Two-spotted Cobweb Weaver and kin

Colorado Springs is blessed with a number of concrete bike trails throughout the city, including parks like Garden of the Gods. At this time of year, one can find numerous insects, spiders, and other arthropods parading across these paths. One example I encountered the other day was a male of the Two-spotted Cobweb Weaver, Asagena americana.

Male Asagena americana, Garden of the Gods

This arachnid could easily be mistaken for some kind of ground-dwelling spider instead of a web-spinning one. Males are only 3.2-4.4 millimeters in body length, but they are stout, with short, muscular legs. The underside of the femur of each leg is studded with short teeth and/or spurs. Females are much more delicate in appearance by comparison, but only slightly larger (3.5-4.7 mm).

While most male spiders go wandering in autumn, this is one of those species found most commonly in spring. Males have been collected from May through July, females May through August. The species ranges across most of North America.

Being conspicuous has its price. Male and female Asagena americana are frequent victims of Black and Yellow Mud Dauber wasps that sting them into paralysis and stuff them in a mud cell as food for the wasp's larval offspring.

Another interesting feature of the male is his "singing" mechanism. The rear of the carapace bears a "stridulating organ" on each side of the pedicel (the narrow connection between cephalothorax and abdomen). The front edge of the abdomen has a semicircular hardened plate that apparently rubs against the stridulating organs to create sound.

Female Two-spotted Cobweb Weavers can be found in their webs under stones, among leaf litter, under bark on decaying logs, and in moss. Mated females produce spherical, translucent egg sacs 4-5 millimeters in diameter containing 20-30 pale yellow eggs.

Asagena fulva from Tucson, Arizona

A related species, Asagena fulva, is common in the southwest U.S., north to Oregon and east to Florida. It is slightly larger, females ranging from 3-5.9 mm, males 2.4-5 mm. They appear "redder," with more white markings than their two-spotted cousins. I found them commonly under stones, garbage barrels, and other cover in the Sonoran Desert of Tucson, Arizona. This species might be an ant specialist, as entomologists have found them camped out in webs at the edge of harvester ant nests, presumably ambushing worker ants as they leave, or return to, the nest.

Both of these species were formerly classified in the genus Steatoda, several species of which are frequently mistaken for black widows.

Asagena fulva from Tucson, Arizona

Sources: Bradley, Richard A. 2013. Common Spiders of North America. Berkeley: University of California Press. 271 pp.
Fitch, Henry S. 1963. Spiders of the University of Kansas Natural History Reservation and Rockefeller Experimental Tract. Lawrence: University of Kansas Museum of Natural History. Miscellaneous Publication No. 33, pp. 1-202.
Levi, Herbert W. 1957. "The Spider Genera Crustulina and Steatoda in North America, Central America, and the West Indies (Araneae, Theridiidae)," Bull. Mus. Compar. Zool. 117(3): 367-424.

Ant-hunting spider, Euryopis

To look at a specimen of the genus Euryopis, you would hardly imagine that it is a member of the cobweb weaver family Theridiidae. Apparently it has physical characters that place it there, but when it comes to behavior, this spider departs radically from its tangled-web weaving cousins

I was invited to participate in an informal "bioblitz" back on June 22, 2014, in the upper reaches of the Monument Creek watershed here in El Paso County, Colorado, an elevation of 8,500-9,000 feet. Among our small party were several botanists, one of whom brought me a couple of spiders she found while looking at plants. One of those spiders was a gravid (egg-laden) female Euryopis that I brought home to take better images.

Euryopis is placed in the subfamily Hadrotarsinae of the Theridiidae family based on shared characters like a high carapace (top of cephalothorax) that is nearly as wide as it is long; exceptionally long, flat fangs; certain anatomy of the female genitalia; and the fact that these spiders do not spin prey-catching webs.

There are about twenty species of Euryopis recorded for North America north of Mexico, and all of them are very small. Both genders range from only 1.3-4.7 millimeters in body length, males slightly smaller than females. Most species share the unique sub-triangular shape and bold color pattern on the abdomen as demonstrated by the Colorado specimen shown here.

Typical ambush pose

What is truly amazing about these spiders is their hunting technique. They simply sit with legs outstretched, either on the ground, foliage, or tree trunk, and wait for an ant to brush against them. This triggers a response on the part of the spider, whereby the arachnid runs around the ant, all the while flinging silk from its spinnerets and bonding the ant to the surface of the substrate before the insect knows what happened.

