One of Nature's most interesting facets is mimicry. The natural world is loaded with animals that are not what they appear to be. This deception gives the imitators an advantage, whether it be predator avoidance, ease of capturing prey, camouflage, or some other edge. Through the miraculous fits and spurts of evolution, the mimickers have developed disguises that are often incredibly similar to quite unrelated animals.
Even though mimicry is one of the greatest examples of Darwinian natural selection at work, Darwin himself missed the entire subject. It goes unmentioned in his landmark The Origin of Species. It was English scientist Henry Bates who brought mimicry to the forefront as an outstanding example of Darwinian evolution, shortly after Darwin's book was published in 1859. Far from being jealous or bent out of shape by Bates' elucidation of mimicry, Darwin became a great admirer and went so far as to say "I have just finished, after several reads, your paper. In my opinion it is one of the most remarkable and admirable papers I ever read in my life.... I rejoice that I passed over the whole subject in the Origin, for I should have made a precious mess of it".
Last Saturday, David Wagner, myself and a few others were able to make a two hour foray through an eastern Pennsylvania park. It was a nice place, but certainly nothing out of the ordinary. Meadows buffered by a corn field, some woods and a stream, and brushy edge habitats. We still managed to find some interesting animals, including the mimics that follow.
The ant mimic jumper is about the size of an average ant, and typically moves along the ground, often with ants. It's resemblance to the totally unrelated insects (spiders aren't even insects, they're arachnids) is startling. It may be that the spider gains an advantage by looking like something that potential prey might tend to ignore. Or it may be that potential predators of the spider often find ants distasteful and shun them.