|
|
|
Click on an image |
|
withC. ocreatus worker |
C.ocreatus worker |
On this page I will discuss in turn:
Spiders that occasionally prey on ants at my field site
The spiders that fed on ants at my field site fell into two broad classes. First, there are a number of spider species that, while not necessarily specializing on ants, will prey on them given the opportunity. As illustrated below, examples include black widows and jumping spiders. It should be noted, though, that elsewhere there are jumping spiders and widow spiders that feed primarily on ants (MacKay 1982 and references therein; Robinson and Valerio 1977; Foelix 1996).
|
|
|
Click on an image |
|
withC. ocreatus worker |
C.ocreatus worker |
Several spider species specialized on the ants at my field site. Most conspicuous of these were one and maybe two species in the genus Euryopis (E. texana and another unidentified species -- G. Binford, pers. comm.). Since I was unable to observe any differences in the behavior of these two species, I will speak of them as one in what follows.
Euryopis foraged at night and captured, with one curious exception, all species of extrafloral nectar gathering ants. Unlike other Therididae which use cobwebs to ensnare their prey, Euryopis does not build a web. Instead it begins hunting by moving to an area likely to be visited by foraging ants. This can be on the ground, along the rib of a barrel cactus, or near an extrafloral nectary. Here they spread their legs and wait for their prey to literally run into them. When an ant does this the spider rapidly turns away and uses the comb setae of its fourth legs to help it cast a wet viscid silk onto the ant (Carico 1978; Coddington 1986; Binford, pers. comm.). If the ant is small the silk not only ensnares it but also glues it to the substrate surface. Next, the spider turns to examine its prey. If the ant is thoroughly entangled the spider delivers a quick venomous bite to one of its legs. If not, the spider will turn and spray the ant with some more sticky silk until it is subdued.
An adult Euryopis is usually not content with capturing a single Forelius foetidus or Solenopsis aurea worker. So, rather than feed on the first ant captured, the spider typically moves a little distance away and awaits more. Since most small ants forage in groups the strategy often pays off. Additionally, ensnared S. aurea workers may release alarm pheromone until they are immobilized by the spider's poison. Unwittingly this often brings more prey to the spider. Sometimes, though, enough pugnacious S. aurea workers come to the aid of their fallen sisters to drive the spider off. But the effect is only temporary. The ants do not free their sisters and almost invariably the spider returns when things settle down and continues hunting.
At a good location an Euryopis adult may capture a dozen or so ants before it stops hunting. When it does stop hunting, it goes about collecting and attaching the ensnared ants to its spinnerets. A spider generally finds most of its victims but it isn't unusual for a few to be left behind. The spider then moves away from where it captured its prey to a cactus spine or some other overhanging object. Here, the spider suspends its clumped prey, lowers itself down by a strand of silk, and begins feeding. While this allows the spider to escape harassment from ants, it doesn't always prevent theft by members of their own species. Often, larger Euryopis individuals will steal bundles from smaller ones.
|
Euryopis with |
|
Click on image to enlarge to enlarge it |
When a spider finishes feeding it departs and leaves the cluster of ants hanging. On chainfruit chollas with active ant predation dozens of these little balls may dangle from spines on the lower portions of the main stem.
Euryopis also feeds on ants considerably larger than S. aurea. On my study plot it was not at all uncommon to find these spiders feeding on single Camponotus festinatus or Camponotus ocreatus workers.
|
Euryopis with |
|
Click on image to enlarge it |
It is easy to imagine that there are dangers in trying to catch ants this large. So how do the spiders do it? On one occasion I did see a spider shoot silk at a C. ocreatus worker on a barrel cactus. The partially ensnared ant tumbled to the ground where it hobbled around for a while until it was able to free itself from the sticky silk. Unfortunately, though, I never saw the complete prey capture sequence for either of the two Campontous species. Workers of both C. festinatus and C. ocreatus are quite sensitive to light and I suspect this changes their behavior in ways that make them less susceptible to spider predation.
