Actually, diffusers will often make the glare worse, not better. Instead of a few sharp hotspots the whole animal reflects light. Really dark salamanders can look white.

One of the ways to reduce this is to use a polarizing filter. The filter can reduce the glare although reducing it too much results in a very flat photo of many amphibs. It can make their normally slimy skin look dry.

Somewhere online there is a tutorial on how to use polarizers to reduce glare on amphibs.

Another way to do it is remember the old physics rule - the angle of incidence = the angle of reflection. This means if you hold the flash at a different angle you can bounce the reflection off away from the lens. Unfortunately, with all the rounded edges on an amphibian body, it is hard to eliminate these.
-----
Chris Harrison
San Antonio, Texas