Last week I wrote about one of North America’s largest spiders, the Golden Orb Weaver (Argiope aurantia). While photographing the specimen from Ohio last month (see below), I discovered something amazing: a dewdrop spider was living on the web of the orb weaver.
It turns out that this itty-bitty spider (below) lives as a “kleptoparasite” of the orb weaver.
Kleptoparasites are animals that steal the food of their hosts. Indeed, that is what a dewdrop spider does. Whether or not its activities impact the host spider is debatable. A study of a species that lives in the webs of Nephila orb weavers revealed that host spiders do not gain weight as much as spiders that do not host dewdrop spiders, and that they relocate their webs more often than non-host specimens (Grostal and Walter, 1997).
Dewdrop spiders are in the cobweb weaver family Theridiidae, and genus Argyrodes.
There are three species in North America, all confined to the United States, and mostly the southern U.S. The species imaged here might be Argyrodes elevatus. Interestingly, in this genus the males are usually larger than the females. The modified pedipalps of “my” specimen reveal it to be a male.
Some Argyrodes have been recorded as actually preying on the host spider. I found a paper online that documented this for a species that uses labyrinth spiders (Metepeira sp.) as a host (Wise, 1982). That makes sense. Argyrodes are tiny, only 2-4 mm in body length, and I can’t see them killing something as large as a female Argiope or Nephila. Maybe they can kill intruding males, though. Given that male Argiope and Nephila are several orders of magnitude smaller than females, they could indeed be vulnerable to an ambitious Argyrodes. Both kinds of spiders tend to frequent the perimeter of an orb web, too, where they would easily come into conflict.
Argyrodes may be considered as being “commensal” when it only takes prey in the host web that is too small for the host to bother with (like the tiny winged ant in the images here). Commensalism is defined as a relationship whereby one organism benefits and the other is not affected positively or negatively.
However, another negative impact that Argyrodes can have on its host is damage to the web. Some dewdrop spiders are known to actually eat the silk itself, and others create gaping holes in a snare when they actively remove prey to an area outside of the web where they can dine without fear of detection by the host.
Next time you come across a large orb web, take a minute to scan for dewdrop spiders. Our understanding of the relationships between Argyrodes and their hosts is still in its relative infancy, and observations you make and record could shed light there.
Sources: Grostal, Paul and David Evans Walter. 1997. “Kleptoparasites or commensals? Effects of Argyrodes antipodianus (Araneae: Theridiidae) on Nephila plumipes (Araneae: Tetragnathidae).” Oecologia, 111: 570-574.
Wise, D. H. 1982. “Predation by a commensal spider, Argyrodes trigonum, upon its host: an experimental study.” Jrnl. Of Arachnology, 10: 111-116.