Caterpillars can cling to a surface with remarkable strength when you're trying to pick one up. The more it holds on, the harder you squeeze and pull it. You don't want to damage the caterpillar, so you should know a few things about how to handle one properly.
Rather than try to pick the caterpillar up, place a leaf in front of it and give it a gentle nudge on the rear end. Usually, when a caterpillar is touched from behind, it will walk forward to avoid the touch. The caterpillar should walk right onto the leaf. Carry the caterpillar to a container on the leaf.
Quite a few caterpillars have spines or hairs that appear soft and fuzzy, but can actually produce a nasty little prickle and irritate the skin. Tussock moth caterpillars, for example, may cause a painful rash. A caterpillar with branched spines may sting – don't ever handle one with bare hands!