Caterpillars grow and develop to their maximum size, then prepare to form a cocoon. Many changes are required. The pupa stage of most Lepidoptera is immobile, so the caterpillar must select a site for pupa formation. Many butterfly caterpillars make their pupa (called a chrysalis) while tied to the stem of a plant by a silk thread. Caterpillars of the tobacco hornworm will dig a nest in the soil. Caterpillars reared in captivity will exhibit their wild behaviors and a rearing process must accommodate them. Butterfly caterpillars may be given vertical sticks as sites for pupation. Caterpillars of the tobacco hornworm will start digging through their food in an attempt to make an underground nest. This process is messy, so the food is typically removed and replaced with a tissue. The caterpillar can chew the tissue and make a nest.
Pre-pupa stage of the tobacco hornworm caterpillar
As the caterpillar begins forming its pupa, the caterpillar cuticle separates from the cells below. The caterpillar becomes less active and incapable of crawling. The cuticle of the pupa is formed under the cuticle of the larva. Upon close observation, the newly forming pupa cuticle can be seen through the cuticle of the larva. When it is time to molt the larval cuticle will split along the dorsal surface and the pupa will emerge.
Living With Insects Blog 
Thank you for this post! I have a prolific garden that produces many tomato plants and the past two years I have become fascinated watching the life cycles and habits of Manduca Sexta and Achemon Sphinx (we have grape vines as well). A few days ago, before we were about to have nighttime temperatures dip below freezing, I found a late blooming lone tobacco hornworm. I cut the remaining leaves/branches of living tomato plants, put them in water and secured them in the middle of a large-ish flat planter with a few inches of dirt. I brought the hornworm and his “terrarium without walls” into the house to escape the freeze. He is very large and should be nearing the pupate stage but I am worried perhaps I’ve messed up his schedule-does bringing him into a warmer environment lengthen the time between 5th instar and settling into the soil to pupate?
Thanks again for your post!
-gwen
Bringing the larva inside into warmer temperatures should increase development rate. The transition from larva to pupa is not instantaneous. Under laboratory conditions, the final instar tobacco hornworm larva feeds for about 4 days, then purges its gut and enters a wandering stage that lasts about 2 days. In nature the wandering stage larva drops from the plant and digs a pupation chamber in the soil. The larva then enters a prepupa stage that lasts about 4 days. During this period the larval tissues are liquified and a pupa cuticle is produced underneath the cuticle of the prepupa stage larva. The larva then molts to a true pupa or cocoon. In the laboratory, the pupa stage is about 18 days. However, in nature at this time of year, the light/dark cycle (short days, long nights) causes the pupa to go into diapause (a resting stage) fort he winter. An adult may not emerge until spring. Release of adults is not recommended because they are pests of crops.
Thank you! This particular tobacco hornworm has definitely entered the ‘wandering’ stage and has been burrowing here and there in the large container I allotted for it after bringing it inside. It’s probably time to put the caterpillar out in the garden and let nature takes it’s course. I know they are considered pests but our garden produces so many tomato plants I feel we can spare a few for these little beasts.