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Return to Texas Entomology - Compiled by Mike Quinn
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(female with young in shadow)
Range: This species is native from New England to Northern Argentina (Agnarsson 2006)
U.S. records from: CT, DC, GA, FL, LA, MD, MS, NC, SC, TN, TX, VA (Draney, 2001)
State map from Dean (pers. comm., 2007), Continental map from Agnarsson (2006)
Abundance:
In Texas, "colonies can be found consistently in the branches of live oak trees along the shores of Lake Somerville." (Jackman 1997) (Jackman's bio)
In Florida, large colonies are apparently rare. Muma (1975) studied spiders in Florida citrus groves for 20 years and reported that the communal spider, Anelosimus studiosus "...may enclose from several leaves to a whole twig of the outer canopy."
Similarly, Deyrup et al. (2004) examined 503 webs of A. studiosus at the Archbold Biological Station, Lake Placid, FL and found that "the webs are always on outer twigs of trees and shrubs."
Time of activity: (Allen Dean, pers. comm., Aug 2007)
Males: March, April, May, June, July, August, September
Females: April, May, June, July, August, September, October, November, December
Habitat: Webs often found near water (Brach, 1977; Jackman 1997).
Size: Adult spiders are relatively small, usually about 8 mm or 5/16" long.
Behavior:
Cobweb Spiders in the genus Anelosimus display varying levels of sociality from temporary webs with multiple individuals (subsocial) to semi-permanent webs or nests composed of multiple generations of spiders (quasisociality).
Sociality among spiders is extremely rare, having been found in only 23 (primarily tropical) species in eight families among the world's 38,000 known spider species. Unlike social insects, with their division of labor where the workers are sterile and only the queen lays eggs, all social spiders in a colony are able to reproduce. In addition, social spiders all cooperate in, and benefit from, web maintenance, prey capture and brood care (Jones & Parker 2002). Social spiders have been likened to a pride of lions as they are both exceptions to the norm of predacious and anti-social behavior of their respective groups. (Helmuth 1999)
Female Social Cobweb Spiders, Anelosimus studiosus, communally guard the egg-sacs and open them when it's time for the spiderlings to emerge as the spiderlings are unable to emerge by themselves. The immature spiderlings are then fed by the adult spiders through regurgitation (Viera et al. 2007). Social Cobweb Spider juveniles occasionally remain in their natal webs with their mothers until maturity, forming multigenerational colonies in which all individuals cooperate in web maintenance and prey capture.
Florida researchers studied 99 Social Cobweb Spider nests and found that on average, juveniles in colonies survived longer, developed faster, and had more resources per individual than did solitary juveniles of the same species. Mothers in a communal groups survived longer and produced second broods earlier than mothers in the non-communal groups. In addition, larger colonies captured more and larger prey. Jones & Parker (2002)
Group living permits spiders to capture prey as large as 10 times their size, whereas an individual spider can usually only manage to subdue an insect twice as big as itself (Helmuth 1999). Uetz (1989) suggested that large colonial spider webs are more effective due to the "Ricochet effect", where prey escaping from one web are deflected onto others, resulting in higher rates of prey capture.
Interestingly, social spider colonies often have a significantly female-biased sex ratio, sometimes as high as 9:1 (!) (Avilés et al. 2000). Forming a colony with a preponderance of females facilities population growth during times of plenty as perhaps occurred in the photograph above. In times of scarcity, massive social webs are never found.
Given that delayed juvenile dispersal benefits both juveniles and mothers (Jones & Parker 2002), the question naturally arises, why don't more species of spiders practice communal living? One reason may be that the non-dispersal of juvenile spiders from their natal nests results in colonies that are highly inbred, an unusual condition among cooperatively breeding animals (Bilde et al. 2005).
So the collective living traits of social spiders, while advantageous in the short term, may ultimately result in a lower genetic diversity and thus an evolutionary dead end. (Agnarsson et al. 2006)
While sociality is quite rare in spiders, there is a rich and growing body of literature on these fascinating spiders.
Social Cobweb Spider Photos:
Etymology: Anelosimus studiosus (Hentz, 1850)
stud, -en, -i (L). Be diligent; zeal
Biography: Nicholas Marcellus Hentz (1797 - 1856) - Wikipedia
Similar Species: There are 2281 species (in 96 genera) of Theridiidae spiders (Platnick 2000-2007)
There are three species of Anelosimus in the U.S. (Agnarsson 2006), but only A. studiosus occurs in Texas. (Dean, 2007)
Anelosimus analyticus (Chamberlin, 1924) - Southern California
Anelosimus arizona Agnarsson, 2006 - Southeastern Arizona
formerly considered to be: Anelosimus jucundus (O. P.-Cambridge, 1896) - Mexico to Columbia
Anelosimus studiosus (Hentz, 1850) - Southeastern U.S.
