Monarch caterpillar hanging in a "J"
I found this caterpillar in the garden at my school with my students in late September. About 12 hours before the caterpillar becomes a pupa or chrysalis, it spins a web of silk on a branch, leaf, fence post, or other hard surface. It attaches its back prolegs to the silk mat and hangs head down (note the tiny black hooks on the prolegs, they are called crochets; the 6 true legs are on the thorax near the head). Then for several hours the caterpillar pulls air into its' body through the tiny breathing holes in the abdomen, called spiracles. The skin begins to loosen and the white stripes begin to take on a light green cast. Just before the big transformation takes place, the whiplashes ("antennae") begin to shrivel and the caterpillar straightens out. Suddenly, the skin begins to split just behind the head and the caterpillar wriggles violently to loosen the skin. The skin then slides up to where the caterpillar is attached by the black stalk called a cremaster. After a minute or two of hard wriggling the skin pops off and a light green chrysalis hangs where a caterpillar had been! (See next picture)