The Zoologist/4th series, vol 2 (1898)/Issue 688/Notes on the South African Social Spiders (''Stegodyphus'')
THE ZOOLOGIST
No. 688.—October, 1898.
NOTES on the SOUTH AFRICAN SOCIAL SPIDERS
(STEGODYPHUS).
In one of his interesting papers on the zoological features of the Transvaal (Zool. ante, 157), Mr. W.L. Distant makes reference to our curious little Social Spiders (Stegodyphus gregarius, Camb.), and perhaps a few further remarks on them may not be out of place. Mr. Distant has raised the question as to the size of the nests constructed by this species. This is of course largely dependent on the age of the colony, the structure being gradually enlarged to accommodate the increasing progeny. In the spring months I have often found single chambers, about the size of a shilling, containing a solitary female, the snare consisting of two small, irregular but closely woven screens diverging on either side of the chamber. From this simple structure a graduated series may be traced up to the fully developed nest. I have seen many hundreds of these nests both in Natal and Mashonaland, and, so far as my experience goes, I should say that the one reproduced in Mr. Distant's excellent photograph is well below the average of a fully developed nest, which I should estimate to be at least twice the size, though they are sometimes notably larger. In this country there is a larger and paler species of the same genus, which I do not remember to have met with in Natal, and its nests are often built on a still larger scale, attaining the size of a man's head.
The nest itself is composed of a compact mass of closely felted glutinous silk traversed irregularly throughout with tubular passages, sometimes terminating in small chambers. In these latter the egg cocoons are often placed, but by no means always, for there seems to be no organised nursery, as with the social Hymenoptera. The surrounding foliage is worked in with the nest (but I doubt whether this is for protective purposes), the exterior being overlaid with a coating of very adhesive silk, which is likewise used for making the snares, these being highly irregular both in size and shape, but usually taking the form of vertical screens.
I may here digress to express my belief that the nests of some of our Sunbirds, viz. Cinnyris gutturalis, L., C. chalybæus, L., Anthodiæta collaris, V., &c, are built expressly to resemble the nests of Stegodyphus for protective purposes. I have watched the construction in the case of these three species, and the nests are all built in a practically similar manner. No attempt is made at concealment, and they hang suspended from the outermost twigs of bushes or low trees at no great distance from the ground—positions which are equally affected by the Social Spider. The ground work of the dome-shaped nest with its small porch is composed of interwoven grass, and the exterior is covered with leaves, twigs, &c, bound on with cobwebs, the structure when finished having a generally unkempt appearance eminently suggestive of the abode of Stegodyphus; and indeed I have been deceived myself in this respect more than once. I have observed A. collaris and C. chalybæus collecting web from the snares of the large Nephile Spiders in Natal; but a pair of C. gutturalis, which built within a few feet of the door of one of my huts on the Umfali river, used only the web of Stegodyphus.
The food of the Social Spiders consists principally of Coleoptera, for the capture of which their strong glutinous snares are admirably adapted. Their chief victims are the Melolonthidse, such as Anomala, Trochalus, Adoretus, &c, which positively swarm round trees and bushes on the warm spring evenings after the early rains. Among the larger diurnal beetles, the handsome Buprestids of the genus Psiloptera fall a frequent prey to their wiles, and it seems strange how such securely armour-plated insects can afford sustenance to the weak little Spiders; especially when one thinks that an intelligent insect like the Bee is apparently unable to find a weak spot in the less efficient armour of the Cetoniid Hoplostomus fuligineus, Ol., when it enters her hive to rifle the honey. But it would take quite a long list to enumerate all the species of Coleoptera which I have found dead in the nests of these rapacious creatures, for in truth "all is fish that comes to their net," even though it be the evil-smelling Lycus or the caustic Mylabris. To illustrate the strength of the silk, I might mention the capture of such powerful Beetles as Copris, Catharsius, and the large Bolboceras panza, Pér.; moreover, our large Migratory Locust (Acridium purpuriferum, Walk.) is, I am pleased to say, occasionally to be found in the larder, and anyone who has received a kick on the finger from the spiny leg of this objectionable insect will realise that it would take something pretty strong in the way of Spider's silk to secure him.
At one time I had thought these Spiders never entirely nocturnal in their habits, and so far as the construction of their snares is concerned they are no doubt crepuscular, like most Spiders of procryptic colouring. But I find they are by no means loth to emerge in broad daylight to capture their prey; and when a luckless Beetle becomes enmeshed, two or three rapidly rush out and tackle it, keeping as much beneath it as possible, apparently to prevent any attack from above. Should the insect be too large for them, other Spiders come out to assist, and it is hauled off with all speed under shelter, no attempt being made to bind it up in any way as the geometric Spiders do. Judging by the position of the dead bodies of their prey, it is probable that at night many are devoured in situ, but even then the majority must be carried inside the nest.
