Subfamily V. CAMPONOTINÆ Forel. Gizzard with a 4-sepaled straight, recurved or reflected calyx, which however, is always covered with circular muscles that separate it from the cavity of the crop. Pedicel 1-jointed. In the worker and female the poison gland forms a flat or oval cushion in the back of the vesicle, with a large tube but without accessory tubules for each cell. Poison vesicle large and elliptical. Sting transformed into a small vestigial apparatus which serves to support the orifice of the vesicle. All the gastric segments visible from above. Terminal segment conical, bearing at its apex the small, round, ciliated cloacal orifice. Anal glands lacking.-Pupæ usually enclosed in cocoons, but sometimes naked. The following tribes are established mainly on peculiarities in the structure of the gizzard. Tribe 1. PLAGIOLEPIDII. Cosmopolitan but mostly paleotropical. Plagiolepis (Fig. 87), Acropyga, Rhizomyrma, Acantholepis (with subgen. Acantholepis and Stigmacros), Brachymyrmex, Myrmelachista, Melophorus (with subgen. Melophorus and Lasiophanes), Notoncus, Aphomomyrmex, Rhopalo Ecophylla (Fig. 123), Gigantiops, Gesomyrmex (Fig. 100). Cosmopolitan. Tribe 5. FORMICI. Prenolepis (with the subgen. Prenolepis, Euprenolepis and Myrmecocystus (with the subgen. Myrmecocystus and Cosmopolitan. Tribe 6. CAMPONOTII. Camponotus (with the subgen. Camponotus and Colobopsis), CHAPTER IX. THE GEOGRAPHICAL DISTRIBUTION OF ANTS. "These craggy regions, these chaotic wilds, Their tribes, till we behold a spacious plain Built up of life, and food, and means of life!" Few circumscribed groups of animals have a more significant geographical distribution than the ants. As colonies they are fettered to the soil or vegetation, but their winged females, though feeble flyers, may be wafted long distances by the wind and thus overcome mountain and water barriers of considerable magnitude. In these respects ants resemble plants, which, though rooted in the ground, are able nevertheless greatly to extend the range of their species by means of windor animal-borne seeds. That ants are often carried by air currents to great distances beyond their normal range is attested by a number of facts. Annually numbers of female ants are wafted out to sea or into ⚫our great lakes to be drowned and eaten by fishes, or conveyed to desolate mountain summits where they perish in futile attempts to found colonies. Occasionally however such widely dispersed females do succeed in establishing themselves and in rearing their offspring. According to Forel (1901m) the occident ant (Pogonomyrmex occidentalis), a species peculiar to the Great Plains, has been taken in Hawaii, and King (1901a) has found in Massachusetts a single colony of Formica neoclara, an ant restricted, so far as known, to the mountain valleys of Colorado. This method of dispersal is, of course, denied to all ants like the Dorylinæ, certain Ponerinæ and Myrmicinæ, whose females are wingless, since these insects cannot cross bodies of water nor high mountain ranges. But as the Dorylinæ are migratory ants, and, as a rule, do not inhabit permanent nests, their colonies compensate, to a certain extent, for the apterous condition of their females. There is, however, a passive displacement or dissemination of whole colonies in certain species like the fire-ant (Solenopsis geminata), which often nests in low-lands subject to frequent and sudden inundations. Von Ihering (1894) has made the interesting discovery that when a nest of these ants is flooded, they agglomerate to form a ball 16-25 cm. in diameter, which encloses the brood in the center. This ball is borne along on the surface of the water while its living units keep shifting their position to avoid too prolonged immersion, till the shore or some projecting rock or tree-trunk is reached, when the colony scrambles out of the uncongenial element. I am informed by a gentleman from Louisiana that this same ant resorts to the same method of saving its colonies in the flooded bayous of the Southern States. Similar observations have been made by Savage (1847) on the African driver ants (Anomma arcens) and by Ern. André (1885) on European ants. Finally, ant colonies or fertile female ants are often transported by man from land to land as stowaways in the cargoes of ships and' railway trains. Every botanical garden annually receives several species of these insects from the tropics in the pseudobulbs of orchids, among the leaves of aroids or tillandsias, or in the soil and moss adhering to the roots of plants, and some of the smaller species thus unintentionally imported manage to establish themselves permanently in the hot-houses. Owing to these various means of dissemination, the species of ants have become more widely distributed than any other insects, with the possible exception of the Diptera. Some of our American forms, for example, Dorymyrmex pyramicus, range from Illinois to Argentina. Many species, like Eciton cæcum and Solenopsis geminata, are coëxtensive with the tropical and subtropical portions of America, and the latter also occurs in the tropics of the Old World. The former, being a Doryline ant, does not occur in the West Indies. Still other species, like Camponotus herculeanus, Formica fusca and sanguinea, extend over the whole north temperate portion of the globe, and C. maculatus is represented by subspecies or varieties on every continent and on many of the outlying islands. The distribution of ants may be studied either from a faunistic or from an ethological point of view. In faunistic studies the emphasis is placed on the areas or ranges covered by the various species, subspecies and varieties and on the bearing of such distribution on the genesis or descent of taxonomic groups as units. And since the existing fauna is unquestionably derived from previous faunas, which must have determined its character and composition, we are compelled to seek for antecedent explanatory conditions in geology and paleontology. In ethological studies, on the other hand, one turns at once to the adaptations of the living forms to their specific environment and works back from these adaptations to the geographical and geological conditions by which they are influenced. Of these two methods, which necessarily supplement each other, the latter leads to more detailed and positive results, since our knowledge of previous faunas is in all cases. more or less vague and problematic. A résumé of what has been ascertained concerning fossil ants will be given in the next chapter, but owing to its fragmentary character, will be used rather as a confirmation than as a foundation for inferences drawn from our existing fauna. Emery (1893-'94) and von Ihering (1894) have shown that there is a very significant parallelism between the distribution of mammals and that of ants. Both groups appear to have arisen simultaneously during the Triassic or possibly during some previous period, and to have spread over the earth's surface in much the same manner, although, if we except the bats, few mammals have possessed such power of dispersal as the ants. A study of the mammals indicates that during the Mesozoic era there were extensive land connections between the present continents of Eurasia, Africa, America and Australia, and that these various regions were inhabited by a primitive, widely-distributed but now extinct fauna. During this era New Zealand was cut off from Australia and during the following Eocene epoch Africa, South America and Australia were in turn separated from the great continental mass. During the Oligocene a boreal and Indian fauna became differentiated in Eurasia and their separation was emphasized during the Miocene and Pliocene periods by an upheaval of the boundaries between the respective regions. During the early Tertiary, also, the connection between North and South America was severed and was not restored, according to some paleontologists, till the Pliocene epoch. It is highly probable, as Emery has suggested, that the Ponerinæ correspond to the primitive, widely-distributed mammals of the Mesozoic era, and together with certain Myrmicinæ, like Solenopsis, Pheidole, etc., represent an ancient cosmopolitan ant-fauna. Ponerinæ occur even in New Zealand, which appears to have been isolated ever since the Jurassic. Since the Doryline are well developed in the tropics of both hemispheres, these ants must have arisen before Africa was separated from South America, probably from some primitive and widespread Ponerine forms like the Cerapachysii. The almost complete absence of Dolichoderinæ in Africa shows that this subfamily must have made its appearance after Africa had been separated from Eurasia. That the Dolichoderinæ came from Ponerine ancestors is indicated |