Once the prey is immobilized, the spider bites it, usually on a leg joint, and waits while the venom takes effect. The spider then fashions a silk "sling" that it uses to haul the victim to a place where the spider can feed without disturbance from other ants.

Ants are aggressive and pretty vicious, so few predators bother them. It is a testament to the toughness of Euryopis to mess with them. At least one species feeds almost exclusively on harvester ants, which are among the most pugnacious of all North American ants, equipped with stingers that they deploy without hesitation (Porter and Eastmond, 1981)

My little female spider soon spun an egg sac in captivity. The size of the bundle was surprising considering how small the spider was. It was white, hemispherical in shape, and covered in little tufts of silk as shown in the image above

Euryopis are probably not uncommon spiders, but easily overlooked due to their diminutive size and rather cryptic appearance when presented on the ground or other natural surface. Try seeking them out around ant mounds and places where ant traffic is high.

Sources: Adams, R.J. 2014. Field Guide to the Spiders of California and the Pacific Coast States. Berkeley: University of California Press. 303 pp.
Agnarsson, Ingi. 2004. "Morphological phylogeny of cobweb spiders and their relatives (Araneae, Araneoidea, Theridiidae)," Zool. J. Linn. Soc. 141: 447-626
Bradley, Richard A. 2013. Common Spiders of North America. Berkeley: University of California Press. 271 pp.
Porter, Sanford D. and David A. Eastmond. 1981. "Euryopis coki (Theridiidae), a spider that preys on Pogonomyrmex ants," Research Notes in J. Arach. 10: 275
Ubick, D., P. Paquin, P.E. Cushing, and V. Roth (eds.). 2005. Spiders of North America: an identification manual. American Arachnological Society. 377 pp.

Candystripe Spider

Our most familiar cobweb weavers, like the Common House Spider and black widow, seem most abundant in or around buildings, so it might come as a surprise to learn that many, if not most, members of the family Theridiidae live outdoors in the strictest sense. One of the larger “wild” cobweb spiders is the Candystripe Spider, Enoplognatha ovata. It is also known as the “Polymorphic Spider” because it has several color variations.

This species is probably native to Eurasia, introduced to North America long ago. It is now a very common spider in New England, the Great Lakes region, and the Pacific states inland to Montana and Utah. They spin their tangled snares in understory vegetation in woodlands, and among wildflowers and herbs in meadows and fields. The spiders usually hide by day under a curled leaf. Females guard a white or bluish egg sac in late summer.

Individual specimens vary from entirely white, cream, or yellow in color with or without parallel rows of black spots on the abdomen, to having paired red stripes down the back, or even a broad central red stripe. Adult females measure 4.3-7 millimeters in body length. Mature males, 3.5-5.2 millimeters, are distinguished from females by their modified pedipalps and the elongated jaws (chelicerae).

Male

Spiderlings emerge from egg sacs in autumn, and overwinter in leaf litter and other protected niches on the ground. As they grow they are subject to parasitism by larval mites (ironically, mites are also arachnids) of the families Trombidiidae and Erythraeidae (Reillo, 1989).

I vividly recall finding this species commonly during my childhood in Portland, Oregon. Years later I found more during a visit to various forest preserves in suburban Chicago, Illinois. I find it interesting how some species are emblematic of one’s life and interests, be it birds, reptiles, or insects and spiders.

Sources: Reillo, Paul R. 1989. “Mite Parasitism of the Polymorphic Spider, Enoplognatha ovata (Araneae, Theridiidae), from Coastal Maine,” J. Arachnol. 17: 246-249.
Sollfors, Stefan. 2008. “Enoplognatha ovata,” Eurospiders.com.
Anonymous. 2013. “Comb-footed Spider, Enoplognatha ovata,” NatureSpot.

Spider Sunday: Triangulate Cobweb Weaver

One of the spiders most frequently encountered in basements, garages, and cellars over much of North America is the Triangulate Cobweb Spider, Steatoda triangulosa. It is a member of the cobweb weaver family Theridiidae, and therefore related to black widows. Not surprisingly, it is sometimes mistaken for a widow.

This species is widely distributed in the U.S., from Massachusetts to Nebraska, Colorado, Utah, Oregon, and California, and all points south (though nearly absent in the southwest and in Florida). It is also recorded from southern Ontario, Canada. It is also known from central and southern Europe, the Mediterranean region, and what was formerly southern Russia. Indeed, it is suspected that S. triangulosa was introduced to North America from overseas.

