|Bold Jumping Spider - Phidippus audax|
Family Salticidae - Jumping Spiders. Also commonly called daring jumping spider.
Live jumping spiders photographed in the wild at Winfield, Illinois, and San Antonio, Texas, USA.
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The bold jumping spider is one of the largest and certainly one of the most frequently encountered of the Salticids. Their active lifestyle, conspicuous dark-colored body, and frequenting of the flower or vegetable garden environment brings them to the attention of casual observers and gardeners of all stripes, many of them dedicated arachnophobes whose first reaction is to kill the unfortunate beast.
Big mistake. Spiders of all varieties are of great benefit to the gardener, helping to keep down the burgeoning population of insects that are doing real damage to the plants. They eat bugs and flies and just about any small soft-bodied insect that blunders within range of its highly accurate, leaping capture and venomous fangs, which are used to quickly immobilize the prey so it may be devoured without struggle.
Unfortunately, most people (myself included) have a deep aversion to spiders, especially big, black, hairy spiders - of which the bold jumper certainly is a fine example. And there's a reason for their common name: they will often stare you down. At least that's the impression we get when such a scary critter turns and faces us. However, she just thinks you're a predator; spiders don't live long who ignore large, moving animals in their vicinity. If you look closely and have good eyesight, you might also see her threat display - she'll wave her pedipalps rapidly in front of the iridescent green chelicerae, creating a surprisingly effective flashing warning signal.
Our apparent inability to view the natural behavior of spiders without anthropomorphizing into them malicious intent has lead to this spider's stigmatization in popular culture as an aggressive, bite-soon-as-look-at-you churl of the rose garden. Instead, the opposite is true. These spiders will always choose escape if they have a choice. You'd have to work pretty hard to be bitten by one.
I think most people who are convinced they have been bitten by a bold jumper (or any spider for that matter) are those who remember seeing one in the hours before they notice they've been bitten or stung by something, anything unseen. The big bad black hairy spider takes the rap for mosquito bites, bee stings, fly bites, and I'll wager thorn-pokes that went unnoticed at the time. (Hell, even lady bugs bite these days.) Then later, when something itches and swells, the bold-jumper-picture comes easiest to mind, and the cognitive story we weave for ourselves readily incorporates all the nasty biases we have against our 8-legged spider companions. It makes sense! I've been bitten by a spider!
Bold jumping spiders are variable in their coloration and markings. Bugguide.net shows pictures of many variations.
Salticids, with rare exceptions, do not build webs to snare prey, they only spin small silken retreat webs for moulting or hibernation. When hunting, jumpers always trail a silken strand from their spinnerets. If they are disturbed, they will rapidly descend on this lifeline to the ground and out of sight, or if they miss their jump, they can climb the thread back to their previous perch. These little guys are very adept at hiding, and if they do not want you to see them, you won't. You can find an extensive article on the family at the Tree of Life Website.
Although a jumping spider can jump more than fifty times its body length, none of its legs has enlarged muscles. The power for jumping comes from a quick contraction of muscles in the front part of the body increasing the blood pressure, which causes the legs to extend rapidly much as the hydraulics in a low-rider car.
I have noticed almost all spiders that live on human structures have "dandruff"
Jumping spiders have excellent vision, among the highest acuity in invertebrates. The eight eyes are grouped four on the face (the two big anterior median eyes in the middle, and two smaller anterior lateral eyes to the side), and four on top of the carapace. The two large, forward-facing eyes (AME) are tubular behind the lens, with a well-developed musculature, unique to salticids, that supports and moves the retina - the opposite arrangement of our own eyes. 
Spider musculature is also different from ours: in the spider, muscles operate from the inside to move external skeletal elements; our own skeletal muscles surround the elements they operate. But even these glaring differences are nothing compared to the jumping spider's brain and digestive system - their esophagus passes right through the brain, and one branch of the gut (analogous to our intestines) actually sits on top of the eyes and brain. 
Class Arachnida / Order Araneae: Spiders are the largest group of arachnids. They are easily recognized by their eight legs, and there are few creatures great or small that elicit such irrational fear in mankind. The vast majority of spiders are completely harmless and offer beneficial services, chief of which is keeping the burgeoning insect population in check. I am continually amazed at the resourcefulness of these supremely successful predators.
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