I’ve been quiet for a few days, which is a good sign in field work: many new specimens to process will keep me away from the computer. To break the silence I’ll show two ants, which despite Heather’s gasteropost and subsequent comments, have a certain cuteness to them. But of more interest to me than the two ants (sorry, myrmecologists) are some jumping spiders that look a whole lot like the ants.
First is a cephalotine ant, which I find rather stylish with its grey robotic body. Below are a Bellota and a Peckhamia, both of which have independently evolved a resemblance to it.
Bellota. I like how the head of the spider seems to correspond with the thorax of the ant, with the thickened first legs of the spider corresponding with the head of the ant. (Why the spider’s name comes from the Spanish word for “acorn”, I don’t know.)
Peckhamia. While many antlike jumping spiders wave around the first legs as if they were the antennae of their model ants, Peckhamia belongs to a group that waves around its second pair of legs.
The second ant is a Pseudomyrmex, the long, thin, quick ants common here in Jalisco.
And this is not an ant but Synemosyna. Even after decades of looking at jumping spiders, when I find a Synemosyna I have to look closely to confirm that it’s actually a spider. This particular species has black spots corresponding to the eyes of ant — within the black spot is one of the spider’s eyes, but the eye is much smaller than the spot.
Why is it a good idea for a jumping spider to look like an ant? This hasn’t been well enough studied to answer the question with confidence. For most antlike jumping spiders, the mimicry does not seem to have evolved to fool the ant (e.g. to eat the ants, like a wolf in sheep’s clothing). One possibility is that it’s a good idea to make a predator think you are an ant, as many predators avoid ants because they can sting or bite and are accompanied by many friends who will do the same. Check out Heather’s blog for some insects that have evolved to look like ants.
Reflections on a Spider's Eyes 
Did you know that the first instar of the giant Australian stick insects are also ant-mimics? Presumably because the stick insect eggs are mimics of snack-adorned seeds that ants feed on and dump by their nests.
Cool! There are so many ant-like things here and in other tropical areas.
Lovely post and pictures! If I could comment on something, I will say that Cephalotes species are curious models compared to many other ants (including the pictured Pseudomyrmex). They have a very weak bite, a non-functional sting, and live in small colonies (typically a few hundred). Consequently, the likelihood that looking like a Cephalotes keeps away predators due to the normal ant intimidation factors is slim. Instead, their heavy armor and low forager density seems to make them of little interest to predators of arboreal arthropods. I suspect that mimicry of Cephalotes therefore confers protection via lack of predator interest, not fear and intimidation.
Indeed, ants in general have lots of exoskeleton to fight through for a fairly small reward. I’ve wondered also if some of the selection pressure comes not from predators avoiding ants, but specialist spider predators failing to see them as spiders — e.g. spider wasps or mantispids. In that case, looking like something else common in the environment may raise the cost of attempting to distinguish.
Avoidance of specialist spider predators sounds like a good reason to look like an ant to me. I especially like the idea because it gives due respect to the visual systems of insect dupes. I get the sense that the predator/dupe of ant mimics is so often assumed to be birds. I’m sure they are important in many instances, but I also think the role of insect predators is underplayed.