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I have a large stratiomyid larva which I think is Stratiomys but possibly not full grown. What do they eat, please? I've put it with some filamentous alga and Zannichellia in very shallow water from the brackish pool it was in, but not seen any sign of it feeding. It just wanders around the pot. It has some damp soil too. Is this what it needs for pupation?

Thanks
John