Notable Cobweb Family Relatives:
Cobweb Family, Theridiidae, includes the well known Widow Spiders, Genus Latrodectus and the Common house spider - Achaearanea tepidariorum
Social Spider Web Associates:
Interestingly, a wide variety of other arthropod species live in association with social spiders. Seibt & Wickler (1988) found two species of social spiders in Africa to "exhibit extreme intra as well as interspecific social tolerance." An example of this tolerance was found by researchers in Florida who examined the contents of 39 social cobweb spider webs, Anelosimus studiosus, and found 13 species of spiders (in 9 families) and 11 species of insects coexisting with the social spiders. (Deyrup et al. 2004)
Here are the scientific names of the other spiders the researchers found. The number following the name indicates the number of samples (out of 39) that had the spider; numbers of individuals are not tallied.
- Araneidae: Metazygia sp. (3), Eustala sp. (1);
- Linyphiidae: Florinda coccinea (Hentz) (2);
- Salticidae: Hentzia palmarum (Hentz) (1), Hentzia sp. (5), Peckhamia sp. (1);
- Anyphaenidae, unidentified to genus (8);
- Clubionidae: Castianeira sp. (1);
- Theridiidae: Dipoena sp. (1); Argyrodes trigona (Hentz) (4);
- Tetragnathidae: undetermined to genus (6);
- Mimetidae: Mimetus sp. (2);
- Oxyopidae: Peucetia viridans (Hentz) (2);
The caterpillar of one species of pyralid moth, Watson's Tallula Moth - Tallula watsoni, was considered by the researchers to be an obligate associate (inquiline) of the social cobweb spider. In laboratory experiments, when the adult moths emerged from their pupal stage, they were always able to escape from the spider's web. The caterpillar seemed to have no positive or negative effect on the social spider and the researchers offered no reason why the spiders tolerated the edible caterpillars in their webs. (Deyrup et al. 2004)
There are five species of Tallula snout moths in North America, but it's not known if the caterpillars of any other species are obligate associates of social spiders.
Tallula atramentalis Lederer 1863
Tallula atrifascialis Hulst 1886
Tallula baboquivarialis Barnes and Benjamin 1926
Tallula fieldi Barnes and McDunnough 1913
Tallula watsoni Barnes and McDunnough 1917
Other U.S. Social Spiders:
Metepeira spinipes, Orb Weaver Family Araneidae, is the most widely-distributed of the colonial web-building spiders in its genus, ranging from the central valley of Mexico to Northern California and Oregon.
Metepeira species in Texas (may be nonsocial):
Metepeira arizonica Chamberlin & Ivie, 1942
Metepeira comanche Levi, 1977
Metepeira foxi Gertsch & Ivie, 1936
Metepeira labyrinthea (Hentz, 1847) - Labyrinth Orbweaver
Metepeira minima Gertsch, 1936
Social Spider Websites:
Other Social Spider Web Photos:
World's Largest Communal Spider Web?
Spiders weave huge natural wonder in B.C.
November 27, 2002 - CBC News
PRINCE GEORGE, B.C. - A biology professor [since retired] in northern British Columbia has spotted a clover field crawling with spiders.
Brian Thair of the College of New Caledonia in Prince George said he saw a silky, white web stretching 60 acres across a field. [It was first spotted in the week of Oct. 27.]
"When you see horror movies with spider web festooned from this place to that place and so on, it comes nowhere near approaching what occurred in this field," Thair told CBC Radio's As It Happens.
A typical barbwire fence on wood posts surrounded the field about six kilometres east of McBride in the Robson Valley. Thair said it looked like the whole area was covered with an opaque, white plastic grocery store bag.
The thin, elastic coasting was not soft and fluffy like webs built by individual spiders. There were about two spiders per square centimetre laying the silk, which first appeared in early October.
Thair said the web showed great tensile strength – enough to put a handful of coins on it without them falling through.
There were "in the order of tens of millions of spiders running frantically back and forth." <snip>
This massive spider web proved not to be from social spiders, but rather sheet weaver spiders of the family Linyphiidae undergoing a "large-scale autumnal migration." Bennett (2003) reported "critical examination of over 250 of the specimens revealed that all but one are adult erigonine linyphiids and the great majority are Halorates ksenia (Crosby & Bishop), listed as Collinsia ksenius (Crosby & Bishop) by Platnick (2002)."
"Although apparently not previously recorded in Canada, large-scale autumnal migrations of erigonine spiders have attracted considerable attention elsewhere. Similar events have been described from a wise variety of north and south temperate locations including the United States, the United Kingdom, continental Europe, Greenland [!], Argentina, and Australia." (Bennett 2003)
Closer to home, in North Texas, Salmon & Horner (1977) studied the aerial dispersal of spiders collected in a suction trap located on the roof of the Science Building at Midwestern State University, Wichita Falls, TX,
"In a 365 day period 3400 ballooning spiders belonging to 14 families were collected in a suction trap. The four most common ballooning families were Erigonidae, Thomisidae, Oxyopidae, and Tetragnathidae. These four families make of 77% of the total. Peak periods of spider aerial dispersal occurred during early spring through early summer and late summer through fall."
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08 Oct 2007 © Mike Quinn / mike.quinn@tpwd.state.tx.us / Texas Entomology
