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by AMNH on
Tarantulas make up about 2 percent of the 44,500 known species of spiders, but they boast a particularly outsized profile thanks to movies where they're cast to signal mortal danger, elicit shrieks of fear—or both.
Off-screen, species like this vividly-colored Mexican red knee spider is actually relatively harmless. It does produce venom that is toxic to its prey—insects, small frogs, lizards, and mice—but it's not a threat to humans. In fact, it's a coveted pet.
Just in time for Halloween, here are ten highlights of this alluring arachnid's anatomy. Click the image below to enlarge.
1. Venom Glands
The spider’s venom paralyzes prey. Bites are relatively harmless to humans, comparable to the sting of a bee or wasp.
Like most spiders, this species has eight eyes but poor vision. The spider depends more on sensitive hairs on its legs and body to orient itself.
These two appendages end in fangs that inject venom into and hold onto prey. Females also carry egg sacs between the fangs.
Two leglike feelers on either side of the mouth are used for touching, tasting, and handling prey. In males, pedipalps also deliver sperm to the female.
5. Sucking Stomach
After regurgitated digestive juices finish liquefying prey, the sucking stomach draws in the resulting “milkshake” of proteins and fat.
6. Book Lungs
Spiders have an open circulatory system in which blood seeps out into the body and flows back to the heart. Book lungs, named for alternating spaces of air and hemolymph that resemble leaves of a book, diffuse oxygen into the bloodstream.
A female lays 200 to 400 eggs in spring, fertilizing them with sperm stored in her body since mating, sometimes months before.
8. Silk Glands
Males weave a silk web on which sperm is deposited and collected on the pedipalps during copulation. Females weave a silk mat on which to lay eggs and fashion into an egg sac.
Silk is liquid within but becomes solid as it leaves the body. Internal pressure pushes silk out of the two pairs of spinnerets (one pair visible above), where it is pulled by the hind legs.
10. Urticating Hairs
Barbed hairs on the top abdomen are cast off by the hind legs, causing irritation in predators. One type, concentrated around the periphery, seems effective against other arthropods; A different type in the center is deployed against small vertebrates.
A version of this article appears in the Fall 2014 issue of Rotunda, the Member magazine.