Spider Sunday: Dewdrop Spiders

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.

Wasp Wednesday: Pachysomoides fulvus

Sometimes, the worst enemy of a wasp is another wasp. Paper wasps in North America have much to fear from a stealthy little ichneumon wasp named Pachysomoides fulvus. I had the good fortune of finding one biding her time in the vicinity of a nest of Northern Paper Wasps, Polistes fuscatus in Adams County, Ohio last month.

This parasitic wasp ranges from New York to Florida, and west to California and British Columbia, but does not appear to be abundant anywhere.

The female wasp lays her eggs on larvae or pupae of paper wasps inside the cells of the exposed paper comb. Young nests populated only by the foundress are especially vulnerable since the “queen” must leave the nest to hunt food for her offspring. Interestingly, an image over at Bugguide.net appears to show one of these wasps ovipositing in the wall of a cell that has at most only an egg in the bottom. Perhaps the larval ichneumon would emerge once the larval paper wasp had hatched.

I found more detailed information on the life history of Pachysomoides fulvus difficult to come by online, at least without buying access to articles from journal warehouses like Jstor. I have also packed up all my books now in preparation for moving. I might revisit this unique wasp, and will alert you if I am able to add more.

There is a second species of Pachysomoides in the U.S. that bears the unfortunate name of P. stupidus. Considering it is willing to confront a population of much larger wasps, the epithet might be appropriate! P. stupidus occurs in the southeast, from North Carolina to Florida and Texas and southern Illinois. It is an ornate, black and white banded insect with orange legs.

In this species, the female wasp lays several eggs on a pre-pupal Polistes grub. The larvae that hatch from those eggs then feed gregariously as external parasites on the pupa. Because the larval paper wasp spins a silken cap over its cell before pupating, the parasitic ichneumon larvae can feed undetected by the adult wasps on the exterior of the nest. Up to thirty-one larvae have been observed on one paper wasp pupa (Pickering, 1980).

Sources: Pickering, John. 1980. Larval Competition and brood sex ratios in the gregarious parasitoid Pachysomoides stupidus. Macmillian Journals Limited.

Spider Sunday: Black and Yellow Argiope

The Facebook page for SpiderIdentification.org is really busy these days. It is no surprise. Spiders in the northern hemisphere, especially orb weavers, are reaching maturity now. Larger spiders spin larger, more conspicuous webs, often in situations where people notice them more often. The chief attention-getter in the United States right now is the Black and Yellow Argiope, Argiope aurantia.

The females are very large spiders, with a body length of 19-28 mm. Their bright Rorschach pattern of black and yellow might set off your “dangerous arachnid” radar, but no orb weaver of any kind is considered by scientists to be dangerously venomous to people or pets. Males, by contrast, are tiny, only 5-9 mm in body length, and magnitudes smaller in terms of body weight. Females need to build up energy reserves to be able to produce eggs.

The female Black and Yellow Argiope spins a rather small orb web given her size, usually in tall grass or shrubs no more than two or three feet off the ground, and usually lower. There, her snare can intercept large insects like grasshoppers.

A distinctive signature in the webs of most Argiope species (there are at five species north of Mexico) is a thick, zigzag band of silk running down the center of the web. This structure is called a “stabilimentum,” and its function remains something of a mystery. It may serve to shield the young spiders, which confine the stabilimentum to the hub of the web, from harm. The young spider quickly zips to the other side of the web when it feels threatened. Another hypothesis is that the stabilimentum is like a beacon on a tall building: it advertises the presence of the web to birds in flight so that the avian animal won’t destroy the web by accident. This comes at a cost, however. The presence of a stabilimentum can reduce prey-catching success by as much as thirty percent (Blackledge and Wenzel, 1999). That statistic also flies in the face of yet another hypothesis: that the stabilimentum most likely functions as a lure. The silk band stands out bodly in the ultraviolet end of the light spectrum, and many insects seeking flower nectar may mistake it for a raceme of flowers. Not all individual spiders spin a stabilimentum, and one might assume that webs spun higher in the vertical plane would be more likely to have one if the purpose was to deter bird strikes.

