Dragonfly nymphs are mostly aquatic and inhabit ponds, slow-flowing rivers or riparian vegetation. They are predatory, mostly feeding on other aquatic insects (including other dragonfly nymphs), although some larger species have been known to feed on small fish and tadpoles.
Dragonfly nymphs capture their prey using their extendable labium, positioned underneath the head. At rest, the labium is folded away, but when prey approaches the nymph, the labium is thrust out faster than the prey can react, resulting in its capture. The labium is equipped with gripping jaws and sharp bristles which ensure that once captured, prey is retained securely in the grip of the nymph.
90-95% of the lifecycle of a dragonfly is spent as an immature and depending on the species, development can take a few weeks to 3 years. In that time a dragonfly larva may moult between 6 and 15 times. The photos above show the shed skin of a dragonfly nymph with the labium still in tact and folded under the head. The ventral view shows just how large the labium is.
To view a short video of a dragonfly nymph catching a worm, follow this link: http://everest.ento.vt.edu/~carroll/insect_video_feeding.html#dragonflyvideo
Photo of dragonfly nymph courtesy of David Jones http://www.mybitoftheplanet.com/