomorphic workers, and that genera like Pheidole and Pheidologeton, so prominent in the Old World tropics to-day, are conspicuous by their absence. In the Pleistocene, however, genera like Pheidole and Anomma have their worker polymorphism fully developed, as I have observed in the Zanzibar copal, so that this condition must have made its appearance during the late, if I am right in concluding that it was absent during the early Tertiary.

CHAPTER XI.

THE HABITS OF ANTS IN GENERAL.

"La fourmi, qui n'est point dédaigneuse et accepte toute nourriture, est, pour cela même, moins inquiète et moins égoiste. C'est bien a tort qu'on l'appelait avare. Loin de la, elle ne semble occupée qu'a multiplier dans sa ville la nombre des copartageants. Dans sa maternité généreuse pour ceux qu'elle n'a pas enfantés, dans sa sollicitude pour ces petits d'hiers qui deviennent aujourd'hui de jeunes citoyens, nait un sens tout nouveau fort rare chez les insectes, celui de la fraternité."-Michelet, "L'Insecte," 1857.

Before proceeding to a more detailed account of the extraordinary habits and instincts exhibited by certain groups of ants, it will be advisable to say something about the activities that are more generally manifested by these insects as a group. And as the ants, like all other living organisms, pursue the three-fold aim of securing food, perpetuating their species, and shielding themselves and their offspring from enemies and the inclemencies of a changing physical environment, I may properly include my remarks under the general heads of nutrition, protection and reproduction. The activities implied by these terms, which must, of course, be taken in an elastic sense, necessarily complicate and supplement one another in the most manifold and intimate manner.

Permanent social life is, generally speaking, possible only for animals that have access to an abundant food supply. Species that have great difficulty in securing food or succeed in finding only a scanty and precarious amount, are compelled to lead solitary lives, or at any rate, can never form populous communities of long standing. It is evident, moreover, that only vegetable food is ever really abundant and that animal food is in the majority of cases limited in amount, difficult to obtain, or abundant only during certain seasons or in circumscribed localities. Predatory animals like the mammals, birds and insects of prey, are, therefore, solitary in their habits, whereas vegetarians, like the rodents, ruminants and many plant-eating insects, are prone to be more or less social. Ants, at first sight, would seem to be an exception to this rule, but this is only conditionally true. Although primitively carnivorous, these insects are unreservedly such only in the lower subfamilies like the Ponerinæ and Doryline. The colonies of the former are usually rare, like those of the social wasps, and of small size, and the colonies of the Doryline, though often very

populous, lead a nomadic existence, since they must continually seek fresh hunting grounds in order to obtain the requisite amount of food. The ants of the three remaining subfamilies, though often predatory, have adapted themselves to a more varied diet and many of them have come to rely almost exclusively on vegetable food. The following are the sources from which these insects as a family derive their nourishment:

1. The original food of ants consists of other insects, especially helpless larvæ and other terrestrial arthropods such as spiders, myriopods and isopods, the dying imagines of the countless insects which fall to the earth when their life-work is completed, those which are just leaving their pupa-cases, and the fragments rejected by insectivorous birds and mammals.

2. The larvæ and pupæ of ants are a favorite food of certain species of Eciton and Formica, which are sufficiently intrepid to pillage the nests of other species. And, in fact, in times of need many species will eat their own offspring, which may, therefore, be considered as an ever-present and available food-supply stored up against periods of famine.

3. The excretions of plants, such as the sweet liquids exuding from the leaves and especially from the floral and extrafloral nectaries, the sap escaping from wounded stems, etc.

4. The honey-dew excreted by plant-lice (aphids), mealy bugs (coccids) and leaf-hoppers (membracids), and the secretions of the caterpillars of the butterfly family Lycænidæ. These liquids are, of course, plant juices that have undergone certain changes in the alimentary tract or glands of the insects.

5. The seeds of plants, especially of grasses and berries, drupes and fruits of all kinds, that have been injured by birds or other insects or by falling to the ground, for the ants are unable to gnaw through the tense skins or rinds of fruits. Some hypogæic species also feed. on bulbs or tubers, the tender bark of roots, or the cotyledons of germinating seeds.

6. One tribe of ants, the Attii of tropical America, lives exclusively on fungus hyphæ, which they cultivate on vegetable substances carried into the nests.

Probably no single species of ant is able to draw on all of these sources of nutrition, but many species are sufficiently adaptable to utilize several of them. The fungus-growing ants are the most highly specialized in their diet and next to these some of the seed-storing. or harvesting species. Many ants, however, are more or less omnivorous, and many find it an easy matter to pass from one kind of food