as long or even longer in many species of Camponotus and Prenolepis, whose sexual forms do not mate till the following spring. The longevity of the workers is certainly much greater than that of the males. Lubbock (1894, p. 12) had workers of Formica cinerea that lived nearly five years, workers of F. sanguinea that had lived at least five years, and some individuals of F. fusca and Lasius niger that attained. an age of more than six years. That the workers of the Myrmicinæ are almost or quite as long-lived may be inferred from the fact that Miss Fielde has kept those of A. fulva under observation for a period of three years. But even greater than the longevity of the worker is that of the female, as would be expected from the larger size and vigor of this caste. Janet (1904, p. 42-45) records the age of a female Lasius alienus as fully ten years, and Lubbock kept a female F. fusca alive from December, 1874, till August, 1888, “when she must have been nearly fifteen years old, and, of course, may have been more. She attained, therefore, by far the greatest age of any insect on record."

Closely related to the longevity of adult ants is the question of their resistance to adverse conditions. On this subject, which is of considerable importance in connection with the economic treatment of these insects, Miss Fielde has published (1901 to 1905) a number of interesting and painstaking observations. Although the optimum temperature for our northern ants lies between 70° and 80° F., the minimum and maximum to which they can be subjected and still survive, are very widely separated. Miss Fielde froze females, workers and a brood of Aphanogaster fulva for twenty-four hours at — 5° C. (23° F.). The insects were then gradually thawed and all survived. Even the frozen eggs, larvæ and pupæ subsequently developed in a normal manner. When the temperature was raised to 30° C. (86° F.) the ants began to show signs of discomfort, at 35° C. (96° F.) the smallest individuals swooned, and even the most vigorous ants with which she experimented succumbed after two minutes' exposure to 50° C. (122° F.). It has been known for some time that female ants can go without food for the greater part of the year while they are founding their colonies. Miss Fielde has demonstrated that large. workers can fast for almost equally long periods. She succeeded in keeping F. subsericea and Camponotus americanus workers alive without food for from 7 to 9 months. Ants are also able to endure long submergence in cool or cold water. Miss Fielde found that Lasius latipes survived 27 hours of this treatment; C. pennsylvanicus, 70 hours, and Aphænogaster fulva eight days! This explains how ants that sometimes nest in the beds of streams, like the Texan Pogonomyr

mex barbatus, can survive a flood of several days' duration. She did not test the resistance of ants to drought, but that this is considerable in many species is shown by the rich ant-fauna of many deserts like the Sahara and the deserts of the Southwestern States and northern Mexico. Miss Fielde also found that ants exhibit considerable resistance to the action of very violent poisons such as corrosive sublimate, potassium cyanide and carbolic acid. Their tenacity is best shown, however, in the number of days they are able to live after severe maiming, like decapitation. Janet (1898g, p. 130) kept a beheaded F. rufa alive for 19 days, and Miss Fielde kept a beheaded worker of C. pennsylvanicus alive for 41 days. And this ant walked about to within two days of its death! In their experiments Janet and Miss Fielde found that the males are least, the females most resistant to adverse conditions, and that the vitality of the workers varies directly as their size.

These facts, with others to be produced in the sequel, show that ants are made of remarkably tenacious protoplasm. Chained to the earth as they are, they have come to adapt themselves perfectly to its great thermal vicissitudes, its droughts and floods, and its precarious and fluctuating food-supply.

CHAPTER VI.

POLYMORPHISM.

"Ce peuple de Pygmées, de Troglodytes, est, en effet, digne de toute notre admiration. Peut-on voir une société dont les membres qui la composent aient plus d'amour public? qui soient plus désintéressés? qui aient pour la travail une ardeur plus opiniâtre et plus soutenue? Quel singulier phénomène! Je ne vois dans la très-grande majorité de ce peuple que des êtres sourds à la voix de l'amour, incapables même de se reproduire, et qui goûtent néanmoins le sentiment le plus exquis de la maternité, qui en ont toute la tendresse, qui ne pensent, n'agissent, ne vivent en un mot que pour des pupilles dont la Nature les fit tuteurs et nourriciers. Cette république n'est pas sujette à ces vicissitudes de formes, à cette mobilité dans les pouvoirs, à ces fluctuations perpétuelles qui agitent nos républiques, et font le tourment des citoyens. Depuis que la fourmi est fourmi, elle a toujours vécu de même; elle n'a eu qu'une seule volonté, qu'une seule loi, et cette volonté, cette loi ont constamment pour base l'amour de ses semblables."-Latreille, "Histoire Naturelle des Fourmis," 1802.

There is a sense in which the term polymorphism is applicable to all living organisms, since no two of these are ever exactly alike. But when employed in this sense, the term is merely a synonym of "variation," which is the more apt, since polymorphism has an essentially morphological tinge, whereas variation embraces also the psychological, physiological and ethological differences between organisms. In zoology the term polymorphism is progressively restricted, first, to cases in which individuals of the same species may be recognized as constituting two or more groups, or castes, each of which has its own definite characters or complexion. Second, the term is applied only to animals in which these intraspecific groups coëxist in space and do not arise through metamorphosis or constitute successive generations. Cases of the latter description are referred to “alternation of generations" and "seasonal polymorphism." And third, the intraspecific groups of reproductive individuals existing in all gonochoristic, or separate-sexed Metazoa are placed in the category of "sex" or "sexual dimorphism." There remain, therefore, as properly representing the phenomena of polymorphism only those animals in which characteristic intraspecific and intrasexual groups of individuals may be recognized, or, in simpler language, those species in which one. or both of the sexes appear under two or more distinct forms.

As thus restricted polymorphism is of rare occurrence in the animal kingdom and may be said to occur only in colonial or social species where its existence is commonly attributed to a physiological division

of labor. It attains its clearest expression in the social insects, in some of which, like the termites, we find both sexes equally polymorphic, while in others, like the ants, social bees, and wasps, the female alone, with rare exceptions, is differentiated into distinct castes. This restriction of polymorphism to the female in the social Hymenoptera, with which we are here especially concerned, is easily intelligible if it be traceable, as is usually supposed, to a physiological division of labor, for the colonies of ants, bees and wasps are essentially more or less permanent families of females, the male representing merely a fertilizing agency temporarily intruding itself on the activities of the community at the moment it becomes necessary to start other colonies.

FIG. 50.

We may say, therefore, that polymorphism among social Hymenoptera is a physical expression of the high degree of social plasticity and efficiency of the female sex among these insects. This is shown more specifically in two characteristics of the female, namely the extraordinary intricacy and amplitude of her instincts, which are thoroughly representative of the species, and her ability to reproduce parthenogenetically. This, of course, means a considerable degree of autonomy even in the reproductive sphere. But parthenogenesis, while undoubtedly contributing to the social efficiency of the female, must be regarded and treated as an independent phenomenon, without closer connection with polymorphism, for the ability to develop from unfertilized eggs is an ancient characteristic of the Hymenoptera and