Popular Science Monthly/Volume 38/December 1890/Defenses of Burrowing Spiders
By HENRY C. McCOOK, D. D.
THE simplest form of burrow is that of the Tarantulas, which represent the largest known spiders. These huge araneads appear to depend wholly upon their size to resist the assaults of enemies who invade their den. At least I have not found satisfactory evidence that they erect any artificial barrier over the entrance to their tunnels.
A more complicated burrow, and one better serving for defense, is that of Leptopelma cavicula of northern Africa. The drawing (Fig. 1) shows a section view of the upper part of the burrow, the entrance to which is without any door or other defense as in the Fig. 1.—Burrow of Leptopelma cavicula. Section view of upper part. case of the tarantulas. The burrow descends perpendicularly for a little way, but at the top a special branch diverges laterally, which curves and again descends perpendicularly for a considerable distance. At the summit of this second and parallel perpendicular tube another branch issues, inclining upward toward the surface. A glance at this structure, if we suppose it to be characteristic of the species, and not an accidental formation, will show that it makes an admirable protection against heavy rains, which sink away into the first burrow as a kind of reservoir, enabling the spider to escape by the diverging branch. Against enemies who pursue it into its den, this structure also presents an effectual defense, for, while an enemy naturally would rush downward into the first direct passage, the spider may escape by the lateral branch. Supposing that the enemy, observing the mistake, ascends and follows along the branches, the spider has the opportunity to push up into the second branch while the pursuer, again following its natural instinct, would rush down the second perpendicular tube. I am here in the region of conjecture, but perhaps no better explanation presents itself.
A third stage in the development of this defensive industry is represented at Fig. 2, which shows the external tube of Leptopelma elongata. This is simply a lily-shaped tube of pure white Fig. 2.—Lily-shaped Tube of Leptopelma elongata. spinning-work, rising directly above the burrow, and supported by surrounding foliage. The purpose of this structure has not been positively determined. As able a naturalist as A. R. Wallace has conjectured that it may be deceptive in its uses, its resemblance to a flower attracting to it insects, which are thus preyed upon by the proprietor. Such elevated objects are certainly apt to attract insects, who are disposed to alight upon them without regard to their promise of providing food. But I am inclined to believe that Leptopelma's silken lily serves as a watch-tower from which she can observe the approach of enemies and make good her escape in time. Moreover, I believe that it is possible for her to pull together the sides of the sheeted turret and thus erect a barrier between herself and some of her feebler pursuers.
Another form of defensive industry is presented at Fig. 3, which is the exterior part of the turret tube of Dolichoscaptus inops (Simon). Fig. 3.—Turret of Dolichoscaptus inops. (Natural size.) This is about an inch in height, and is composed of mingled chippage and mud, a sort of débris of chopped straw and soil.
A still further stage is shown at Fig. 4, which represents a columnar turret of Dolichoscaptus latastei, several inches high. This resembles the tower of the preceding species, but adds thereto a hinged covering after the manner of a trap-door. This turret is also composed of chippage and débris of various sorts gathered from the neighborhood, and is supported upon the surrounding foliage, which in the drawing is a plant of Lavandula dentata. All the uses to which such an elevated structure can be put are served by this ingenious structure, and, in addition, the trap-door is manifestly intended to defend the inmate from the assaults of enemies.
We come now to the trap-door nests of Nemesia meridionalis, and other species making traps of the wafer type, as so fully described by Moggridge. Here we have simply a dropping away of the turret of Dolichoscaptus, and the use of the burrow independently Fig. 4.—Turret, with Trap-door, of Dolichoscaptus latastei, supported on a plant—four inches high. (After Simon.) of the same, but with the trap-door retained. In the species studied by Moggridge a single burrow is the ordinary rule; but there are many variations, some of which are manifestly characteristic of species, and others which are probably occasional and accidental. A variation described by Mr. Simon is shown at Fig. 5, the nest of Stothis astuta, which inhabits the forest of Cartuche, near Carácas, South America. The drawing shows a section of the burrow, indicating the curved course, and also the two wafer-like trap-doors habitually placed at either end. That this peculiar industry is defensive is probable, for we can readily imagine the spider disappearing within its den at one door, and, if its pursuer should succeed in entering the same, escaping at the other. We might, without much stress of imagination, carry the conception a little further, and suppose, again, the enemy making its exit from one door and the spider again descending into Fig. 5.—Section View of Curved Burrow of Stothis astuta, showing Double Trap-door Entrance. its burrow by the other. This game of bopeep might evidently be played to the great advantage of the trap-door spider, and manifest disconcerting of its enemy.
