Proleg

A proleg is a small, fleshy, stub structure found on the ventral surface of the abdomen of most larval forms of insects of the order Lepidoptera, though they can also be found on other larval insects such as sawflies and a few other types of insects. In all the orders in which they appear, mainly Hymenoptera and Lepidoptera, prolegs of any form evolved independently of each other by convergent evolution.[1] They appear to have evolved from ancient inactive genes that has since been activated again.[2]

J: medial prolegs
K: anal proleg
(F, G, and H: true legs)

Lepidoptera: Papilio machaon caterpillar with 4 pairs of medial prolegs and a pair of anal prolegs

Hymenoptera: Craesus septentrionalis caterpillars with 7 pairs of prolegs

Prolegs of lepidopteran larvae have a small circle of gripping hooks, called "crochets". The arrangement of the crochets can be helpful in identification to family level.[3] Although the point has been debated, prolegs are not widely regarded as true legs, derived from the primitive uniramous limbs. Certainly in their morphology they are not jointed, and so lack the five segments (coxa, trochanter, femur, tibia, tarsus) of thoracic insect legs. Prolegs do have limited musculature, but much of their movement is hydraulically powered.