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these critters are immature Tabanidae larvae, most likely first stage larva. headcapsule with eye-spots, large and curved mandibles, striped epidermis and Grabers' organ in the anal segment are typical. The latter organ are these 4 small black spots at the end.
Tabanidae larvae are predators which can be fed with other animals like isopods or soft-bodied organisms.
In this young stage ID to genus or species level is very difficult but as they are retrieved from an aquatic habitat and they are white, it would possibly be something like Chrysops, but not sure though.