Simon gives an interesting example of the ability of a spider of this species to change its habit and adapt its industry to unexpected surroundings. The species commonly seeks dark and damp localities, and digs in vegetable earth a burrow not very deep. The nest was begun underneath a stone in soil which was so rocky as to be impenetrable. Not wishing to change its site, and not to be cheated out of its proposed domicile, Stothis proceeded to erect a cylindrical case about two inches long, composed of a conglomerate gathered from surrounding particles of soil and vegetable chippage. These were cunningly wrought together, the whole structure silk-lined, and the characteristic trap-doors hung, one at either end. Thus, while varying her habit in so far as to build a surface tunnel instead of a subterranean one, Stothis preserved her defensive habit of erecting for herself a back door by which she could retreat in case of invasion at the front door.
The burrow of Stothis cenobita (Simon) is simply a rounded chamber underneath the surface, and closed by a trap-door, which differs in no particular, as far as I can observe, from the ordinary trap-door of the American Cteniza californica.
It is difficult to say what may be the enemies of the trap-door spider against which such ingenious architecture has been reared and such vigilant watch is exercised. But the quite general testimony is that these spiders leave their tubes at night and go forth in search of prey; or, as in other cases, open the lids of their tunnels and spread straggling lines near by, upon which passing insects are entangled and delayed long enough to allow the spiders to pounce upon them from their open caves. If we credit these accounts, we might infer that the enemies which the trap-door spiders most dread are not such as are abroad at night. Evidently the creatures are fearless at that time—a state of mind which doubtless results from their knowledge that they are comparatively free from their worst enemies. The enemies which they most dread may therefore be reasonably looked for among diurnal creatures, and not among those of nocturnal habits. Among these foes, at least one of the most formidable and irresistible is a diurnal insect, the female of the terrible digger wasp, which I do not doubt will be found to store trap-door spiders, as well as tarantulas and lycosids. There is no evidence known to me that Pepsis formosa invades the tunnel of the Mygalidæ, in order to dig them out. Such an act is not, indeed, beyond her powers; and, reasoning from the conduct of Elis 4-notata, it is highly probable. But we are not yet warranted in attributing the habit to her. Some lizard or mammal that might pull open the trap with its claws may be looked for as also a probable enemy against which trap-door spiders erect and defend their ingenious barrier.
At all events, the spider herself is well aware of these enemies. Abbé Sauvages invariably found, when he attempted to open the door of the nest of "the mason-spider" (Nemesia and Cteniza), that the mother was on guard, holding down the lid of her tunnel with great force. In his efforts to pull the trap-door up, the spider would jerk it down, and there would be an alternate opening and shutting of the nest until his purpose was accomplished. It is the habit, according to Moggridge, Simon, and all observers who have noted the point at all, for these animals to hang back downward upon the inner surface of the door. In many nests which I have seen there are holes along the outer or free edge of the door—the part directly opposite the hinge—which mark the points at which, probably, the fangs of the spider had been fixed, in order to give it a strong purchase against intruders.
One of the most curious examples of relation of structure to enemies, or perhaps of the reaction of hostile environment and agents upon structure, is found in a territelarian spider (Cyclocosmia truncata). This aranead, according to Hentz, dwells like others of its kind in cylindrical cavities in the earth. Though many specimens were found, he never saw any lid or closure to the aperture of its dwelling. The very singular formation of its abdomen, which is as hard as leather behind, and is truncated to form a perfect circle, induced Hentz to believe that when in danger it closes its dwelling with that part of its body instead of with a trap-door or lid. This conjecture,
of course, needs confirmation, though it seems not improbable; and one may imagine the intellectual confusion of a pursuing enemy which finds its prey suddenly disappearing within a hole in the ground, but which, when investigated, presents nothing but a level surface where certainly a hole ought to have been! The dorsal view of the spider is given at Fig. 6, the side view at Fig. 7; and a diagrammatic section view of the creature is drawn at Fig. 8, as it probably would appear when closing up the opening to its burrow.