ProLOGUE (Ha, Ha) / Disclaimer
Last month, when I wrote a post all about the osmeterium, I asked for suggestions regarding other anatomy you’d like to learn about. One request was for information about “hydraulics of prolegs.” I’ve been reading up on prolegs for weeks now, and I’m psyched to share what I learned with you. But before we get started, I need to acknowledge three things:
(1) The information in this post comes from a fantastic article called “Locomotion in Caterpillars,” written by L.I. van Griethuijsen and B.A. Trimmer and published in Biological Reviews in 2014. For anyone who wants to delve deeper into this topic, I can’t recommend the article enough.
(2) Most of what we know about caterpillar prolegs and locomotion is based on studies of tobacco hornworms (Manduca sexta). Researchers assume that most of these studies can be generalized to the many species of caterpillars of relatively comparable size that move in similar ways.
(3) Because Sam Jaffe took a phenomenal photo of Luna prolegs, I wanted to feature prolegs during the Luna’s reign as Caterpillar of the Week. Here, we’ve pictured prolegs of several different caterpillars. They aren’t all Lunas. Check photo captions for IDs.
What you actually want to read
~What are PROLEGS, anyway???~
Caterpillars, like any other insect, have six true legs, located on their thorax, right behind their head. In most cases, however, their true legs don’t do much in terms of helping the caterpillar move around. (An example of an exception would be when caterpillars drop down on a silk thread—they usually use their true legs to crawl back up.)
Instead, caterpillars use their prolegs, which are attached to their abdomen, to aid in locomotion. Not the way you and I use our legs, but we’ll get to that. Caterpillars have different numbers of prolegs, from two pairs to five pairs. The hindmost pair is called the anal prolegs. Noting the number of pairs of prolegs on a caterpillar, and how they’re configured, can help you identify its species.
When a caterpillar becomes a moth or butterfly, it will continue to have six true legs. But it won’t have any prolegs. Prolegs are an adaptation that’s just for larvae.
~How do PROLEGS work?~
Caterpillars use their muscles to move. But muscles can’t contract on their own. People and other vertebrate creatures have skeletons—hard structures that help us contract our muscles. Many soft-bodied animals (like octopuses) can harden their bodies by pressurizing their internal fluids, which works like a skeleton to help their muscles contract.
Caterpillars, on the other hand, have no skeleton and can’t pressurize their bodies to harden like a skeleton for the purposes of walking. So how do they use their muscles? By attaching their bodies to a hard surface. How do they do that? They use their prolegs!
Prolegs aren’t actually used to propel caterpillars forward. Instead, they act as anchors that hold the caterpillars tightly in place during movement of other body segments. Prolegs bind the caterpillar to a rigid substrate, such as a twig. That twig acts like an external skeleton, which provides the stability caterpillars need to contract their muscles.
There are many, many muscles involved in caterpillar locomotion. As caterpillars crawl forward, they gradually detach their prolegs from their substrate, beginning with the anal prolegs in back, and ending with the frontmost pair. They use a system of muscles to pull their bodies forward, and then reattach their prolegs to secure themselves in place so they don’t lose any ground.
At the tip of each proleg is a pad called a CROCHET, which is covered in tiny barbs. If you look at the attached picture of Luna prolegs, you’ll see translucent pads with tiny brown barbs attached directly to the twig. Those are its CROCHETS. The tiny barbs work like Velcro to tightly attach the prolegs to all kinds of substrates, including leaves, twigs, and fingers. If you’ve ever held a caterpillar, you’ve probably felt those sticky crochets on your skin!
To disengage the crochets, each proleg has two muscles. When these muscles contract, the crochets release and the prolegs tuck up, while the caterpillar’s body propels itself forward.
During movement, prolegs’ PLANTA HAIRS collect sensory information about their substrate. In the attached picture of Luna prolegs, you’ll see black hairs protruding from its green prolegs. Those are its PLANTA HAIRS. Each of these hairs has its own sensory neuron, which provides information to the caterpillar about the substrate. If the planta hairs sense an obstacle in a proleg’s path, that triggers a response that lifts the proleg higher, and sends a message to other muscles to lift the other prolegs higher when they reach that obstacle. Interestingly, these messages usually aren’t transmitted through the caterpillar’s brain, so the messages move more quickly to their intended recipients.
When the prolegs’ body segment has moved forward completely, it’s time to reattach the prolegs. The proleg muscles relax to their resting position, which extends the prolegs. Their barbed crotchets automatically attach to the substrate, and the caterpillar is once again anchored to its external backbone. Proleg anchoring provides rigidity for future muscle contractions, so the caterpillar can continue to crawl.
Who knew a caterpillar taking a couple of steps could be so complex? This continues to confirm what we at The Caterpillar Lab have known all along… caterpillars are AMAZING!
Call to action!
Curious about any aspects of caterpillar biology? Write your questions in the comments and we’ll look into using them in future posts!
We’d also love your feedback on these long posts. Do you read them in full? Would you prefer them to be broken into smaller posts? Are they accessible to folks with varying levels of scientific background? Please let us know—your input matters to us!
--Liz Kautz, Education Director
The Caterpillar Lab