A Burrowing Spider

Dr. Alfred C. Stokes

In the somewhat heavy soil of certain fields, where but a scanty herbage thrives, the cave-­making spider (Tarantula arenicola, as identified by the Rev. Dr.  H. C. McCook) has excavated so many of the nearly perpendicular and cylindrical burrows, that the place is almost honeycombed, and the surface is conspicuously dotted by the irregularly five-sided towers erected above each opening. The burrows vary from one-quarter to three-quarters of an inch in diameter, and in depth from eight to twelve, or even twenty, inches; the smaller being formed, it is said, by the young, which enlarge them with their growth. The walls are compact and smooth, but without lining. Tow­ers in other localities have been observed two inches high: none I have seen are above one inch, the majority being still less.

Arachnids

Tarantulas

Other spiders

Scorpions

Other arachnids

From: Science, August 8, 1884, pp. 114-116

Among my captives, the most active work­ers are an adult and a half-grown individual, between whose actions, while digging, slight differences are observable. In a glass jar they refused to do more than attempt to escape by unavailing efforts to scale the sides, but, when set free in the garden, they at once began to exhibit their manner of burrowing, and dispos­ing of the excavated earth. Most of the labor is performed by the large and strong mandi­bles, with the probable assistance of the fore­legs. A pellet of earth, frequently a third of the worker's cephalothorax in bulk, is loosened as the spider labors head downward, and is seized by the mandibles. The young spider turns at the bottom of the burrow, and ascends, head first, to the edge of the aperture, where the pel­let is held just above the surface; then, by a blow from both fore-legs it is thrown to a dis­tance varying from four to twelve inches, usually falling in particles, so that no fresh earth is noticeable near the burrow-entrance. The half-grown individual then backs down the tube, and resumes work below. The mature spider, while the pit is shallow, ascends backward with the load, comes entirely out of the orifice, turns around, and, having popped the abdomen into the opening, throws away the pellet. She rests for a few moments, again turns within the cave, and descends, head foremost. Before returning to work below, however, she often carefully examines the edges of the burrow ­entrance, and, if the earth has become dry and friable, strengthens it by threads of web, applied by longitudinal strokes of the spin­nerets; and, if her movements have broken down the margins, she places her head under the edge, pushing and lifting the earth in a way suggestive of a dog's method of heaping dirt on a bone with his nose. She then applies more web, and resumes her digging. But, as the burrow deepens, the mature spider also turns while below. I have, however, never observed a young individual bring up a pellet backward.

That the spinnerets of this species take any part in pellet-making is improbable. Mrs. Mary Treat, while studying Tarantula turri­cula, observed their application to the earth­-mass before its ejection. It is likely that Tarantula arenicola relies solely on the cohe­sion of the moist particles, without the addition of strengthening web, as I have repeatedly witnessed the dry soil of the field crumble to sand before the spider could get the pellet quite out of the tube.

The young specimen brought up a load at intervals varying from two to five minutes; and a cavern half an inch across and about one inch deep was excavated in an hour and a half. While deepening a burrow, a young spider in the field worked somewhat faster. Assuming a pit to be of the uniform width of three-quar­ters of an inch and twelve inches deep, the Tarantula must carry out the comparatively enormous amount of 5.31 cubic inches of earth.

The towers are usually composed of short pieces of grass (fig. 1) placed above and across each other in an irregularly five-sided wall. Occasionally small twigs are used. In­deed, almost any light object will be utilized if within reach, for the spider will not leave the burrow to search for materials. If nothing is attainable without such an effort, she will erect a low wall of earth. In several instances tow­ers have been destroyed, and the ground cleared for a space of three inches radius; and from another place the sod was removed: but, in every case, the spiders raised a bulwark of earth, one having attached a single sliver of pine shaving, the only thing within her reach. At times the grass is curved around the open­ing, as if a wisp had been taken, and the tower formed at almost a single stroke, without the labor involved in placing each blade separately. Near the fa­vorite field, a house­wife, in the annual frenzy of house­cleaning, had thrown out a quantity of coarse straw, which some of the Tarantu­las utilized by erect­ing towers (fig. 2) of comparatively im­mense straw logs. Two miles from the latter was found a lofty edifice (fig. 3) built of large pieces of brown, partially decayed wood from an old railroad tie. Mrs. Treat has witnessed their construction by another species. I have not observed the entire pro­cess.

