The Ichneumons which thus regard the Spider’s eggs as convenient food for their own larvae are probably very numerous. Nor are they themselves always free from parasites. Occasionally the larvae of minute Hymenopterous insects are found to be parasitic upon the eggs of an Ichneumon which have been laid in a Spider’s cocoon.
It sometimes happens that the development of the young spiders has so far advanced at the time of the Ichneumon’s intrusion that the latter’s intention is frustrated, and its offspring, instead of devouring, are themselves devoured. Again, some few of the eggs in an infested cocoon occasionally escape the general destruction and reach the adult condition, but there can be no doubt that Ichneumons are largely instrumental in keeping down the numbers of most species of spiders. The perils which attend the Spider after leaving the cocoon are no less formidable, and much more numerous. The whole newly-hatched brood may be destroyed by a heavy rain-storm. If there is not a sufficient supply of food suitable to their feeble digestive powers they perish of inanition, or eat one another. This cannibalistic propensity is a considerable factor in the mortality among young spiders, and the adult animals frequently prey upon one another.
Argyrodes piraticum, in California, invades the webs of larger spiders of the family Epeiridae, which it seizes and devours. A. trigonum, common in the eastern states of North America, has the same habit.[[288]] Hentz found in Alabama a spider, which he named Mimetus interfector, of still more ferocious and piratical habits. Its special quarry is Theridion tepidariorum. Sometimes the Theridion overcomes the invader, and one case was observed in which a second Mimetus was devouring a Theridion beside the dead body of its predecessor, who had come off the worse in the combat.
The eggs of Theridion tepidariorum are also sometimes devoured by this spider, and a similar propensity has been observed in some English species, for Staveley[[289]] states that it is common to see certain spiders of the genus Clubiona feeding upon the eggs which have been laid by their neighbours. The larvae of some Hymenopterous insects are parasitic upon Spiders themselves, and not upon their eggs. Blackwall found this to be the case with the larvae of Polysphincta carbonaria, an Ichneumon which selects spiders of the genera Epeira and Linyphia on which to deposit its eggs.[[290]] The spider thus infested does not moult, and is soon destroyed by the parasite which it is unable to dislodge from its back. Menge, in his Preussische Spinnen, enumerates several cases of parasitism in which the larva, as soon as it has developed from the egg, enters the spider’s body, there to continue its growth. Spiders are also subject to the attack of a parasitic worm, Gordius (cf. vol. ii. p. 173).
Some of the most deadly foes of Spiders are the Solitary Wasps. There are many species of Pompilus (vol. vi. p. 101), which, having excavated holes in clay banks, store them with spiders or other creatures which they have paralysed by their sting. They then deposit an egg in the hole, and immediately seal up the orifice. This habit is found to characterise the solitary wasps of all parts of the world. Belt[[291]] relates the capture of a large Australian spider by a wasp. While dragging its victim along, it was much annoyed by the persistent presence of two minute flies, which it repeatedly left its prey to attempt to drive away. When the burrow was reached and the spider dragged into it, the two flies took up a position on either side of the entrance, doubtless with the intention of descending and laying their own eggs as soon as the wasp went away in search of a new victim. Fabre[[292]] gives an interesting account of one of the largest European Pompilidae, Calicurgus annulatus, which he observed dragging a “Tarentula” to a hole in a wall. Having with great difficulty introduced its burden into the cavity, the wasp deposited an egg, sealed up the orifice, and flew away. Fabre opened the cell and removed the spider, which, though completely paralysed, lived for seven weeks.
The same indefatigable observer describes the method adopted by the comparatively small Pompilus apicalis in attacking the formidable Wall-spider, Segestria perfida. The combatants are well matched, and the issue of the battle would be doubtful if the wasp did not have recourse to stratagem. Its whole energies are directed towards forcing the spider away from its web. At home, it is confident and dangerous; when once dislodged, it appears bewildered and demoralised. The wasp darts suddenly towards the spider and seizes it by a leg, with a rapid effort to jerk it forth, releasing its hold before the enemy has had time to retaliate. The spider, however, as well as being anchored by a thread from its spinnerets, is clinging to its web with its hind legs, and if the jerk is not sufficiently energetic, it hastily scrambles back and resumes its defensive position. Before renewing the attack the wasp gives the spider time to recover from the excitement of the first onset, seeking, meanwhile, the retreats of other victims. Returning, it succeeds, by a more skilful effort, in drawing the spider from its retreat and hurling it to the ground, where, terrified and helpless, it falls an easy prey. Should the insect bungle in its first attack and become entangled in the web, it would itself become the victim. Certain wasps thus appear to seek out particular species of spiders as food for their larvae. Others are less discriminate in their tastes. Again, some, as in the cases cited above, store their egg-nest with a single spider, while others collect many for the purpose.
The American “blue digger wasp” (Chlorion caeruleum) excavates its nest in the ground, and inserts a single large spider of any species.[[293]] Another wasp, of the genus Elis, selects the Wolf-spiders, and especially Lycosa tigrina, for the use of its larvae, while Priocnemus pomilius shows a preference for the Crab-spiders, or Thomisidae.
One of the most remarkable instances is that of Pepsis formosa, which preys upon the gigantic spider Eurypelma hentzii, wrongly styled in America the “tarantula,” but really belonging to an entirely different family, the Aviculariidae.
Fabre’s most interesting researches have established the fact that the instinct of the wasp leads it to sting the spider in a particular spot, so as to pierce the nerve-ganglion in the thorax. The precision with which this is effected is absolutely necessary for the purpose of the insect. If stung elsewhere, the spider is either incompletely paralysed, or it is killed outright, and thus rendered useless as food for the future larvae of the wasp. On the one hand, therefore, the Tarantula has acquired the habit of striking the wasp in the only point where its blow is instantaneously fatal, while on the other the wasp, with a different object in view, has been led to select the precise spot where its sting will disable without immediately destroying the spider. The latter case is, if anything, the more extraordinary, as the insect can hardly have any recollection of its larval tastes, and yet it stores up for progeny, which it will never see, food which is entirely abhorrent to itself in its imago state.
Spiders taken from the egg-nests of wasps by M‘Cook survived, on the average, about a fortnight, during which period they remained entirely motionless, and would retain any attitude in which they were placed.