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.

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: 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: 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.