Friday Cat-erpillar Blogging: Get a Grip

Caterpillars are excellent and sure (pseudo)footed climbers. Caterpillars have special extensions of the abdomen for gripping plants called prolegs. At the tip of the proleg are rows of small hooks called crochets. The crochets will tightly grip a surface. A caterpillar clinging to your finger must be pried off, similar to separating velcro strips.

Top: Hydrangea Sphinx Caterpillar clings to a stem using its prolegs (extensions of the abdomen) while its true legs dangle in the air.
Bottom: The tip of a proleg showing the row of crochets used for gripping the plant.

The prolegs of the caterpillar are powered by well studied muscles, the planta retractor muscles. When a planta retractor muscle contracts, the proleg is lifted off the plant. In the retracted position the proleg hovers in the air off the plant and is easily moved. The caterpillar swings its leg forward through the air. After the movement is complete, the retractor muscle relaxes and the proleg returns to the plant, with crochets extended to grip the plant. The crochets grip the plant much the same way that a cat’s claws will grip your hand. However, the crochets are far smaller than the claws of a cat and do not penetrate deep into the plan (or penetrate your skin the way a larger cat’s claw might).

The grip of the caterpillar crochet is the bio-inspiration for a “Compliant Gripper” for use by a robot. The Compliant Gripper is currently under development by the Seoul National University Biorobotics Laboratory. The gripper uses the same principles of the caterpillar crochet. The gripper features sharp metal pins that will grip the object. The pins are mounted on a structure with shape memory alloy coil actuators and sides that can buckle under pressure. The buckling enables the “crochets” of the gripper to engage the surface. The gripper is demonstrated in the video below and is capable of lifting a small brick.

About jjneal

Jonathan Neal is an Associate Professor of Entomology at Purdue University and author of the textbook, Living With Insects (2010). This blog is a forum to communicate about the intersection of insects with people and policy. This is a personal blog. The opinions and materials posted here are those of the author and are in no way connected with those of my employer.

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2 Responses to Friday Cat-erpillar Blogging: Get a Grip

Na'Tashia Henderson says:

February 9, 2012 at 1:56 am

A ‘bug’ is not an insect if it doesn’t have three legs. I know the “prolegs” don’t count as real legs, and as you stated are only extensions of their abdomen, but in my mind I just cannot consider a caterpillar to be an insect because they have more than three legs that are used for locomotion. Why are these legs considered “prolegs” instead of just regular legs? Is it because a butterfly or moth only has three legs and not the extra ones the caterpillar has before metamorphosis? This is a topic that has always sat in the back of my mind ever since I began taking this course.

jjneal says:

February 9, 2012 at 2:02 am

Legs and prolegs are both extensions of the body. However, legs are jointed appendages. Prolegs lack joints. Morphologically they are very different.

They both function in walking, but they function differently. The true legs walk and dig into the surface with claws. The proleg have rows of hairs (crochets) that grip the surface. Look closely at them and describe the differences.