Art. XXIII.—A Fly and a Spider (Pompilidæ, Salius monachus, Sm.; and Porrhothele antipodiana).
[Read before the Wellington Philosophical Society, 18th March, 1903.]
It is proverbial that spiders are cunning and ferocious, and it is popularly supposed that their physical strength surpasses the strength of flies, yet just as flies are helpless under a spider's bondage, so, literally speaking, in the hands of some flies are spiders helpless.
To begin with, a class of flies having four wings, a sting, and which construct a nest wherein they deposit eggs and their young afterwards mature, are known to naturalists as Hymenoptera Aculeata. Examples of this class are the bees and wasps. In my young days I derived amusement from the antipathy that spiders have for flies of this class. A strong cunning spider will lie low and concealed when some one or other of these four-winged flies happens to be in its vicinity. Put a wasp in the web of a spider—one of the Epeira group—and watch results. (The species on which I used to experiment was E. diadema.) The spider almost invariably would be most anxious to be relieved of her visitor, to effect which she will frequently sacrifice part, often a large part, of her laboriously constructed web. In feverish haste, keeping all the time well out of reach, the spider severs the strands above its captive, the weight of the insect breaking away the lower strands.
I question much whether E. diadema gained knowledge of the formidable character of its captive from actual individual experience; certainly they fear the consequences of coming to close quarters. Is the antipathy instinctive? Can it be that generations of spiders have witnessed the hapless fate of their neighbours when some four-winged Hymenopteron, descending from above, like an eagle upon a lamb, has carried away a spider captive, to incarcerate her in a cell or mud-hole, paralysed and helpless, doomed to form the fresh meat on which the young of its captor will feed and mature?
The advantage possessed by the fly is in the powerfully poisonous secretion which it injects by means of its sting into some portion of its victim's anatomy, with instantaneous paralysing effect. Peckham,* having broken off the leg of a small crayfish, induced a wasp to sting it at the exposed part
[Footnote] * “Instincts and Habits of the Solitary Wasps.”
of the stump; immediate paralysis ensued, and a few hours afterwards the crayfish was dead.
Naturalists are divided in opinion as to whether the fly purposely paralyses its captive without killing in order that the meat shall remain fresh until the eggs deposited in the same nest have hatched, or whether paralysis is a fortuitous result.
When collecting other insects I have not unfrequently noticed the Pompilidæ foraging for spiders; but a recent case was particularly striking in the remarkable disparity in size between the fly and its captive.
At a place where the bank at the side of a road had fallen I noticed, when driving by, what appeared to be a large black insect fluttering down, now stopping on a small ledge, now descending until it nearly reached the bottom. Investigation showed that a fly (Salius monachus)* had captured a great hairy spider (Porrhothele antipodiana), * whose weight was so great as to drag the fly down the steep bank. When I reached them the fly was some distance away, apparently searching for a suitable hole in which to bury the spider. It soon returned, and, taking firm hold with its fore feet, commenced to drag the body along with comparative ease, the fly walking backwards. A comparison can be made by imagining a man, single-handed, dragging the body of a large full-grown dray-horse. The fly would not leave its captive, and I had no difficulty in getting both into one of my boxes.
At home I examined the spider, and unhesitatingly declared it to be dead. The legs were limp, and so expanded as to look as though it had attempted to run when attacked. The pair of posterior appendages were also relaxed, and extended horizontally. There was no alteration in the condition of the spider for twenty-four hours, but after that interval the spider showed evidence of life in having drawn its legs into a crouching posture, and the anal appendages were elevated vertically. In a day or two the spider, if irritated, would move its legs slightly; but the anal appendages at all times showed more sensitiveness. Exactly seven days after the date of its capture the legs and anal appendages were again relaxed, and it was dead indeed.
Against this fly the spider could have no chance, the whole of the former presenting a smooth, hard surface. It is, indeed, a regular Ned Kelly of the insect world, and quite without vulnerable spots. Had the fly not been interfered with it would certainly have buried the spider, which would have revived, and most probably, before it died, would have had the young of its captor gnawing at its vital parts, it being mean-while unable to resist—a fearful fate indeed.
[Footnote] * Identified by Captain F. W. Hutton, F.R.S.
Kirby, quoting Consul Krug, says of a large West Indian Hymenopteron (Scolia atrata) that it digs its nest, “then goes in search of a grasshopper. Having partially disabled it with its sting, it mounts on its back and rides it up to its own grave, where it buries it. If the grave proves to be too small, the wasp drives the grasshopper away while it enlarges it as much as is required, and then brings the grasshopper back to the hole.”
There is a great deal of interest in the life around us, of which we know very little.
Explanation of Plate XXIX.
- Fig. 1.
- Fig. 2.
Both natural size.