Worm

Underground at any hour, and across the surface the moment it starts raining. Rain is by far the fastest source. They also jump out of things you break, so clearing grass and the small dirt and stone clutter underground turns up a steady trickle. Graveyards swap them out for maggots, so do not go looking in one.

They wander, and they are quick enough to be irritating with a net. Dig single-tile pits across flat ground before a storm and they drop in and stay put, which turns collection into walking a line and swinging. Both blocks beside the pit have to be full and unsloped or they climb straight back out.

Bait, mostly, and near the bottom of the bait ladder. Fine for casual fishing and not worth carrying once better bugs turn up. One can also go into a cage if you would rather look at it than fish with it, though that is a one-way trip without Shimmer.

A small fraction of them generate gold instead of pink. Those sell for a great deal more and behave far better on a hook, so it is worth glancing at what you netted before you spend it.