Today I have a short post for you! I was out collecting with my students during class last week and one of them took us to a nice spot on campus, a secluded little courtyard of one of the old buildings with a handful of citrus trees. We looked around and found some stink bugs on a tree, some butterfly cocoons hanging off the buildings, and some spingtails. One of my students found this:
If you think this looks like something that was ejected from the back end of a bird, you’re not alone! This is the caterpillar of the giant swallowtail (Papilio cresphontes). As you might imagine, looking like bird droppings has its advantages. This caterpillar blends very well into the background and it is hard to recognize that it is an insect at all. That’s the whole point of looking like bird droppings! Any insectivore (an organism that eats insects) looking for a tasty caterpillar to eat is likely to pass right by this one because it looks so much like something else – and something most animals wouldn’t consider eating. The appearance of this caterpillar is part of its defense against predators. If it stays still, most predators won’t even notice it’s there.
But say something happens to bump into the caterpillar (such as an insect systematics student looking for insects for her collection) or otherwise detects the caterpillar’s presence. Then the caterpillar brings out it’s backup defense! It’s depicted in this video:
That little orange slimy looking thing that pops out of the caterpillar is called an osmeterium. Normally, it’s hidden in a pouch inside swallowtail caterpillars, right behind the head. When disturbed, the caterpillar can squeeze some of it’s hemolymph into the osmeterium, causing it to pop out of the pouch. The everted osmeterium is then waved at the predator. Now how might this little organ be useful in deterring predators that might want to eat the caterpillar? It’s covered in potently stinky chemicals! Any animal that gets a big whiff of a foul smelling substance from something it’s considering eating, especially from something that looks a whole lot like bird poop in the first place, is probably going to pause for a moment and consider whether it’s worth eating. Most things will leave the caterpillar alone rather than eating it. And when the predator wanders off and leaves the caterpillar to itself, it can pull the osmeterium back into the pouch behind its head until the next time it’s needed.
Pretty fun, eh? If a caterpillar that looks like bird poop isn’t fun, I don’t know what is! :)
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