Lacking direct observation of how Euryopis spiders captures large ants, we might hazard some inferences based on how another species of Euryopis captures its prey. In the eastern United States E. funebris preys on Camponotus castaneus, an ant similar in size and disposition to C. festinatus (Carico, 1978). For the most part, the prey capture sequence is similar to that described above for smaller ants. There are a couple of minor differences, though. First, rather than cover the body haphazardly, E. funebris appears to shoot most of its sticky silk at the legs of its victims. Second, the spider will frequently circle its entangled prey rapidly two or three times while paying out non-sticky silk. The first difference could simply be the trivial result of casting out silk at a constant angle with regard to the substrate: a cast that would hit a small ant in the body would hit a large a large one in its legs. As for the second difference, I didn't pay close enough attention to determine whether or not Euryopis uses non-sticky silk to catch small ants. But I certainly never observed the circling behavior that Carico describes. Why might this difference exist? When capturing large ants the spider not only has to entangle its prey but also needs to keep it from rolling or tumbling away. While sticky silk alone might glue down a small ant this strategy alone seems less likely to work with larger ants. Perhaps, then, circling while paying out non-sticky silk increases the likelihood that a large ant will become tethered to some part of its substrate like a cactus spine or piece of upturned bark.
Earlier I mentioned that Euryopis spiders feed on all but one of the species of ants that visit extrafloral nectaries on my study plot. The exception is Crematogaster opuntiae, easily the most common EFN using ant in the Tucson area. Euryopis clearly avoids these ants: I never found the spider on plants used solely by C. opuntiae. Occasionally, though, I did find C. opuntiae workers that had been killed by this spider. But invariably these ants were on plants also used by S. aurea and the corpse was always surrounded by dead S. aurea workers. Presumably these spider-caused deaths were the result of mistaken identification.
Why Euryopis refuses to prey on C. opuntiae is not known. But that it doesn't is interesting particularly when considering the regular annual turnover of extrafloral nectary using ant species (see S. aurea's annual cycle of invasion and abandonment of EFN-bearing cacti). Could Euryopis predation affect this cycle or even drive it? While there are many other possible explanations for this cycle spider predation as a cause cannot be summarily ruled out. Its been shown several times that predation, and in particular spider predation, can alter aboveground ant colony activity. By removing foraging workers, Gentry (1974) showed that colonies of the Florida harvester ant (Pogonomyrmex badius) may respond rapidly to perceived predation by sending out fewer workers to collect food. In a few instances the affected colonies even stopped sending out workers altogether for periods up to several weeks. Likewise, another harvester ant, Pogonomyrmex rugosus, may stop foraging for 7-10 days in response to predation by the western widow spider Latrodectus hesperus (MacKay, 1982). Finally, the spider Steatoda fulva, who builds its web very close to the nest entrance of P. badius colonies, may force these ants to stop aboveground foraging for several days until a new nest entrance is excavated (Holldobler, 1970).
The other ant specialist at my field site was a species of Dipoena. More specialized than Euryopis, it only preyed on the two Camponotus species and always captured them while on a cactus.
|
Dipoena "waiting" |
|
Click on image to enlarge it |
Beyond this I can say little about Dipoena's habits or method of prey capture. The spider shown in the two photographs spent a lot of time very near the extrafloral nectaries of one of the barrel cacti on my study plot. Also, as the video clip demonstrates (not yet available), its behavior around these nectaries suggests that it went there to prey on ants. Nevertheless and despite my best efforts, I was unable to video prey capture by this or any other Dipoena individual. Almost certainly this was because my camera lights were changing the two ant species behavior in a way that made them less susceptible to predation. On a number of occasions I turned off my camera lights for 10-15 minutes while I surveyed other plants -- only to return and find the subject of my observations consuming a Camponotus worker it had captured in my absence.
|
Dipoena with captured |
|
Click on image to enlarge it |
Carico, J.E. 1978. Predatory behavior in Euryopis funebris (Hentz) (Araneae: Theridiidae) and the evolutionary significance of web reduction. Symposia of the Zoological Society of London 42: 51-58.
Coddington, J. 1986. In Spiders: Webs, Behavior, and Evolution. Edited by William A. Shear. Stanford University Press, Stanford, California.
Foelix, R.F. 1996. Biology of Spiders. Oxford University Press, Inc., Oxford.
Gentry, J.B. 1974. Response to predation by colonies of the Florida harvester ant, Pogonomyrmex badius. Ecology 55: 1328-1338.
Holldobler, B. 1970. Steatoda fulva (Theridiidae), a spider that feeds on harvester ants. Psyche 77: 202-208.
MacKay, W.P. 1982. The effect of predation of western widow spiders (Araneae: Theridiidae) on harvester ants (Hymenoptera: Formicidae). Oecologia (Berl) 53: 406-411.
Robinson, M.H. and C.L. Valerio. 1977. Attacks on large or heavily defended prey by tropical salticid spiders. Psyche 84: 1-10.
Copyright (c) 1998 Barry Sullender
Rice University
All rights reserved