Answer to the Monday Night Mystery

What was that inexplicable bit of chitin hiding away in a hole in a twig?

This photo should help:

It's the heavily sclerotized head shield of a Cephalotes varians turtle ant. Ants in this mostly Neotropical genus inhabit pre-existing cavities in trees and branches, a limiting resource that spurs intense competition among colonies of various tree-dwelling species. Workers come in two size classes: small and relatively lithe, and tank-like with a dinner plate for a head.

Turtle ants aren't fighters. Rather, they're all about defense. If a colony gets hold of an old beetle burrow, the heavily armored majors will plug the entrance with their head shield and sit tight, budging only to let their nestmates pass. They are literally living doors.

As usual, the commentariat picked this one within minutes. Points are awarded as follows:

• 8 points for JasonC., who nailed the order, family, genus, and the behavior.
• 2 points for MarekB, who picked the species
• 2 bonus points for James Trager, who submitted a fine tall tale about diptera that made me laugh

As a word of thanks, I borrowed the ants over the weekend from Corrie Moreau, an ant specialist at the fabulous Chicago Field Museum. Corrie has been studying the surprising endosymbionts of these beautiful insects. I'll be posting more about turtle ants later in the week.