A new word (at least to me) for the rear end! It's not yet in OED.

Parasites are among nature's great manipulators, taking advantage of their unfortunate hosts in myriad ways. Perhaps none are more twisted, however, than the tiny nematode worm, that makes its host, a tropical ant, look like a bit of fruit. This is not good for the ants. It makes them enticing to birds.

Stephen P. Yanoviak … and his colleagues discovered this bizarre parasite-host relationship in Panama … . They noticed that some of the ants, which are normally black, had gasters, the rear segment of their bodies, that were bright red. … the researchers found that the gasters were full of hundreds of tiny eggs containing developing worms.

These ants … collect and eat bird feces, … sometimes infected with the nematode eggs … So worms develop inside them, and … congregate in their gasters. … with the [worm] eggs inside, the covering of the gaster becomes thinner, so what normally appears black becomes translucent and looks red in sunlight. What is more, infected ants tend to hold their gasters high when walking, making them even more conspicuous.

… researchers … were fairly certain that birds, attracted by what they think are bright red berries, eat the ants, completing the cycle so it can begin again.

Dr. Yanoviak said this was an extremely rare case of parasite-induced mimicry. "Turning an ant into something that looks like a piece of fruit is pretty remarkable," he said.
– New York Times, Apr. 8, 2008, p. F.3

Now that is just the sort of nature story I'm glad I didn't read at breakfast!

Are "gaster" and "keister" related? They sound as if they could have derived from some common bottomly ancestor.

Wordmatic

I'd not heard of the word, but I've long used a made up word, "snallygaster" as a monster of some type.