B

paratively small number of species, they have considerable economic importance. The following have been brought to our shores and have succeeded in gaining a foothold, especially in dwellings where they do

FIG. 86. The Argentine ant (Iridomyrmex humilis). (Courtesy of Mr. W. Newell, drawing by Miss Charlotte M. King.) A, Worker; A', head; A", petiole of same in profile; B, deälated female; B', head; B", petiole of same; C, male; C', head; C", petiole.

not come into competition with our native species: Monomorium pharaonis, salomonis, destructor and floricola, Solenopsis rufa, Pheidoie megacephala and flavens, Tetramorium cespitum, guineense and

simillimum, Prenolepis fulva and longicornis, Plagiolepis longipes, Tapinoma melanocephalum and Iridomyrmex humilis. All of these, with the exception of the pavement ant (T. cespitum), are of tropica! origin, and nearly all of them have come from the Old World. T. cespitum of Europe is now common about New York, Washington and Philadelphia, but it is so sporadic that we must conclude either that it is of comparatively recent importation, or is prevented from spreading by competition with our native ants.1

All of the other species cited above require considerable warmth and even Monomorium pharaonis, the tiny yellow house-ant, which is often a pest in ships or in the dwellings of sea-port towns, does not nest out of doors except in southern latitudes. Some of our tropical ants (Neoponera villosa, Camponotus floridanus and Pheidole flavens) manage to live for considerable periods of time in our northern hothouses. At least one species from the American tropics (Iridomyrmex humilis (Fig. 86)) has acquired a much wider range, having recently made its appearance in New Orleans. In this locality, where its habits. have been carefully studied by Titus (1905) and Newell (1908a), it has become a serious pest and is driving out the native ants. That it is spreading rapidly over the warmer portions of the globe is shown by the fact that I have recently received specimens from various localities in California and from Cape Colony. It has also become a pest in Portugal (Martins, 1907), and, according to Stoll (1898) has been imported into Madeira where it has supplanted another previously introduced species, Pheidole megacephala, which was the house-ant of the island in the days of Heer (1852).

Some idea of the abundance of this ant in the middle of the last century may be gained from the following extract from Heer's work: 'It occurs throughout the southern portion of the island of Madeira up to an elevation of 1,000 feet in prodigious numbers, especially in hot, sunny places, where it is to be found under eight out of every ten stones that may be overturned. In the city of Funchal there is probably not a single house that is not infested with millions of these insects. They climb to the top stories, issue in swarms from the cracks in walls and floors and keep crossing the rooms in regular files in all directions. They creep up the legs of tables, along their edges and into cupboards, chests, etc." This ant is very common in the Bermudas and West Indies and will probably be found in Florida. There can be little doubt that wherever it gains a foothold in tropical or

1According to Marlatt (1898) this species has long been a resident of the Eastern States. He believes that it may be the species referred to by Kalm as occurring in the houses of Philadelphia as early as 1748.

subtropical countries it is able to propagate very rapidly and to exterminate the indigenous ant-fauna. This seems to be the case in Bermuda, and I have recently seen a good illustration of its habits in the Virgin Islands. During March, 1906, I devoted ten days to a careful study of the ant-fauna of the little island of Culebra, off the eastern coast of Porto Rico, without seeing a single specimen of Ph. megacephala. This island is, however, completely overrun with a dark variety of the vicious fire-ant (Solenopsis geminata). One day, on visiting the island of Culebrita, which is separated by a shallow channel hardly a mile in width from the eastern coast of Culebra, I was astonished to find it completely overrun with Ph. megacephala. This ant was nesting under every stone and log, from the shifting sand of the seabeach to the walls of the light-house on the highest point of the island. The most careful search failed to reveal the presence of any other species, though the flora and physical conditions are the same as those of Culebra. It is highly probable that Ph. megacephala, perhaps accidentally introduced from St. Thomas, a few miles to the east, had exterminated all the other ants which must previously have inhabited Culebrita. The absence of megacephala on Culebra is perhaps to be explained by the presence of the equally prolific and pugnacious fire-ant.

The recent displacement of Ph. megacephala in Madeira and of our native ants in Louisiana by Iridomyrmex humilis is analogous to the

L

FIG. 87. Worker of Plagiolepis longipes, now spread over the tropics of both hemispheres. (Bingham.)

well-known displacement in Europe and America of the black houserat (Mus rattus) by the brown species (M. decumanus). In a similar manner, according to Stoll, another ant, Plagiolepis longipes (Fig. 87), introduced into the island Reunion from its original home in Cochin China, has driven out some of the primitive autochthonous species. We may also look forward to the appearance of this same ant within the warmer portions of the United States, since it has already been recorded. by Pergande (1894) from Todos Santos, in Lower California.

