Wednesday, August 17, 2016

Spined Micrathena - Spiny Orbweaver Spider

     If you've taken a walk in the woods lately, you've likely run into a spider web. Chances are pretty good it was the orb web of the Spined Micrathena (Micrathena gracilis) or Spiny Orbweaver Spider. It is one of the most common woodland spiders, and it commonly builds webs across trails. Why does it always build its web across a trail where something will run into rather than parallel to the trail? Because those same trails used by people and larger animals are also the most likely to be used by the flies and other small insects that make up the vast amount of prey for these bizarre shaped spiders. Though they are a nuisance, they are harmless to people. 

Spiny Orbweavers are spiny and hard indeed.

     Female Spined Micrathena Spiders have 10 spines or points above their bodies and are cone shaped below. They can come in a variety of colors, but often are darker below and lighter colored (often white, yellow, or orange) above. This is a form of counter-shading, helping to camouflage them in the dappled light of the forest. They hang head down in the center of their webs, and from above match the darker forest floor, but from below match the lighter colored sky. They can get to about half an inch long and only the females build webs.

When looking at the bottom portion, the body is cone shaped.

     The males are rarely seen. They only have two spines and are very slender, being only half the size as adults than the females. They often station themselves near a female's web seeking an opportunity to mate with her. They have to be careful or the larger female will eat them. 
     The Genus Micrathena is derived from the Greek adjective for "small" and the Greek goddess Athena, who was the patron of the spinning/weaving arts. There are about 100 different species in this Genus, but only 3 in Eastern North America (with one additional one in the West). Micrathena gracilis, the Spined Micrathena, is the most common and ranges all the way to Costa Rica. Its specific name means "slender, slight, or graceful." A neat little name for these web builders. 
     The hard spiny body of the Spiny Orbweaver is likely a protection against predators such as birds and lizards. Since the spiders rarely if ever bite, this is important. They will often try to scurry away to a corner of their web if disturbed. They will also sometimes play dead, lying there immobile with their legs tight against their armored bodies. But their most interesting defense is the low pitched buzz (stridulation) they emit when scared. Next time you approach a web, blow on it and see if you can hear it.

When frightened, Spined Micrathena Spiders may pull their legs tight and rely on their armor for protection.

     Micrathena Spiders are in the family Araneidae, the Orbweavers. Their spiral webs are usually less than a foot across, but are placed 3-7 feet up high, perfect height for someone to walk into. When sunlight hits them, they can be really showy, giving them another common name: CD Spiders, since it might appear as though someone had strung one up across the trail.
     Unlike many other orb building spiders, Spined Micrathenas are diurnal, active during the day. They actually eat their webs (all but the supporting frame threads) every night. It takes them about an hour to build their webs again the next morning. About two thirds of their prey are Diptera, or flies. Spiny Orbweavers are a bit slow and clumsy, so their prey gets away a lot. They also differ from many other spiders in that they bite their prey first before wrapping them up, just the opposite of what most other orb web spiders do. 
     These small and common orb web spiders only live about a year (the males not even that long). By late summer and early Fall, the females will lay their eggs in a fuzzy egg sac hidden off to the sides of their webs. The adults all soon die, but the eggs will overwinter and young spiderlings will hatch out to spread through the forest the next spring. 
     So the next time you run into a web in the woods, it may be that you just destroyed the handiwork work of these little spiders. Before you curse them too badly, remember that they're harmless and doing you a favor by catching flies and other small flying insects. If you see one of their webs across your path, enjoy the craftsmanship along with the unusual color and shape of their maker. 


  1. Alonso, I really appreciate the work that went into this excellent post.I was looking for information to share with our small but enthusiastic central Massachusetts nature club. We learned a lot from this piece. Fascinating creature. Thank you for posting this!

  2. Found one of these in my front yard in Detroit, MI. Took me nearly two hours to find this post. Thank you so much for the information.