I have tried feeding E. posticus all sorts of insects, from fully alive to halfdead and dead, but nothing worked. But they prefer the high proteine fish flakes and they eat the dried sikworms that is also rich on proteines.
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About the ant question, then they will not eat the ants, but some species live in a sort of mutualism.
Some ants will bite off the antennas of certain roaches and keep them in the ant nest. The roaches get confused and loose orientation. So they wander around like cattle and is kept alive as long as the ants wants it...
Certain Diptera (Conopidae) follow raiding army ants and paralyze cockroaches and lay eggs in them. Quite specialized.
Some roaches live with the fungus growing ants where they live from licking secretions in connection with the chewing of the leaves.
The common fact is that such species are adopted to their specialized behaviour over many many generations.
However, to experience such mutualism in ones own hobby setup would be more than a challenge... unfortunately.
BR/ Ole