[2019] What do Glyphids eat?

Besides Greenbeards, I mean.

Let's look at the facts. Due to their teeth, Glyphids are quite obviously carnivores, but there isn't much in the way of life down there that could be considered prey. Unless of course they eat the Mactera, who appear to be herbivores(?) and eat whatever insects would. But this doesn't explain why Mactera can be seen fighting side by side with the Glyphids whenever the dwarves are around.

The only other prey animals I can think of would be loot bugs, silicate harvesters, and cave cruisers, but they seem to be doing pretty well for themselves in a cave where they're outnumbered 1000 to 1.

What do you guys think?