Bees are very supple and can be posed easily when they’re fresh. Over time, a pinned bee will become dry and brittle; its legs and other body parts can snap off with the slightest bump or jolt. For this reason, it’s important to arrange a bee’s legs, wings, and antennae in just the right position when you’re first pinning it. But sometimes that doesn't happen.
If you identify your bees after they've dried and become brittle, sometimes you'll find that a leg is tucked in such a way that a crucial identifying feature is hidden, or -- particularly in the case of male bumblebees -- you need to extract the genitalia in order to positively identify the bee to species. In these cases, attempting to move the dried leg or extract the genitalia will almost certainly result in a bunch of broken bee pieces.
It is possible to rehydrate a dried bee to the point where you can manipulate its body parts without breaking them off. Here's how.
Use an airtight container that is at least three or four inches tall. Add about ½" of water to the container. Make a styrofoam raft for your pinned bees -- you want the raft to be wide enough that the bees won't make it top-heavy and be at risk of tipping over. The point is to keep your bees out of direct contact with the water so their fur doesn't get matted down. Once you've pinned your bees to the raft, put it in the container with the water, seal the lid, and set it in a safe place for 24 hours. After 24 hours, check your bees. Gently try manipulating the body parts. If the bee still seems brittle, put it back in the hydration chamber a little longer, for up to a total of 48 hours. The longer they're in the hydration chamber, the greater the risk of mold and mildew growth. Once you've rehydrated your bee, move its parts to where you want them and carry on with your identification work.