The eggs are usually laid about February or March, being placed in small flat circular cocoons of a yellow colour. The young Spiders are much more rotund in appearance than their parents, and of a yellow colour. In the early winter the nests may be noticed to fall considerably into disrepair, and the damaged snares are no longer mended, and eventually disappear. On investigation I found this to be due to the fact that about this time the older generation dies off entirely, the nests then being found to contain only young Spiders. The dead bodies of the parents may be seen lying about in the passages amidst the débris of deceased Beetles. Whether the older Spiders are actually killed and devoured by their own progeny, I have not so far sufficient evidence to decide. For the present, however, I incline to this view, for otherwise I can see no reason why the parents should suddenly die off as they appear to do. It is true that at this period their food supply almost comes to a standstill, for during the winter months Coleoptera cease almost entirely to venture out on the wing; but this would affect the young ones equally, if not more. Even if this supposition be correct, it is difficult to understand how the young subsist through the winter, for, so far as I have seen, they do not emerge at all from the nest, and they certainly construct no snares during that season for the capture of insects.
One of the most interesting features in the economy of these creatures has yet to be dealt with. In the winter of 1895 I examined a number of deserted nests along the Umfali river in hopes of finding Coleoptera harbouring therein, and I was surprised to find in several instances large balls of grass, wild cotton, or even feathers, right in the middle of the nest. I was quite at a loss to understand how or why the Spiders should accumulate these materials, and I did not find the solution until early in the present year.
It happened thus. On one of my entomological rambles some miles from Salisbury, I found myself suddenly enveloped in a regular winding-sheet of sticky Spiders' silk, which was evidently that of my friend Stegodyphus. I therefore walked a short distance up wind to find whence it came, and soon descried a bush on a termite heap, on the summit of which were some hundreds of these Spiders, apparently engaged in constructing a new nest, and evidently in a great state of perturbation. It at once struck me as very curious that these wary creatures should be thus exposing themselves wholesale in broad daylight, and I therefore proceeded to search for the disturbing element. The bush was placed on the side of the termite heap, and was connected by several strands of about four feet long with a bush on the top, on which was a smaller lot of some fifty Spiders. These were again connected with another shrub about six feet away on the far side of the ant heap. Here was found the original nest, and there were still one or two Spiders on the outside of it. Within a foot of the nest was an open bird's nest, apparently that of a finch, containing two eggs, with which, however, I was unacquainted, they being very similar in appearance to those of the European Bullfinch. It seemed hardly probable that this could be the cause of the commotion, and so it proved, for the eggs were clearly deserted, being quite cold and hard set. I then examined the Spiders' nest, and was surprised to hear sundry squeaks inside, so, placing my butterfly-net beneath it, I tore the nest open, and out dropped four little Dormice (Myoxus nanus, De Wint.). Here, then, was the cause of this twofold domestic tragedy! Here, too, was the explanation of the facts I had observed on the Umfali, for in the centre of the nest was a chamber lined with soft grassheads, feathery flower-seeds, and one or two feathers. Subsequent observation has convinced me that this is a normal habit on the part of the Mashona Dormouse, for I have since found two full-grown specimens in similar positions, and, besides, a large proportion of disused nests show clear signs of having been thus occupied. I expect the primary attraction will be found to be the Beetles caught in the Spiders' web, the Dormice having gradually learnt to utilise these snares for their own purposes, and finally evicting the inhabitants. Whether they usually occupy deserted nests, or themselves oust the lawful proprietors, cannot yet be decided for certain; but the above instance clearly demonstrates that the latter method is sometimes employed, and from the evidence before me I am inclined to believe that this is the usual course. It is pretty clear that in this case the mother Dormouse must have brought her young to the nest, for they were fully a week old, and perhaps more, and the condition of the nest showed that it had been occupied only for a short time. Likewise it seems probable that the Spiders resisted the invasion for some time, as it was evident that they could only have evacuated a few hours before, whereas it must have taken the Dormouse some little time to construct her nest and bring her young there.
To return to the victims—they were busying themselves all this while in setting their house in order, though a large proportion were apparently of opinion that it was desirable to put a still greater distance between them and their persecutors; for some fifty of them were standing together with abdomen in air pouring forth a regular stream of silk in hopes of connecting with another tree. In one case a few threads caught on to a tree fully twelve feet away; the near ends were then promptly fastened down, and a Spider would advance cautiously along, strengthening the thread and hauling in the slack as she went, but in every case the thread broke. After many attempts to thus retreat further they gave it up, and went on with the work of making a new nest. The foundations of this were made by forming a dome-shaped canopy some eighteen inches in diameter over the top of the bush, the Spiders running backwards and forwards in all directions laying down the silk. I presume that eventually the edges of this canopy would be drawn together to form the outer shell of the nest, but unfortunately I was unable to remain longer to watch the process, and as I never found an opportunity to revisit the spot, I never learnt the sequel of this interesting chapter in the life-history of Stegodyphus.
In conclusion, I may mention that I have recently discovered a singular messmate of the Social Spiders. This is none other than one of the Micro-Lepidoptera! The larvæ in their frass-covered cases reside among the débris of dead insects, on which I presume they feed. I have not yet reared the imago, but hope to do so before long. In a nest I opened yesterday I found six empty pupa-cases, from which the moths had evidently emerged; how they managed to escape right from the heart of the nest seems little short of a marvel. Truly these venturesome insects pass their lives in the very jaws of death, and the struggle for existence must be keen indeed to compel them to resort to such an abode.
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