Adult females are only 3.6-5.9 millimeters in body length, males even smaller at 3.5-4.7 millimeters. Both genders usually sport the same distinctive pattern on the abdomen: a background color of dirty white or beige with a pair of bold, wavy, rusty or purplish brown lines down the back. The cephalothorax is normally dark reddish brown, and the legs pale brown or yellowish with dark bands at the joints. Males have a more slender, leggy appearance than females.

The Triangulate Cobweb Weaver spins an irregular, tangled web with a more-or-less sheet-like central platform. It seems to prefer dark recesses where it can dash into a crevice if disturbed. Look for them in your basement, cellar, garage, water meter box, or under bridges and in culverts, where the entire web may be protected from the elements. The spider hangs upside down in its web.

Mating occurs from late spring through early autumn. Females deposit about thirty eggs at a time, wrapping them in an opaque, white, spherical sac. The eggs may be seen through the sheer fabric enclosing them. A female may produce six or more egg sacs in her lifetime.

This species preys on a variety of insects, including the Red Imported Fire Ant (“RIFA”) in the southern U.S. Unfortunately, it is an unlikely candidate as a biological control agent of that pest (MacKay and Vinson, 1989). This is apparently a spider that “plays well with others,” as it may build its web in relatively close proximity to the webs of other cobweb weavers, cellar spiders, and even brown recluse spiders.

Don’t be alarmed by the presence of this species. It is not recognized as being dangerously venomous to people or pets. Indeed, it is such a small spider that its bite is unlikely to puncture human skin anyway

Sources: Howell, W. Mike and Ronald L. Jenkins. Spiders of the Eastern United States: A Photographic Guide. Boston: Pearson Education, Inc. 361 pp.
Jackman, John A. 1997. A Field Guide to Spiders & Scorpions of Texas. Houston: Gulf Publishing Company. 201 pp.
Jones, Dick. 1983. The Larousse Guide to Spiders. New York: Larousse & Company, Inc. 320 pp.
Levi, Herbert W. 1957. “The Spider Genera Crustulina and Steatoda in North America, Central America, and the West Indies (Araneae: Theridiidae),” Bull Mus Comp Zool 117(3): 367-424.
MacKay, W.P. and S.B. Vinson. 1989. “Evaluation of the spider Steatoda triangulosa (Araneae: Theridiidae) as a predator of the red imported fire ant (Hymenoptera: Formicidae),” J NY Entomol Soc 97: 232-233.
Moulder, Bennett. 1992. A Guide to the Common Spiders of Illinois. Springfield: Illinois State Museum Popular Science Series, vol. X. 125 pp.

Spider Sunday: Common House Spider

One thing about spiders: You don’t have to stray far from your home to find them. There is perhaps no better example of this than the Common House Spider, Parasteatoda tepidariorum. This species is so inextricably tied to human habitations that it is seldom found “in the wild.” It also owes most of its cosmopolitan geographic distribution to our global commerce in plants and other products.

Indeed, it is suspected that while the Common House Spider was first described from specimens collected in Germany, the species probably has its origins in South America where its closest allies reside (Edwards, 2001).

The species was initially assigned the name Theridion tepidariorum by Karl Ludwig Koch in 1841, but U.S. scientist A. F. Archer transferred it into his newly created genus Parasteatoda in 1946. Herbert W. Levi, a Harvard University arachnologist, then placed it in the genus Achaearanea in 1955. The current placement is back in Parasteatoda, where it is one of two species in that genus in North America, 46 species total in the world (Howe, 2012). Depending on the reference you consult, tepidariorum could be in either Parasteatoda or Achaearanea.

The pinball path of scientific nomenclature is almost matched by the number of aliases the species has in the English language: Common House Spider; American House Spider; Domestic Spider; Common Gray House Spider (in Myanmar and Pakistan).

The Common House Spider is a member of the family Theridiidae, collectively known as “cobweb weavers” or “comb-footed spiders.” Most possess a series of barbed spines on the last segment of the fourth leg. The spines help to “comb” silk from the spinnerets. The webs are large, three-dimensional structures that seem haphazard and tangled. Abandoned webs collect dust and appear as the classic “cobwebs” of scary movies.

The webs are very effective traps, however, and while flying insects can be intercepted by the irregular network of threads in the body of the web, the lines that anchor the web to the ground, or lowermost substrate, are the real traplines. Studded with sticky blobs of liquid silk, they adhere to passing ground-dwelling insects, other spiders, or even small vertebrates, holding them fast. The spider then hoists its victim into the body of the web where the spider swaths it in additional silk and renders the prey immobile. A bite is then administered and the venom acts to both paralyze or kill the prey and begin the digestive process from the inside out.

