When people find hawk-moth caterpillars (or moth pupae) they often ask us what's the best way to rear them into adult moths. Ideally, the process is best handled by nature, but if you really want to witness the process in captivity then here's a few tips...
The caterpillar should be placed in an airy container and given the correct food plant. Depending on the species it's usually best to line the base of the container with a layer of potting compost and some dry leaves.
It's really important to make sure that your caterpillar always has plenty of fresh food. This needs to be picked daily. The stems of the food plant not only provide food, but also give the insect something to climb on and a place to hide itself. Stand the stems inside in a jar of water inside the container. Place some sponge or cotton wool or kitchen tissue in the neck of the jar to prevent the caterpillars falling in and drowning.
Caterpillars are eating machines, and while a lot of food goes in the front end of the caterpillar, a similar volume of matter comes out of the back end, so you'll need to clean the base of the container daily.
When the caterpillar is fully grown, it will very often turn a darker colour, lose interest in eating, and start running around looking for somewhere to pupate. When it does, the pupa should be placed on a layer of earth in a small sealed (but well ventilated) container.
The container should be kept in a cool but frost-free place until next spring. If you keep it indoors the heat will make it emerge too early and (depending on the species) it'll likely die. Winter = no flowers = no nectar = starvation.
In spring the pupa should be placed between the grooves of a sheet of corrugated cardboard. It should be misted with water occasionally to produce a humid atmosphere and to induce the emergence of the adult moth. Avoid over-spraying as too much moisture can promote the growth of mould which could kill the developing insect. I know, I know - too much water - bad, too little water - also bad. It's not easy.
When the adult is about to emerge place a number of twigs and stems in the emergence tank. The twigs are required by the moth to climb up before expanding and drying its wings. If no suitable supports are provided then your moth will have deformed wings and be unable to fly.
You should release the emerged adult in the same area as you collected the caterpillar. When releasing the moth during the day make sure it's in a place with some foliage to shelter in. If it's out in the open it could be spotted by a bird and be eaten.