Slugs are terrific source of nutrients for a Venus flytrap. They are particularly hand for manual-feeding as they’re so slow moving that you can make sure they’re nicely positioned inside the trap to prevent them getting “half-digested”.
Any bug caught by a venus flytrap must be fully digested inside the trap. The clenching power of the traps leaves locks the bug inside and the plants digestive juices do the rest of the work. However, some times mother nature doesn’t work as intended! If you find a half consumed bug stuck to the outside of your trap, then it maybe wise to pry the bug loose. Firstly you are saving a poor bug from dying via starvation and thirst, but you are also doing your plant a favour. If the bug has a few legs stuck inside the trap, the trap may not seal adequately and it could potentially leak. When this occurs the trap can be damaged, so for this reason it is often a wise idea to remove the bug manually to save the trap from damage. Just make sure you don’t damage the trap in the process!
Here is a video demonstrating a fly half-stuck in a trap.