Look for male spiders lurking on the outskirts of a female’s web. They may be attracted to the female by a pheromone she emits (Olive, 1982). Approaching cautiously, a male may eventually be able to couple with the gargantuan object of his affection. He inserts one of his pedipalps into one side of the female’s epigynum (female external genitalia), and quickly pumps his sperm into her, hopefully before she renders him a meal. He will repeat the process with his other pedipalp on the opposite side of the female’s epigynum, if he is able. Should he succeed even once, he usually breaks off the tip of the pedipalp, which remains stuck in the epigynum. This “mating plug” therefore prevents any competing male from inserting into that opening (Foellmer, 2008).

Once mated, a female produces one or more egg sacs, each about the size of a large marble, and covered in tough, papery silk. Inside are 300-1,400 eggs. The eggs hatch in late autumn or early winter, but the spiderlings do not exit the egg sac. Instead, they go into diapauses, a dormant state with lowered metabolism. They emerge the following spring and reach adulthood by late summer. Various parasites and predators can take their toll on the egg sacs and spiderlings, however. One study found that 19 species of insects and 11 species of other spiders emerging from the egg sacs of Argiope aurantia. Chief among the parasites were the ichneumon wasp Tromatopia rufopectus, and the chloropid fly, Pseudogaurax signatus. The overwhelming predators of the egg sacs are birds. Nearly every egg sac found in the wild during the study had suffered damage from birds (Lockley and Young, 1993).

The Black and Yellow Argiope is also known as the “Writing Spider,” named perhaps for the stabilimentum, Golden Orb-weaver, and Yellow Garden Spider, among other aliases. I grew up learning it as the “Golden Garden Spider.” The species ranges from coast to coast in the United States, but is absent from arid regions.

Enjoy your encounters with these remarkable spiders while you can. The first frost is likely to claim their lives, if they do not perish in some other way before that date. See if you can determine what kinds of insects they are preying on, and whether their web location changes. There remains much to learn about even our most common and conspicuous arthropod neighbors.

Sources: Blackledge, Todd A. and John W. Wenzel. 1999. “Do Stabilimenta in Orb Webs Attract Prey or Defend Spiders?” Behavioral Ecology. Vol. 10, No. 4, pp. 372-376.
Foellmer, Matthias W. 2008. “Broken genitals function as mating plugs and affect sex ratios in the orb-web spider Argiope aurantia.” Evolutionary Ecology Research. Vol. 10, pp. 449-4462.
Lockley, T. C. and O. P. Young. 1993. “Survivability of Overwintering Argiope aurantia Egg Cases with an Annotated List of Associated Arthropods.” Journal of Arachnology. Vol. 21, No. 1, pp. 50-54.
Olive, Cader W. 1982. “Sex Pheromones in Two Orbweaving Spiders: An Experimental Field Study.” Journal of Arachnology. Vol. 10, No. 3, pp. 241-245.

An Entomologist in Hollywood?

Yesterday I applied to be a host for a new television series being produced by Asylum Entertainment "and a major cable network." Wish me luck. I did share the link to my YouTube demo reel, but I fear they may think I'm not animated enough. I'm no Steve Irwin, but I am no stick in the mud, either. Maybe they will notice the stand-up performance in my YouTube account....

I have a gut-level feeling this is a more legitimate media venue than some of the other opportunities that have crossed my desk, so I really want to make it to the next level in the audition process. My style is to deliver accurate information, allay fears, refute urban legend and folklore, and do so with empathy to those not as sympathetic to the arthropod realm as I am. There have been far too many over-the top hosts already.

Stay tuned, and feel free to offer your support here, on Facebook, and any other public outlets you can think of. Thank you.