Apple #387: Ant Muscles

I'm supposed to do another entry about the plants I saw while on my trip to California. But I'm short on time tonight, so I'm doing a shorter entry instead. I'll get to those sequoias this week, I promise!

So I was walking with my co-worker to the next building where we have to go to pick up the mail when I spotted an ant zooming at lightning speed across the sidewalk. I pointed this out, saying something very articulate like, "Look at that ant go!" My co-worker responded by saying something about the ant's muscles. "Those ant muscles are really working," or something to that effect.

"Ants have muscles?" I said.

"Sure," said my co-worker. "How do you think they move?"

"I don't know. I thought they just had exoskeletons."

"But all animals have muscles. They couldn't move without them."

Here again, words failed me. Nobody in my science classes had ever said anything about ants having muscles. All anybody ever told me about ants were that they had 3 parts to the body -- head, thorax, abdomen -- and those were covered in exoskeleton. Never a peep about any muscles. And those ant legs are so skinny. Are there really muscles somewhere in there?



We all recognize that ants are strong and fast. So where are their muscles?
(Photo of leaf-cutter ant from DangerousWildlife.com)

• Ant muscles are a fairly well-kept secret; even now, with all this free online knowledge, people don't talk about them much.

• Most of the time, what scientists have to say about ant muscles is that they are not, in fact, super-strong compared to human muscles. A lot of people think that, since ants can carry loads that are way huger than the size of their bodies, ant muscles must be phenomenally strong. But this is not the case. The reason ants can carry such huge loads (compared to what humans can carry) is that their body weight is relatively light while the proportion of muscle-to-body-mass is greater. Their muscles don't have to work as hard to carry around their ant bodies, so the muscles have lots of strength-capacity left over to carry other stuff. If our muscles didn't have to work so hard carrying us around, we'd be able to heft enormous things, too.
  

• Even as scientists are answering these sorts of questions about ant muscles, they never say where those ant muscles are.

• Other ant scientists answer questions about how ants eat, how they walk, and that sort of thing. In service of answering those questions, they typically provide anatomical diagrams of ants, like the one below. But nowhere on those anatomical diagrams do they point out the freakin' muscles.
  

This particular anatomical chart of a worker ant gets very specific, even to the point of indicating the femur and tibia. But it does not mention anything about leg muscles.
(Diagram from Wikipedia)

• Biologists do talk sometimes about a particular set of muscles -- the ones in the ant's head which operate the ant's mandibles. These muscles are huge, in ant terms. Some ants in particular, like the trap jaw ant, have especially enormous mandibles and correspondingly enormous muscles that work the mandibles.

• Mandibles are the ant's jaws. Ants trap their food in the mandibles and then they literally squeeze the life out of the food. Then they keep squeezing out all the moisture from the food. They drink that liquid and then toss aside the solid matter. So the ant needs really strong muscles to accomplish all that squeezing. Trap jaw ants also use their mandibles to help them escape from predators [movie]. (For more about that movie see my entry on the speed of ants.)
  

Drawing from 1927 of the muscles in an ant's head which operate the mandible. But these aren't leg muscles.
(Diagram from Wikipedia)

• One pair of scientists, Birgit Ehmer and Wulfila Gronenberg, got way interested in the muscles that operate an ant's antennae. Birgit and Wulfila had 10 pages' worth of observations to make about ant antennae muscles. While no doubt fascinating, this was not the kind of ant muscle action I was after.

• After some time of not finding anything about ant muscles, I started to guess. I looked at close-up photos of ants and I wondered if the places on their legs that get wider are the places that contain muscles. For example, what amounts to the thigh on this ant looks relatively muscular.

You don't want to meet one of these ants in real life. This is a bullet ant, native to Belize. Its sting is so powerful, people say it feels like you've been shot with a bullet. The pain from the sting feels like "waves of burning, throbbing, all-consuming pain" that lasts for 24 hours.
(Photo from Guide to Belize)

• Finally, I found the answer I was looking for. The second section of the ant's body, called the mesosoma, "is packed full of muscles" that operate the ant's six legs.

In this diagram, the pink section called the mesosoma is where most of the ant's motion-related muscles are contained. In the diagram at the top of this entry, that same section is called the Alitrunk. Most of us call that section the thorax.
(Diagram from ASU's Ask a Biologist)

• According to a book on entomology, the muscles in the thorax (a.k.a. the mesosoma) extend out to the first joint of the ant's leg, called the coxa. There is a pair of muscles (adductors and abductors) on either side of the coxa that help the ant move the legs back and forth. There is another pair of muscles (levators and depressors) that help the ant lift the leg and put it back down.

• In the ant, most of the muscles are around the coxa and the femur, but a few of them are also around the tibia. So my guess based on looking at the bullet ant photo wasn't that far off.
  

• I also learned this other ant muscle fact: when the queen ant has lain her eggs and is nesting, she doesn't leave the eggs, so the worker ants bring her food. But she needs still more food. So the muscles that once supported her wings, which are also in the thorax, get absorbed into her body to supply her with extra nutrition.

There you have it. Ants do have leg muscles. They're even somewhat similar to the leg muscles that you and I have, albeit very tiny. The cloak of mystery that has shielded us from the truth about ant muscles has been thrown back, thanks to your Apple Lady.


If you liked this, you might also be interested in this entry about the speed of ants.


Sources
Rob Campbell, MadSci Network, How can ants carry so much weight in proportion to their size?
Howstuffworks, Ant
Arizona State University, Ask a Biologist, Face to Face with Ants
Cedric Gillott, Entomology, second edition, pages 424-426. Thanks, Google Books!
Encyclopedia Britannica, insect, Form and Function, External Features, thorax
Encyclopedia.com, Dictionary of Zoology, coxa
C. Claiborne Ray, "Ant Power," The New York Times, November 20, 2007