An Unusual Type of Web Constructed by A Samoan Spider of The Family Argiopidae
The spider whose peculiar web is to be discussed is one of the most conspicuous of the spiders of the island of Upolu, Western Samoa. It has not yet been possible to identify it, but it appears to belong to the subfamily Araneinae.*
Each web consists of a wide-meshed tangle of threads in the middle of which is suspended a flat dome; in a large web 8–10 in. in diameter. The dome is composed of a thread running in a very close spiral and attached to numerous threads radiating from the centre. The radial threads branch, so that the meshes are much the same size all over the dome, measuring roughly 2 × 1.5 mm. No part of the web is constructed of sticky silk.
In spinning the web the spider first constructs the large tangle. It then clears a space in the middle and begins to put in the radii, outlining a very obtuse cone. It spins the spiral from the centre outwards, circling steadily round below the sheet with its head towards the centre and the legs on the forward side bent and moving rapidly, the impression produced being that of a sewing machine. At frequent intervals new radii are inserted. The spiral thread is attached along a short length of each radius so that when closely examined the thread has a slightly notched or zig-zag appearance. While spinning is going on the sheet is attached only by the apex and margins. When completed, a process taking two or three hours, the spider attaches some vertical threads below the sheet. Then it moves to the upper side, slackens the sheet by cutting most of the threads at the apex, and finally spins a fine-meshed tangle between the sheet and the main tangle, working from the centre outwards. This pulls the conical sheet into the final dome or saucer shape. The resting position of the spider is upside down beneath the dome.
The whole web may be up to about two feet across. Frequently many of all sizes are attached to one another and fill quite large volumes of space. One mass which was observed covered an area at least twenty feet across and extended some twelve feet up trees and bushes. It contained at least 200 large spiders. The small males and several other species of spiders are also found in these webs, some parasitic in that they eat the young and small prey of the large spider. The females lay their eggs in plano-convex brown coccoons hung
[Footnote] * The spider is Cyrtophora moluccensis Doleschall. I am indebted to Dr. Willis J. Gertsch, of the American Museum of Natural History, for the identification.
above the centre of the dome, sometimes in strings of five or six. One was opened and found to contain 366 eggs.
A similar web has been described in the United States, spun by a small and apparently rare spider, Hentzia basilica. Some authorities place it in the Metinae, others in the Araneinae.
The most specialised web constructed by spiders is the orb web of the Argropidae. It consists of threads radiating from a central hub and supporting a spiral thread of sticky silk. Sometimes there is a barrier web also in the form of a tangle of thread on one or both sides of the plane of the orb. In spinning the web, after the framework and radii are in place, the spider constructs the hub and notched zone, and attaches a thread, known as the scaffold spiral, outwards towards the edge. Then working inwards, it puts on the sticky spiral and removes the scaffold spiral.
One theory of the evolutionary origin of the orb web is that the ancestral spider acquired the habit of spinning a tangle of threads in which to hang its egg coccoon. From this starting point the sheet web of the Linyphiidae can be imagined to be derived by the addition of a sheet to the tangle, and in some cases the eventual loss of the tangle. The Theridiidae have a tangle web, sometimes containing a little sticky silk. By rearrangement and specialisation of this the orb web of the Argiopidae might have arisen.
The web of the Samoan spider differs from the orb web in being spun from the centre outwards and in having no sticky silk. It does, however, resemble the notched zone and scaffold spiral, and might be regarded as a specialisation of the half-constructed orb web, a sort of paedomorphic form. In this connection the web of the Nephelinae is of interest, as it retains the scaffold spiral when completed as well as the sticky spiral. It also has the branched radii giving the uniform size of mesh all over, but does not otherwise resemble the domed web. On the other hand, the domed web might be regarded as resembling a stage in the evolution of the orb web in which a sheet has been placed in the primitive tangle, more regularly constructed than the sheet of the Linyphiidae, but not yet replaced by one made of sticky silk. It seems most likely that it actually is a modification of an orb web, rather than a primitive type, but it is of considerable interest none the less, as departures from the usual orb pattern are extremely unusual amongst Argiopid spiders.