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Ant nests in plants

Ant nests in plantsAnts, through their very intimate relations to plants, have come to take every advantage of cavities found in these organisms. This is especially true in the tropics where the prevailing temperature is so high that no special contrivances for incubating the brood are needed, and where sufficient moisture and protection are abundantly afforded by the walls of vegetable cavities. Indeed, many tropical plants present cavities so admirably suited to the accommodation of ant colonies as to appear to have been developed for this particular purpose. Such, for example, are the hollow stems of trees like Cecropia and Triplaris, the hollow petioles of Tococa, the hollow thorns of Acacia, and the bulbs

of Lecanopteris, Hydnophytum and Myrmecodia. Other plants, like certain epiphytes of the genus Tillandsia are often inhabited by colonies of ants which nest in the spaces enclosed by their overlapping leaves. All of these cases may be more properly treated in the chapter on the relations of ants to plants.

Another set of cases, in which, however, symbiosis is out of the question, comprises the nests of ants in the woody portions of plants, in cavities that have been wholly or in part excavated by wood-destroying larvae, beetles, etc. A number of species which live in small colonies have learned to take advantage of such cavities in the bark of trees, in twigs, woody galls, etc. These cavities may be modified and extended to suit the convenience of the ants. Some of our species of Leptothorax (canadensis, schaumi, fortinodis) prefer the cork-like bark of dead or living tree-trunks, our Colobopsis species and numerous varieties of the circumpolar Camponotus fallax inhabit the solid wood of hickory, pine or oak twigs 3 to 4 cm. in diameter.

The dead wood of standing or prostrate trunks is often extensively riddled by the galleries of Cremastogaster lineolata and Camponotus pennsylvanicus, noveboracensis, ferrugineus, and levigatus. These insects, which are popularly known as carpenter ants, apparently start their intricate galleries in spots where the wood has decayed or has been in part destroyed by other insects. The galleries are often continued down into the underlying soil, especially in arid regions where the wood dries out in summer.

In some parts of the country the old woody galls on oaks furnish the ants with exceptionally convenient quarters in which to start colonies or even for the permanent accommodation of small communities. This is especially true of the galls of a Cynipid (Holcaspis cinerosus) on the live-oaks of central Texas. These spherical galls, which measure from 2-4 cm. in diameter, after being deserted by the insects that produce them, remain attached to the twigs for several years. Much of the interior has been eaten out by the Cynipid larva, the imago of which leaves the gall by a circular aperture in its side.

Hundreds of recently fertilized females of several different species of ants annually take up their homes and start their formicaries in these hollow spheres. On examining several thousands of these galls in the vicinity of Austin, Texas, I found a considerable percentage of them inhabited by colonies of the following ants which are here enumerated in the order of increasing frequency: Leptothorax fortinodis, Leptothorax obturator, Colobopsis etiolata, Cremastogaster clara, Camponotus rasilis.

The species of Leptothorax and Colobopsis, which never form large colonies, inhabit the galls permanently, the Cremastogaster and Camponotus use them mainly during the incipient colonial period. Two of the species, L. obturator and C. etiolata are especially interesting on account of their methods of modifying and guarding the nest entrance. The former ant is very small, and the solitary female on entering the gall finds the opening made by the gall-fly inconveniently large. She therefore plugs it up with wood-fillings mixed with saliva. When the small workers of her first brood hatch, they perforate the middle of the diaphragm with an opening just large enough to permit their bodies to pass in and out. Occasionally this same ant nests in twigs of the hop-tree that have been hollowed out by a small carpenter bee (Ceratina nana). In such nests the larger opening at the end of the twig is occluded and then perforated in the same manner.

Even more interesting is the behavior of Colobopsis etiolata when it nests in Holcaspis galls. This ant has strongly marked soldiers, with peculiar truncated, roughened, stopper-shaped heads which exactly fit the circular holes in the galls. These individuals are, therefore, told off to act as animated portals to the nest. When a worker wishes to forage on the branches of the oak, it approaches the stationary soldier from behind and palpates its gaster. The soldier moves aside to let the worker pass out and then at once moves its head back into the circular aperture. In order to enter, the returning worker has to stroke the soldier's truncated forehead, and the guardian again steps aside for a moment. Though most abundant in the oak-galls, C. etiolata occasionally nests in the hard wood of tree trunks and branches (Carya myristicaefolia), apparently in preformed larval burrows. This seems to be the usual method of nesting of C. pylartes of Texas.

Forel has described very similar nests and habits for the European C. truncata which nests in twigs of the walnut tree. More recently I have seen another species (C. culmicola) nest­ing in the hollow culms of sedges in the "swashes" of the Bahamas. In this case the nest often extends over several internodes of the plant and each is perforated with a circular opening occluded by the head of a soldier. The slightly truncated heads of the soldiers of many wood-inhabiting species of Camponotus suggest very similar habits. The same inference may be drawn from the structure of the head in the soldiers of a peculiar Texan Pheidole (Ph. lamia) which nests in slender galleries under stones. In this insect the anterior surface of the head is remarkably like that of Colobopsis and may be said to present a striking case of convergence.