And if they are insect larvae, so much the better to add some spice to the frogs' diet. Frogs eat insects, not the other way around. There are myriads of different soil nematodes that live on decaying organic matter or fungi, so positive id would be difficult. If they aren't found around and burrowing inside the plant roots, or actively eating leaves, which would happen with a root nematode or other harmful insect larvae, they are probably harmless to plants as well.
There is a VERY, VERY remote possibility something like this could be a free-living rhabdidiform frog parasite, and I've had a recent discussion of this with someone who works in a zoo and has a bit of knowledge about frog parasites in Ranus and northern frogs, but nothing pertaining directly to darts. I've requested some literature on amphibian parasites and will share anything of interest I find in the future.
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Patty
Lost River, Idaho
D. auratus blue
D. galactonotus
D. imitator
D. leukomelas
D. pumilio Bastimentos (soon)