ARTHROPOD MUSEUM NOTES
Number 24 • October 1, 2003 • Jeffrey K. Barnes
Genus and species: Micrathena gracilis (Walckenaer)
The dark-spotted, whitish abdomen surrounded by 5 pairs of black-tipped spines distinguishes the female of this common woodland spider. As in other Micrantha species, the fourth femur is longer than the first, and the book lung covers have stridulating files. Males do not resemble females and are only a fraction of the size, have a flattened, elongate, whitish abdomen. There is one generation per year. Individuals mature in July in the southeastern United States.
The genus Micrathena contains over 100 species of mostly Neotropical woodland orb-weavers (Levi 1985). M. gracilis is found in dense deciduous forests in eastern North America south to Costa Rica. Males do not construct webs after attaining sexual maturity. The female’s web is a small orb, 3.0 – 7.5 inches in diameter, typically three to seven feet above the ground in the understory. The viscid spiral may be vertical or sloped skyward up to 45 degrees off vertical. Webs are found in large open spaces in the shaded understory, where they are exposed to a diversity of flying insect prey. Females rest in the open hub during daylight, sensing vibrations from prey striking the web. They hang head down in the center of the web, with the abdomen horizontal and parallel to the ground. The brown and yellow undersurface of the abdomen faces upward and blends with ground litter and vegetation. The light colored upper surface of the abdomen faces downward and camouflages the spider against the light blotches of the canopy. The orb is renewed daily, but the triangular or rectangular silk frame may persist for days or weeks in the same position. At dusk, the female ingests virtually every strand of the web except frame threads, on which she remains until morning. She rebuilds the orb at dawn.
Webs can be exposed to different light regimes during the course of the day, and spiders can encounter heat stress at times. Spiders in microhabitats where solar radiation is high exhibit an east/west orientation that reduces heat load. Spiders in closed, cool microhabitats exhibit a north/south orientation that increases body temperature. This behavioral thermoregulation allows the species to exploit a variety of deciduous forest microhabitats. Web orientation appears to be a behavioral adaptation that allows the species to maximize its time on the web in all forest microhabitats, thereby maximizing its chances of taking prey (Biere and Uetz 1981).
The webs are selective for prey size, retaining mostly Diptera larger than 3 mm, even though most insects striking the web are smaller. Females are slow moving and almost clumsy, allowing many insects to escape their webs. Of the insects retained by the web, the spiders elect to attack and consume mostly larger Diptera. About two-thirds of the prey is Diptera. Hymenoptera and Coleoptera make up most of the remainder of the diet. Unlike other Araneidae, the spined micrathena bites it prey first, then wraps it in silk (Uetz and Biere 1980, Uetz and Harstock 1987).
After a female molts to adulthood and constructs a viscid spiral, males build mating threads on which they court. For a complete mating, after copulating the first time, the male dismounts and approaches the female again to inseminate her second reproductive tract. Egg sacs have a fluffy appearance. They are placed on vegetation near the web. After the egg sacs are made in September, the female becomes moribund (Bukowski and Christenson 1997a, b; 2000).
The ridges of the stridulating files are at about a right angle to the major axis of the body. Three or four stout setae projecting dorsally from areas near the bases of the hind femora scrape across the files to produce a sound that is said to be a low-pitch buzz or hiss audible to humans at a distance of about two feet. Stridulation is probably defensive in function. The sound is produced when the spiders are disturbed (Hinton and Wilson 1970).
Only three species of Micrathena occur in the eastern United States. M. gracilis females have 5 pairs of conical tubercles on the abdomen, whereas M. mitrata females have only two short posterior pairs and M. sagittata females have three pairs, the posterior pair being the largest. All three species can be found in Arkansas (Levi 1978).
Biere, J. M., and G. W. Uetz, G. W. 1981. Web orientation in the spider Micrathena gracilis (Araneae: Araneidae). Ecology 62 (2): 336-344.
Bukowski, T. C., and T. E. Christenson. 1997a. Determinants of sperm release and storage in a spiny orbweaving spider. Animal Behaviour 53: 381-395.
Bukowski, T. C., and T. E. Christenson. 1997b. Natural history and copulatory behavior of the spiny orbweaving spider Micrathena gracilis (Araneae, Araneidae). Journal of Arachnology 25: 307-320.
Bukowski, T. C., and T. E. Christenson. 2000. Determinants of mating frequency in the spiny orbweaving spider, Micrathena gracilis (Araneae: Araneidae). Journal of Insect Behavior 13 (3): 331-352.
Hinton, H. E. and R. S. Wilson. 1970. Stridulatory organs in spiny orb-weaver spiders. Journal of Zoology 162: 481-484.
Levi, H. W. 1978. The American orb-weaver genera Colphepeira, Micrathena and Gasteracantha north of Mexico (Araneae, Araneidae). Bulletin of the Museum of Comparative Zoology 148 (9): 417-442.
Levi, H. W. 1985. The spiny orb-weaver genera Micrathena and Chaetacis (Araneae: Araneidae). Bulletin of the Museum of Comparative Zoology 150 (8): 429-618.
Uetz, G. W. and J. M. Biere. 1980. Prey of Micrathena gracilis (Walckenaer) (Araneae: Aranaeidae) in comparison with artificial webs and other trapping devices. Bulletin of the British Arachnological Society 5 (3): 101-107.
Uetz, G. W., and S. P. Harstock. 1987. Prey selection in an orb-weaving spider: Micrathena gracilis (Araneae: Araneidae). Psyche 94 (1-2): 103-116.