Phoebis sennae
The caterpillar is in the early stages of pupation.
The caterpillar hangs vertically attached to a silk pad by
the cremaster (a spiny appendage at the end of the abdomen).
The caterpillar and pupae are also supported by a thin silk fiber
extending from the middle of the caterpillar.
For an excellent description of this butterfly, see: http://www.entnemdept.ufl.edu/creatures/bfly/bfly2/cloudless_sulphur.htm