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SPINY-BACK SPIDERS

At the Center such avoidance behavior means slow going during the dog days of August. Every few paces on every trail there's another web impeding our progress, and getting past some of them with out damage to web or lumbar vertebrae requires a bit of physical agility. The majority of trail-blocking webs are three-sided, with a tightly woven spiral of sticky silk attached within the triangle. Most often there is a long horizontal line at the top that connects two trees on either side of the trail. From one end of this topline another strand of silk angles downward--sometimes anchored to the ground, sometimes to the base of one of the vertical trees. The third line connects the other two. A dry, non-sticky escape line, which allows the spider to evacuate quickly in crisis situations, often leads from the top of the web to an overhanging branch or leaf (right).

Even though webs slow our trail- walking, it makes sense they would go across a path rather than paralleling it; insects that comprise the spider's food are more likely to fly down an open pathway than into trees on either side. If we manage to spot a web on the trail before running into it, there's usually a spider sitting near its center. Around Hilton Pond the largest but least common of these are Black and Yellow Argiopes--big black and yellow eight-leggers that are sometime called "writing spiders" because of a white zigzag they weave into their webs. Slightly more numerous are several sizes of brown orb-weavers we haven't yet learned to identify. But by far the most prevalent spiders blocking our trails are those with spiny abdomens: the Micrathena group.

Micrathena spiders occur across the U.S. and Canada and into the American tropics. The females are characterized by spines or bumps on their abdomens that give them an angular appearance. These projections--quite sharp in some species--once drew our blood when a spiny-back spider dropped onto our neck. Thinking it was a Deer Fly, we slapped it hard and drove the points through the skin on our neck AND our hand. This was painful to us, but the spider just bounced off and went on its merry way. Although these sharp spines are most certainly not an adaptation to prevent people from hitting at the spider, they probably do keep some predators from eating it. We can only imagine how it might feel to squeeze a Micrathena spider between the tongue and soft palate.

The Micrathena we see most often around Hilton Pond is M. gracilis, whose common name is the not-very-imaginative "Spined Micrathena." This species is widely distributed across the eastern U.S. in open woods that are early into succession. The female has a total length of nearly half an inch--at least half of which is a whitish or yellowish ten-spined abdomen with light and dark striations on the sides (top photo) and white blotches on the dorsal portion (above). Her abdomen is further ornamented by nearly microscopic dimples that apparently have no known function. The smooth-surfaced carapace--to which eight glossy black legs are attached--is dark brown or amber-colored, and the head bears several pairs of tiny eyes remarkably sensitive to light and movement. (NOTE: The female in the photo above had just vacated her web via an escape line to take refuge under a foliose lichen growing on the support tree.)

Compared to his mate, a male M. gracilis (above right) is much smaller at only about a quarter-inch long. He also sports no spines, has a flat and elongated brown abdomen, and is seldom seen. He spends much of his time hiding in or near a female's web, waiting for just the right time to move in to copulate without getting eaten in the process. After mating, the female creates a tan sphere that is soft and knobby and lays her eggs within (below left); this silken case, affixed to vegetation near the web, protects the eggs from desiccation and predators.

The web of a female Micrathena gracilis is a small, delicate orb about 6-8" inches across--just the size for catching small insects such as flies and mini-moths. She weaves the web by exuding silk from her abdomen, using her hind feet to guide the strands into a tight spiral with non-sticky support spokes. This ability is one of the most wondrous events in nature, and we're glad we now have enough sense to avoid webs and observe the spiny-back spiders that toil and spin at Hilton Pond Center.

All text & photos © Hilton Pond Center
M. gracilis male photo © John & Jane Balaban on BugGuide.net

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