Spiny Orbweavers (Micrathena Spiders)

Micrathena spp.

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Photo of a white micrathena spider in her web
The white micrathena, M. mitrata, has 2 short pairs of tubercles and a white abdomen with a few distinct black blotches on the upper side.
David Bruns
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).

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Photo of arrow-shaped micrathena spider
Arrow-Shaped Micrathena
Arrow-shaped micrathena, Micrathena sagittata

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Photo of female spiny-bellied orbweaver (spined micrathena) on a leaf
Spined Micrathena (Micrathena gracilis)
The spined micrathena, Micrathena gracilis, is the most commonly encountered micrathena in Missouri.

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Photo of a spined micrathena in her web
Spined Micrathena in Web
The spined micrathena tends to hang with its "back" toward the ground and the spinnerets pointing upward, with the abdomen looking like a tiny pyramid.

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Photo of a spined micrathena on a twig
Spined Micrathena
There are three species of spiny orbweavers (genus Micrathena) in Missouri.
Habitat and conservation

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.

Foods

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.

image of Micrathena Spiders Distribution Map
Distribution in Missouri

Statewide.

Status

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.

Human connections

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.

Ecosystem connections

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.