Dimorphic Jumping Spider - Maevia inclemens
Family Salticidae
Live spiders photographed at Warrenville, Illinois.
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Dimorphic Jumping Spider - Maevia inclemens
This very active male jumping spider was out hunting on November 1st, near Chicago. He is about 1/8 inch long.
Jumping spiders are distinguished from other spiders by their four big eyes on the face and four smaller eyes on top of the head. Around the world there are probably more than 5000 species of jumping spiders. Jumping spiders are charming lil buggers that look up and watch you.  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.

Salticids are perhaps as old and diverse as mammals, though not many humans know of their world. Many salticids are colorful, they take on a variety of body forms, and some have disguises, looking like ants and other organisms.

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. [1]

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  actually sits on top of the eyes and brain. [3]


Note: the function of the posterior medial eyes is unknown [2]
References
  1. Bugguide.net, Maevia inclemens (Dimorphic Jumper)
  2. Wikipedia, "Pedipalp"
  3. Thomas Eisner, Maria Eisner, and Melody Siegler, "Secret Weapons: Defenses of Insects, Spiders, Scorpions, and Other Many-Legged Creatures" (Belknap Press, 2005)
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