Parasteatoda tepidariorum is not considered to be dangerously venomous to people or pets, but it is frequently misidentified as the Brown Widow. Both spiders are in the family Theridiidae, build similar webs, are roughly the same size, and very similar in coloration. Indeed, I have found both species living virtually side-by-side in Redondo Beach, California. Note that the Brown Widow has a red or orange hourglass marking on the underside of the abdomen. The Common House Spider lacks this distinguishing feature.

Mature females of P. tepidariorum measure 5-8 millimeters in body length. Males average 4 millimeters. Males may share the webs of females as shown in the image below. The male is the slightly smaller, darker individual on the right. Females can live more than a year, and many individuals may occupy a relatively small area, their webs nearly contiguous. That does not mean they are peaceable, however. One that strays into its neighbor’s web may end up as dinner.

Mated females produce numerous egg sacs from late spring through late summer. The tan, papery, pear-shaped sacs may contain from 100 to more than 600 eggs each. They are familiar objects in the web, and easily identify the species.

Look for the webs of this spider in a variety of places: Under the eaves of buildings, in rock retaining walls, window wells, and tree holes. I recall finding a population of about six specimens thriving between panes in a storm window in western Massachusetts. There they fed on stray insects like the unfortunate Multicolored Asian Lady Beetle wrapped up in the image below.

I will bet you can find your own Common House Spiders today. Go take a walk around your home and see for yourself.

Sources: Barnes, Jeffrey K. 2003. “Common House Spider,” Arthropod Museum Notes No. 17, University of Arkansas Department of Entomology.
Edwards, G. B. 2001. “Common House Spider, Achaearanea tepidariorum (C. L. Koch) (Arachnida: Araneae: Theridiidae), University of Florida IFAS Extension, EENY-238.
Howe, Amanda. 2012. Personal communication.

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.

Spider Sunday: Western Black Widow

The first time I encountered a black widow was in the desert of eastern Oregon. I cannot recall the circumstances exactly, but I was rather awed, given that I had read about them but never seen one until then. Thanks to the book How to Know the Spiders, by B. J. Kaston, I learned the species found in the Pacific Northwest is the Western Black Widow, Latrodectus hesperus.

Eventually, I was able to easily identify black widow webs, even if the spider was not present. Black widow webs are large, three dimensional tangles of extremely strong threads. The whole web can easily occupy a cubic foot or more, and you can pluck the silken strands like guitar strings without them breaking. A funnel-like retreat is usually seen going into a rodent burrow, under a boulder, or some other cavity that would take heavy machinery to break into. Black widows do not want anything to do with larger animals. Only at night are you likely to see the spider out on her web.

Believe it or not, black widows begin their lives as largely *white* spiders. They lose the pale spots and stripes as they age, but because males mature more quickly, in fewer molts, they never lose their pale markings entirely. Even the immature spiders sport at least some semblance of an “hourglass” marking on the underside of their abdomen, so you can still identify them as widows. Since widows sit upside down in their webs, that hourglass marking is usually visible.

The Western Black Widow is the largest North American member of the genus Latrodectus. Females have a body length of 14-16 millimeters as adults, males 7-8 millimeters. Mature females usually lack any red markings besides the hourglass, and that may be broken or even wanting in some specimens. Some individuals may be chocolate brown instead of black.

Mature males are recognized by their swollen pedipalps, which resemble tiny boxing gloves located near the spider’s face. He uses these as intromittent sex organs that fit like a key in the “lock” of the female’s paired genital openings. Contrary to popular myth, the female does not always cannibalize her mate, though sex is a risky business for nearly all male spiders.

Mated females can produce several egg sacs in their lifetime, each containing up to 750 eggs. Few of the spiderlings that emerge will make it to maturity themselves, due to cannibalization by siblings and other natural hazards. Adult females can live more than a year, though.

Western Black Widows are able to secure surprisingly large prey. Their webs are designed to trap prey walking over the ground. When a victim stumbles into one of the sticky trip threads attached to the ground, the thread breaks and rebounds, yanking the potential prey animal into the main body of the web. The trip threads are so elastic and strong that even small vertebrates can be captured in widow webs. I personally witnessed a hatchling lizard struggling in a web. Yes, I intervened.

While black widows have no problem killing prey as imposing as large beetles, they flee rapidly when they sense a larger creature such as a human. The speed at which they can scramble back into their retreat is astonishing. I have had many a photo opportunity cut short when I accidentally bumped a thread.

The Western Black Widow is a very common spider found from extreme southern British Columbia south to Mexico and west to Kansas, Oklahoma, and Texas. It is particularly abundant in drier habitats such as deserts and prairies, though it can be encountered in more damp situations as well.