Still another ant that has acquired a footing in tropical Florida, and probably also in other localities in the Gulf States, is Prenolepis longicornis. It has long been a common species in the green-houses of temperate Europe and America. In some of these, as in the Jardin des Plantes in Paris, it has been a permanent resident for more than forty years. In the city of New York it may sometimes be found even on the top floors of the great apartment buildings. Wasmann (1905g) and Assmuth (1907) give good reasons for believing that the original home of this ant is India, and that it has been carried to all parts of the tropics in ships. They show that it has been accompanied in its wanderings by two myrmecophiles, a Lathridiid beetle (Coluocera maderæ) and a small cricket (Myrmecophila acervorum var. flavocincta). The peregrinations of Tapinoma melanocephalum, which also occurs in northern dwellings and green-houses, are similar to those of P. longicornis.

The foregoing sketch of the distribution of North American ants shows that our fauna is very rich in comparison with that of Europe. Nevertheless it must be admitted that we have few distinctive typesapparently only the specialized parasitic genera Epœcus, Symmyrmica, Sympheidole and Epipheidole, the subgenera Dichothorax and Acanthomyops and the ancient relicit Proceratium. Kobelt has been led by his studies on the distribution of other animals to the conclusion that our existing North American fauna, like that of other countries, "apart from the introduced and feral domestic animals and the English sparrow-has shown no evidence of enrichment since the diluvial period. The present is a depauperate diluvial fauna. America, too, proves that we are not living in an incipient, but in a declining geological epoch, not at the beginning of a youthful, creative Quaternary, but at the close of the Tertiary period, whose generative power has been extinguished." This statement may not be strictly true of dominant insect groups like the Formicidæ. Not only is it probable that our fauna is being slowly but continually enriched by accessions from the tropics, but a comparison of the list of North American ants at the end of this volume. with the lists of European species compiled by Mayr, Forel, Emery, Ern. André and others, shows that the related and identical species of both continents have a greater number of subspecies and varieties in North America. This would seem to force us to the conclusion that many of our ants are actually in a mutational or premutational phase.

Turning from this more general, faunistic account to the ethological distribution of ants, we observe considerable differences in the frequency with which the colonies occur within the range of each species. When we thus concentrate our attention on a single form,

we find that the colonies are not uniformly distributed over their whole range, but only in particular stations, or habitats, showing that these insects, like plants and many other animals, depend very intimately for their welfare on precise physical and organic environments, such as the nature of the soil and vegetation, the amount of moisture and the exposure to sunlight. Colonies that happen to be established in unfavorable localities take on a more or less depauperate appearance. This is indicated by their scarcity and the small size of the colonies and individuals, and is particularly noticeable at the very limits or just beyond the limits of the normal range of a particular form. I find that according to the station inhabited by the various species, subspecies and varieties, at least in North America, we may distinguish the following ethological groups, or associations:

1. The woodland, or silvicolous association, comprising the species that inhabit our moist, shady northern and eastern forests. With the extinction or drainage of these forests or the removal of the undergrowth, this characteristic, and in many respects, very primitive fauna rapidly disappears.

2. The glade, or nemoricolous association, comprising the ants that prefer open, sunny woods, clearings or the borders of woods. A portion of this fauna maintains itself even in the gardens and parks of our cities.

3. The field, or cespiticolous association, comprising the ants that prefer to nest in grassy pastures and lawns, in situations exposed to the full warmth and light of the sun.

4. The meadow, or pratincolous association, comprising the ants which inhabit low, grassy meadows or bogs.

5. The heath, or ericeticolous association, comprising the ants that inhabit rather poor, sandy or gravelly soil exposed to the sun and covered with a sparse growth of weeds or grasses.

6. The sand, or arenicolous association, comprising the ants that prefer to nest in pure sand.

7. The desert, or deserticolous association, comprising the ants that inhabit the dry, open deserts and plains.

A few of our species, like Lasius americanus and Formica subsericea, are so adaptable that they occur more or less abundantly in all or nearly all of the above stations. Owing to intergradation of these stations in some places, there is, of course, a corresponding mingling of forms. Thus certain species, like Monomorium minimum, seem to belong indifferently either to the heath or sand fauna. In the deserts of the Southwestern States these two faunas may either mingle or be sharply separated from each other. In the Northeastern and