This study was conducted to see if any marked overlap in prey occurred between three species of web-building spiders which commonly occupy the same part of the habitat in urban Wellington, and the fringe of nearby bush areas.

The three species are:

Achaearanea veruculata (Theridiidae) - the small cobweb spider. (Fig. 1). This species, by far the smallest of the three, reaches to about 5mm body length. It is fawn to brown in colour and builds a cobweb of untidy threads in crevices of rocks or trees, also on dwellings, shrubs and fences. Its captured prey is wrapped up with a technique of throwing threads around the animals. This technique is shared with other theridiids such as Steatoda and Latrodectus (the katipo).

Araneus pustulosus (Araneidae) - the garden orb web spider. (Fig. 2). This species grows to 15mm body length, and shows a wide colour variation from cream through brown and green to near-black. It builds an orb web of radiating threads with a spiral thread laid on to the radials. The web may be found on bushes, dwellings, fences and on rock faces where projecting surfaces are available. Prey are wrapped up by rolling with the legs while silk is dispensed from the spinnerets.

Ixeuticus martius (Dictynidae) - the black house spider. (Fig. 3). This species grows to 15mm body length and while brownish when young, darkens with age until parts of the body are black. Other parts carry whitish hairs giving this species a greyish appearance. The web is a series of struts on which a zig-zag pattern is woven, but as the web ages it becomes disordered and the strut and zig-zag pattern is often hard to see. Prey are grabbed and dragged to the spider's retreat to be dealt with there.

Ixeuticus is the only one of these three species to construct a definite retreat of silken thread. A. pustulosus lies exposed on a surface while A. veruculata hides in a crevice when possible.

Fig. 1. Achaearanea veruculata shown in a typical posture. The dorsal surface faces to the ground.
Fig. 2. Araneus pustulosus shown at the centre of its orb web. The spider normally faces head towards the ground. An insect prey can be seen above the spider, well wrapped in silk.
Fig 3. Ixeuticus martius in a typical posture at night, waiting for prey to contact the web.

Localities Studied

A preliminary study provided the outline for a web-collecting programme in which 100 prey-containing webs were examined for each species in each of the four seasons. Thus a total of 400 webs were found for each species, accumulated at the rate of 10 per week spread over the four locations.

The webs of the species studied do not always contain recognisable prey remains. It was found in the preliminary survey that for every usable web, between five and 10 webs need to be examined. This gave a figure of between 6,000 and 12,000 web examinations over the year in order to find the 1200 prey-containing webs required for the three species comparison. Only prey which were actively being fed on, or had recently been fed on by the spiders were counted in the study. Familiarisation with the collection sites and positions of webs ensured that double counting of remains was avoided. This is an important point because some prey (e.g. beetles) can persist in webs for many weeks after the spider has fed on them.

The prey remains were identified to family level where possible and these data used to draw up results (Tables 1-7).

Spider Size and Prey Captured

Where possible a measurement was made to the body length of spiders found with prey in order to relate the prey captured to the maturity of the spider.

Spiders were placed into one of the folowing groups for this intraspecific comparison:

The dividing line between spiderlings and immature spiders is somewhat arbitrary, but that between immatures and matures was based on the body length of females found with egg sacs. The results of the survey are presented in Tables 1-7.

A. veruculata

In this species, Diptera (35.4%) were numerically the largest prey group, but they were not nearly as significant as they were to the other two species. The Hymenoptera (18.2%) were the next most significant prey for A. veruculata. Here ants (N = 74) were the single largest family. The web of this species seems well suited to the capture of walking insects and this would explain partly the high capture level of ants. Another feature of A. veruculata's feeding is its predation on spiders, particularly on I. martius. Close observations of these two species on the side of the author's house showed that the captures of I. martius were probably related to members of this species changing their web sites. The captured spiders in many instances having moved 30cm or more from their known web site. A. veruculata quickly and readily wraps up other spiders with the silk-throwing technique mentioned earlier. The other unusual feature of this species prey is its feeding on slaters (Isopoda), millipedes (Diplopoda) and harvestmen (Opiliones), all prey groups which are either absent or found in minimal numbers in the webs of A. pustulosus and I. martius.

Table 2 shows some niche differentiation between the spiderlings and the mature spiders, with smaller prey such as psyllids and phorids being important to spiderlings, while mature spiders utilise other spiders as the most numerous prey item. There is thus a shift towards predation on other spiders as A. veruculata matures. The importance of very large prey such as cicadas is discussed below. Seasonal prey differences were not especially marked in this species. Ants are most significant in summer (N = 28); craneflies (Tipulidae) in autumn (N = 21); and through the winter fungus gnats (Mycetophylidae) (N = 30) are an important food source.

A. pustulosus

Diptera (62%) were again by far the most numerous prey group, with Mycetophilidae (N = 351) being the most common item. Of the rest of the prey groups, only the Hemiptera (16%) and the Hymenoptera (14%) exceeded double figures as percent of prey caught. The orb web. which is strung in the flight path of insects, appears well suited to capturing Diptera. Because of its location in the habitat, it is less well suited for the capture of other prey groups which spend either all or most of their time walking. This was well illustrated by the capture of ants which were all winged forms in the orb-webs of A. pustulosus but walking ants in the cob-web traps of A. veruculata. A. pustulosus is definitely more restricted in its diet than A. veruculata, as shown in the relative paucity of items in the “other” prey category.

There is a clear niche shift in prey taken at different stages of development of A. pustulosus. The spiderlings take a variety of small prey such as psyllids, aphids, page 27 ants and psocopterans. The mature spiders capture much larger prey with bees featuring prominently along with larger types of dipterans such as blowflies (Calliphoridae) and cranefiles. The bees were nearly all honey bees (Apis mellifera) with an occasional bumble bee (Bombus sp.) being found. Bees are a problem prey item for A. pustulosus; they yield a large amount of food for the spider and so are well worth capturing, but the hazards of being stung are always present and a number of adult spiders were observed misjudging the wrapping-up of either honey bees or bumble bees - and were seen to be stung. In all cases the spider died as a result (see Table 12).

There were some seasonal differences apparent in this species, but these tended to be obscured by the predominance of mycetophylids in the prey count. In winter this group assumed even greater significance. 169 captures being found. In summer psyllids were numerically important but would be significant only for the small spiders. In autumn aphids and ants (N = 34 and 47 respectively) are important but mainly for smaller spiders.

I. martius

Like A. pustulosus, I. martius relies heavily on Diptera for its food intake. This order made up 57% of the total prey caught. The difference between these two spiders is the wider spread among the families within the Diptera by I. martius. The Mycetophylidae are not the dominant item as in A. pustulosus. Here Muscidae. Acalyptrata and Tipulidae are all very important with Calliphoridae and Stratiomyiidae being significant too. As in A. pustulosus, flying ants were caught and formed a useful addition to the diet in summer and autumn. Cicadas form an important item in summer (N = 15) and because of their large size must be even more important than their numbers alone suggest. In winter prey are scarce and flies such as craneflies, fungus gnats, muscids and blowflies are particularly important for the continued growth and maturation of I. martius. As the spider grows, smaller prey, like psyllids and ants which are present in significant numbers in the food of the spiderlings, become unimportant for mature spiders. The larger flies such as tipulids and stratiomyiids along with the cicadas are very important in the diet of the mature spiders.