This caterpillar is now in its fifth instar. If you haven't done so already, go back and look at the first instar pictures (and the third instar ones if you want); those first instar pictures were taken on September 10th, a mere 13 days ago. What was a one- to ten-milligram bundle of cuteness is now a two- to three-gram tomato plant defoliating behemoth (and, if anything, even cuter).
You may or may not know that insects are supposed to have three pairs of legs (six legs total), while spiders have four pairs of legs (eight legs total). Knowing this, let's count the legs on the caterpillar pictured above. A quick count reveals that the caterpillar has sixteen legs (eight pairs), significantly more than the six it's supposed to have. Are caterpillars not insects? Have we been wrong all along in believing that insects have six legs?
The resolution to this great leg-counting problem is to notice that the first three pairs of legs look significantly different from the last five pairs of legs. Only the first three pairs of legs (the black and white striped ones) are true legs: they have a hardened exoskeleton with joints, and are found on the insect's thorax. The other five pairs of "legs" are actually extensions of the caterpillar's body wall called prolegs, and are not true legs. At the end of the prolegs are a number of small hooks, called crochets, that are used to attach to whatever substrate the caterpillar is currently on. It is these crochets that provide the majority of a caterpillar's "sticking power" on branches.
Hind end of a fifth instar Manduca sexta. The crochets are visible at the very end of the proleg.
The prolegs are not hardened structures, instead they are shaped primarily by the turgor pressure of the insect's hemolymph (blood). When the caterpillar wants to move, muscles on the body wall retract the soft prolegs into the body cavity (which also disengages the crochets), and when the caterpillar wants to reattach the prolegs, it relaxes the retractor muscles, which allows the prolegs to extend using the hemolymph's turgor pressure.
Here's a sequence showing a caterpillar walking forward that demonstrates the proleg retraction and extension:
Two sequential images of a M. sexta walking forward.
In the first (top) image two of the prolegs are retracted into the body cavity. In the second image those two prolegs have moved forward, are extended, and their crochets are attached to the substrate (the crochets are visible at the end of the prolegs). Here's a closer-cropped version of the first image showing the proleg retraction:
The caterpillars are now less than a week away from pupation. Stay tuned for more!