aspen wood... and don't worry. They never moved, it was quite odd. They were wriggling all over the place right up until I dropped them into the tank. Then they froze and the snake ate them one by one. It was less like she was swallowing them and more like she was drinking them. All that's left are some stains. I've been hesitant to handle Tiamat since she's gravid but it might be worth checking on her.