Because of the timid nature of widows, it is relatively easy to minimize the possibility of being bitten. Simply take these precautions:

• Never leave clothing, shoes, or gloves outdoors overnight. A spider can seek shelter during that time.
• Do not put your hands or feet where you cannot see into, such as holes and crevices, inside the mailbox, or behind large objects that have been in storage for awhile.
• Carefully inspect firewood, houseplants, toys, etc being brought indoors from outside in case a spider may be hitchhiking on the object.
• Do not walk barefoot outdoors, especially at night.

Reactions to bites from widows can vary considerably from one person to the next because immune system responses to envenomation are highly individualistic. The spider may not even deliver much, if any, venom. Still, they typical experience is excruciating. The venom is neurotoxic, meaning it affects the nervous system. This translates to triggering constant muscle contractions that result in severe cramps, especially in the abdomen, legs, and other large muscle groups. One friend of mine had back spasms for months following a black widow bite. One should always seek emergency hospitalization immediately, precisely because one cannot be sure how their body will react. Antivenin is available for treatment, though it is recommended as a last resort by most hospital physicians. Antivenin is produced from horse serum, which can carry its own complications including allergic reactions. Obviously, avoiding bites is preferable to treating them after the fact.

Black widows cannot be easily “controlled.” Contact insecticides have a very low probability of reaching a hidden spider, and killing one spider means it will soon be replaced by another anyway (prime web sites are a coveted commodity). Meanwhile, the spiders will kill plenty of pest insects that could potentially be more trouble than the spiders themselves. By all means, do inspect playground equipment and toys before allowing your children to play outdoors. It all comes back to vigilance and prevention. Take care, but value the work your spiders do.

Spider Sunday: Brown Widow

Spiders are very good at exploiting us humans, and using our vehicles and cargo to conquer new territory. One example of this phenomenon is the Brown Widow, Latrodectus geometricus. Exactly where this species is native to is a matter of speculation. It is known from South Africa, the Middle East (Afghanistan), the Mediterranean (Cyprus), Australia, the southern United States, and even Japan.

Here in the U.S., L. geometricus has been well-known in the southeast, but has been extending its range up the Atlantic Coast (to North Carolina so far) and across the entire southern tier of states (Tennessee, Texas, southern California). There have also been reports from Nevada and Colorado. The specimens shown here were imaged along a retaining wall at an undisclosed location in Redondo Beach, California (a friend’s house).

According to arachnologists, despite the virulence of the venom of the Brown Widow, the species should not be considered dangerously venomous to the average, healthy adult person. The spiders apparently inject less venom than black widows, and the effects are more localized than systemic. One can avoid the potential of a bite simply by not placing his or her extremities where they can’t see, and avoiding clutter in yards, sheds, garages, and other situations where the spiders are likely to occur.

The Brown Widow is very likely to be confused with a completely innocuous spider known as the American House Spider, Parasteatoda tepidariorum. The two are nearly identical in appearance, but the House Spider never has a red hourglass marking on the underside, whereas the Brown Widow does. Like most widows, the Brown Widow prefers to sequester itself deep inside cracks or crevices by day, venturing out into its web only at night. House spiders do not have retreats, so are visible at all times, though usually tucked snugly against a wall or other surface during the day. The House Spider also has a much broader distribution, being found across most of the North American continent. Below is a pair of House Spiders, male on the right.

The egg sacs of the Brown Widow are highly distinctive, being spherical but bearing numerous tufts of silk (see images below). Both the Brown Widow and the House Spider belong to the family of "cobweb weaver," Theridiidae, which only adds to the confusion in identifying them. They both build extensive, tangled snares.

The “spread” of the Brown Widow to the west coast is a relatively recent occurrence, the first specimens being noted in the Los Angeles area in February, 2003 as a result of the Los Angeles Spider Survey being conducted by the Natural History Museum of Los Angeles County. Verified accounts of this species from elsewhere along the Pacific Coast would be welcome. It has been suggested that trucks, cars, and recreational vehicles have mostly been responsible for the assisted migration of this species. That seems reasonable, especially in the case of RVs, which often sit idle for long periods, allowing spiders to establish themselves there.

Sources: Brown Widow Spiders
Santana, Fred. 2007. Brown Widow SpidersUF/IFAS Sarasota County Extension
Levi, Herbert W. and Lorna R. and Herbert S. Zim. 1990. Spiders and Their Kin. New York: Golden Books. 160 pp.