Ok, if you already know it is a catalpa worm, then it is NOT poisonous, OR venomous. 'Poisonous' means something is toxic if it is ingested (eaten). 'Venomous' means its bite or sting is dangerous.
Catalpa worms eat ONLY catalpa, so it won't eat if it is not on that tree. Further, it probably won't eat just 'any' catalpa. Plants defend themselves with toxic chemicals in their sap, and some individual plants and trees are more toxic than others, even within the same species, and in the same location! That is why you will see some trees almost completely defoliated, while other trees right next to them are virtually unaffected.
Sphinx moth larvae, including catalpa worms, pupate in the soil, so if you continue keeping your caterpillar, provide a terrerium with a few inches of slightly compacted soil so it can burrow down when it is time for it to graduate to the next part of its life cycle.