The spiders' favorite position is a crouching one at the summit, the legs within the tower, and supported by the walls. At the sight of any approaching object, they dart backward into the burrow. They are not disturbed by surface vibrations. Footsteps, even the passage of a heavy wagon within five yards of the pit, do not affect them; but the slightest movement of the observer, two feet distant, or the sudden swaying of a bush, sends them to the burrow immediately. Dr. H. C. McCook, writing in a popular magazine, says of the use of these erections, that "they probably serve as watch-towers, from which the keeper may observe the approach of her enemies," as an attraction to roving insects, and perhaps to prevent flooding of the cavern by rain. The towers in this locality are far from being water­-proof; they are used exclusively, I think, to facilitate the capture of food. But observers, so far as I am aware, have made no state­ments as to the method of food-capture, when the food fails to voluntarily scale the walls.

The towers are observatories and transmit­ters of signals to the spider when below. From them she scans the field, as the robber barons of the olden time, from their battlements, watched for the com­ing of the caravan. The spider peers through the scanty grass-blades, se­lects her victim, and, as I have witnessed, leaps from the sum­mit to seize the prey.

I have seen her spring at a fly on the ground, missing ­it, of course. But she does not always wait for food until the pit and tower are completed. I have seen her dart from the edge of an unfinished burrow, capture an ant three inches distant, and retire to the shallow cave. Ten minutes later she reappeared empty-handed, and almost imme­diately attempted to seize another near by, but failed to do more by her frantic efforts than scrape up a heap of loose earth.

The towers are so loosely constructed that an ant can scarcely run over the walls without making enough rattling to admonish the con­cealed spider, which at once hurries to the top, and, if the insect is acceptable, takes it in. A black ant running over the foundations almost invariably brings the spider up; and the gentle tapping of a straw, or even dragging a straw across the dead grass in contact with the walls, is quite sure to be followed by the arachnid's appearance. The sense of direction, or the ability to perceive whence the dis­turbance proceeds, is well developed. The spider always ascends on that side to which the straw is applied, and the same individual can be brought to each side in succession. The depth of the cavern seems to have little effect. I have called up the occupant from a burrow which subsequent examination has proved to be eighteen inches deep. Unless she has been deceived several times, she usu­ally runs up rapidly, and will occasionally snap at the end of the straw. While experimenting, it is hardly possible to avoid introducing fila­ments of the tower, or adherent particles of earth, and it occurred to me that these might be the call to which the spider responded; but sand from an anthill, sprinkled in freely, had no effect.

Mrs. Mary Treat, writing of another species of Tarantula, says that all food-remains were ejected in the same way as the earth pellets. Tarantula arenicola is not so neat. The earth beneath old burrows is often darker than the walls, and densely filled with fine rootlets. It is probably darkened and enriched by the spi­der's excrement and food-remains. From bur­rows in the field it is the rule to take masses of debris, which consist of the spider's exuviae, the heads and legs of ants, the elytra and other chitinous parts of beetles, with fragments of insect-wings. It seems that the dead and empty bodies are torn to pieces, and scattered at the bottom. This was done by a captive which would not dig, but which ac­cepted maimed flies. After extracting the juices, the spider tore the body into fragments so small that only careful search could find them. In but two instances have I observed an ejection of food-remains. A mutilated fly was seized from a tower, and twenty-four hours later I did find what appeared to be the desic­cated remains. In the second case, two spi­ders were fighting fiercely when set free at evening, near the burrow of a small specimen in the garden. During the night the occupant of the burrow was dislodged, and the vanquished spider had been dragged into the pit which the conqueror had enlarged, and whence, in the course of the morning, frag­ments of the dead body were thrown out, among them the abdomen severed from the thorax, but not otherwise mutilated. Occa­sionally, also, an elytron can be found near a tower in the field.

This disposition of remnants is somewhat re­markable; since spiders in general are cleanly, and since this one is particularly intolerant of intrusive objects. A straw or stem dropped into the burrow is immediately carried up, and tossed away. The only instance observed, where a young spider ascended backward, was when trying to get a heavy stick out of the pit: having lifted in vain. she attempted in to pull.

Noticing the fondness for ants, a number of bran-cracker crumbs were sprinkled at a distance of six inches from the tower, and an ant was soon struggling under a load larger than itself.  Suddenly the spider on the tower started, erect and rigid: she leaped to the ground, she ran six inches, she seized that bit of cracker, and retreated with it to her bur­row, leaving the emmet on its back in the dust. For two hours she remained below. The following day I twice witnessed the same performance. The spider once overran the crumb, and so lost it. At the third time, the piece of biscuit became wedged in the tower as the spider was running in backward, and I plainly saw her nibbling at it. During a momentary absence for forceps to remove it, to examine for marks of mandibles, the spider carried it down and out of sight. The frag­ments were not touched, except as they were being borne about by the ants. Is it usual for spiders to take any but animal food?

Note: as this article was written in 1884, the systematics are no longer valid. As of yet, I have not identified which burrowing spider is meant by Tarantula arenicola; any suggestions, please forward to Herper at Verizon dot net.