Sunday, September 1, 2013

Arrowshaped Micrathena spider

It must be a banner year for spiders in southern Ohio. We saw large numbers and quite a diversity of orbweavers alone on our recent trip to Cincinnati and Adams County, August 18-26. Among the more remarkable species was the Arrowshaped Micrathena, Micrathena sagittata. We found several individual females on the trail to Buzzardroost Rock, a preserve managed by The Nature Conservancy in Adams County near the town of Lynx.

The spiny orbweavers, which includes the genera Micrathena and Gasteracantha, are mostly tropical, and some species in the rainforests of southeast Asia, Australia, and Africa are even more extreme in the spikes, horns, and other processes jutting out of their armored abdomens. No doubt a spider studded in spines is not terribly appetizing to a predator.

The Arrowshaped Micrathena is not a particularly large spider, ranging from 8-9 millimeters in body length for mature females to only 4-5 millimeters in males. Males are seen less frequently than females by the casual observer and they lack the spines that adorn their mates. The color and pattern of this species is relatively consistent, as shown in these images.

Looking at the underside of the spider, we see that her spinnerets are located in the center of her abdomen, rather than at the posterior where you would expect them to be. Her venter (underside) is mottled black or brown and yellow, which helps camouflage her from predators that might approach from overhead.

The spiders spin circular webs about one foot in diameter with a tight spiral and an open hub (center). The spider hangs onto the threads that frame the hub, and also secures herself to the hub with strands of silk issuing from her spinnerets. She can drop out of the web and into the leaf litter below if she feels threatened, but then reel herself back to the hub when danger passes. A zigzag band of silk called a “stabilimentum” may be present, if only vaguely, immediately above the center of the web. The function of this decoration is still debated, with theories ranging from it representing a fake flower to attract insect prey, to a way to advertise to birds that a web obstructs their flight path.

The web is usually tilted somewhere between the vertical and horizontal planes, which makes it challenging to get decent images of the architect sitting on it. The abdomen of the spider may also sag, so getting the whole animal in focus is problematic. The spiders usually build their webs in shrubs and among other plants in the understory of deciduous forests, or along forest edges and openings in the canopy. Look for them two to four feet off the ground. They aren’t easy to spot even then. Researchers in Kansas found the webs particularly common in stands of Green Dragon (Arisaema dracontium), Pennsylvania Pellitory (Parietaria pensylvanica), and American Lopseed (Phryma leptostachya).

The snare intercepts flying and jumping insects, like leafhoppers, that the spider then feeds on. Interestingly, these spiders do not wrap their prey as other web-builders do. Flies, small wasps and bees, and beetles have also been recorded as prey of this spider.

Mated, mature females construct fluffy, spherical egg sacs of white silk, about 12 millimeters in diameter and containing roughly 90 eggs. The egg sac is the overwintering stage and mature spiders are seen mostly from July to September.

Micrathena sagittata is found from Maine to Florida and west to Nebraska and Texas, but is probably more common in the southern portion of its range.

Right now is prime spider-hunting season, with webs becoming more obvious as the spiders reach maturity, and leaves start to fall during autumn. Enjoy looking for them.

Sources: Bradley, Richard A. 2013. Common Spiders of North America. Berkeley, California: University of California Press. 271 pp.
Fitch, Henry S. 1963. Spiders of the University of Kansas Natural History Reservation and Rockefeller Experimental Tract. Lawrence, Kansas: University of Kansas Museum of Natural History, Miscellaneus Publication No. 33. 202 pp.
Howell, W. Mike and Ronald L. Jenkins. 2004. Spiders of the Eastern United States: A Photographic Guide. Boston, Massachusetts: Pearson Education. 362 pp.
Jackman, John A. 1997. A Field Guide to Spiders & Scorpions of Texas. Houston, Texas: Gulf Publishing Company. 201 pp.
Kaston, B.J. 1972. How to Know the Spiders (Third Ed.). Dubuque, Iowa: Wm. C. Brown Company Publishers. 272 pp.

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