Jumping Spider – Phidippus clarus

Male spiders have an interesting way of inseminating a female: they use their palpi, the little 'feelers' beside the face. In the females, these palpi are simple and leg-like.  But adult males have the palpi swollen and more complex.

He spins a small web and deposits a drop of sperm on it from the underside of his abdomen. He then places the tip of the palp into the sperm, and draws the sperm through the palp's opening into the sperm duct of the palp, where it is stored. The male then goes out in search of females. If he finds one, he performs a courtship dance. If she accepts him, he places his palp against an opening on the underside of her abdomen (her epigynum). He locks it in place by putting a thumb-like projection into a groove in the back of her epigynum. The palpus then expands and injects the sperm into the female.
 

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]

Note: the function of the posterior medial eyes is unknown [2]

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 overlies the eyes and brain! [1]