Spiny Orbweavers (Micrathena Spiders)

Media
Photo of a white micrathena spider in her web
Scientific Name
Micrathena spp.
Family
Araneidae (orb weavers) in the order Araneae (spiders)
Description

Missouri has three species of micrathena spiders, as a group called spiny orbweavers. As orbweavers, they spin intricate, circular webs. All three species have some combination of pointy, conical tubercles on their bodies. Because males are small and rarely seen, the following descriptions refer to females, which are commonly seen resting in their webs. All have glossy black legs.

M. gracilis, called the spined micrathena or spiny-bellied orb weaver, has 5 pairs of black tubercles and a white and black (or yellowish and brown-black) mottled abdomen. It is the most commonly encountered micrathena in Missouri.

M. mitrata, the white micrathena, has 2 short pairs of tubercles and a white abdomen with a few distinct black blotches on the upper side. It an odd way, it looks like it's wearing a turban, which is exactly what the species name "mitrata" refers to.

M. sagittata, the arrowshaped micrathena, has striking reddish, black, and yellow colors and has 3 pairs of tubercles. The pair of tubercles at the back end of the abdomen are rather large, forming two corners of the triangular (“arrow-shaped”) body. With a little imagination, this spider suggests a hard-rocking "Flying V" electric guitar.

Size
Length: to about ½ inch (excluding legs).
Where To Find
image of Micrathena Spiders Distribution Map
Statewide.
Most micrathenas are found in woodland areas. M. gracilis, for example, is found most commonly in central and southern Missouri, where timber is more extensive. Micrathenas can be found in many habitats, including around homes and gardens. They rarely enter houses.
Like many spiders, micrathenas capture insects in the sticky strands of their webs, then deliver a bite of venom sufficient to subdue and to begin digesting the interior of the insect. Then they wait, returning to the prey to ingest its liquified contents. The close spacing of the circles in micrathena webs enables them to specialize in tiny flying insects such as mosquitoes. Micrathenas are ferocious predators to small insects, but they are completely harmless to humans.
Common.
Life Cycle
Like many spiders, micrathenas live for only a year. They hatch from egg cases in spring, disperse, and undergo molts as they grow. Females are twice as big as males and are the ones most of us see, for they are the ones that spin conspicuous webs. Males visit females in their webs, and courtship often proves fatal to them; their bodies provide nourishment to their mate and future offspring. Females spend the season eating insects and creating egg cases, then die on the onset of cold weather.
Spiders are unfairly feared and hated by many humans, and we would do well to try to conquer our unreasonable phobias. Spiders may be creepy, but they do us a tremendous service in natural, nontoxic pest control. We can choose to view micrathenas as terrifying, or as tiny, ornate exterminators.
Micrathenas and other spiders help control populations of insects, particularly flying insects, many of which are obnoxious to hikers. Additionally, many birds steal trapped insects from spider webs to eat for themselves. Hummingbirds harvest spider webbing to use in building their own nests.
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About Land Invertebrates in Missouri
Invertebrates are animals without backbones, including earthworms, slugs, snails, and arthropods. Arthropods—invertebrates with “jointed legs” — are a group of invertebrates that includes crayfish, shrimp, millipedes, centipedes, mites, spiders, and insects. There may be as many as 10 million species of insects alive on earth today, and they probably constitute more than 90 percent all animal species.