It is possible when collecting daphnia to include a couple of dragon-fly larvae in the net. Regrettably, any strainer that lets daphnia via will also pass the little larvae.
There are numerous species, the ringed club dragon-fly (Cordulegaster annulatus) and also the giant dragon-fly (Aeschna grandis), are two of the very best recognized, but they can all be considered under one heading. Their method of attack would be to lie and wait until a fish comes within striking distance, then rapidly extend the mask which fastens tightly on to the fish.
The mask is the organ carrying the mouth and pincers which could be thrust forward at will. The pincers hold the victim whilst the fish is eaten into. Propulsion is by means of a stream of water violently squirted from the body, even though some species get along by wriggling the tail like an oar.
The life of these larvae lasts about a year. The larva then becomes like a squat beetle, at which stage it leaves the water to turn out to be a dragon-fly.
They're especially dangerous to young fry, and also the only prevention would be to maintain a close scrutiny on live foods.