Ya know, I have a theory. In the spring I bought a culture of african nightcrawlers, a relative of the red wiggler. I dumped them into my pen and they've become established in the substrate. Occasionally I'll see one of my snakes eating them, usually Fatty, Lime, or Lemon when she was still with me *With Don now, not dead. XD*. The other thing I noticed is that both of the snakes who I've seen eat the worms have a bit of swelling near the base of their heads. I know that one snake species that's related to garter stores toxins from toads it eats, so perhaps garters can store toxic materials in specialized glands?