Fence lizards, fire ants, and wood frogs are certainly the current stars of the Langkilde Lab. While their names may be on the marquee, there are a number of other players with small – but important – parts in our lab. Eastern newts, chiggers, valley-and-ridge salamanders, green iguanas, and jumping beans are just a few of the critters who have turned up in various projects. My favorite supporting character in my work, however, is the dragonfly larva.
Most people are familiar with dragonflies as aerial predators, welcome almost everywhere for their voracious appetites for mosquitoes and other pesky insects, which they deftly intercept in mid-air with remarkable accuracy (link). But their predilection for hanging around wetlands and ponds hints to a lesser known aspect of their lives. Their young, called larvae or nymphs, hatch from eggs laid on submerged aquatic plants and develop under water before climbing out and emerging from their shed exoskeleton as a flying adult. During the larval stage, however, dragonflies spend their lives feasting upon small aquatic animals with similar gusto as the adults. (In fact, baby dragonflies also like to eat the aquatic baby mosquitoes, giving mosquito populations a one-two punch). Some types of dragonflies, particularly the “darners” (family Aeshnidae), are also some of the most voracious predators of my primary study subject – tadpoles. In several projects, I’ve used the larvae of Anax junius (aka, the green darner) as a predator of tadpoles. Anax dragonflies grow rather large and are perfectly equipped to eat even fairly large tadpoles. And the tadpoles know it – I and other researchers have documented well how tadpoles that detect a dragonfly feeding on other tadpoles exhibit strong antipredator responses, becoming less active, hiding more, and growing larger tails. But how do these dragonflies, with their apparently small jaws and bodies that lack any fins for swimming, manage to feed on tadpoles?
This is the cool part. First thing to know is that dragonfly nymphs dart very quickly through the water by means of jet propulsion. They draw water into their expanded abdomen to breathe, but when they need to move quickly they force this water rapidly out of their anus by compressing the abdomen and increasing the internal pressure. A stream of water jets out with enough force to rapidly launch the dragonfly forward and, if it is out of the water, can wet your face like a squirt gun. This helps them catch tadpoles, small fish, and other prey that move much faster than the dragonflies can crawl. I’ve watched dragonflies shoot out from their perch into the open water to snatch a tadpole, and then casually wiggle their legs to help them drift back to their resting place.
When it comes time to actually grasp the prey, the larval dragonfly has another trick up its sleeve. Beneath its mandible (mouthparts proper) is a labium – an arm-like structure with sharp hook-like claws at the end. (In some dragonflies, the labium has a less menacing shape more suited to catching plankton). The labium remains neatly folded under the dragonfly most of the time. When prey is near, the dragonfly again compresses its abdomen to produce hydraulic pressure. This time, however, a sphincter muscle in the anus prevents the water from exiting and propelling the dragonfly. Instead, the labium is forced forward by this pressure, unexpectedly extending the reach of a dragonfly that seemed to be at a safe distance. The claws at the tip of the labium grab the prey and pull it back, where the mandibles can begin to tear it apart to be swallowed. The labium holds onto the prey with its recurved hooks so the dragonfly can handle a very large prey item (even one still struggling) while the mandibles go about eating it at a relatively relaxed pace.
As someone who loves tadpoles, you might think Anax wouldn’t be at the top of my list of favorite creatures. But I’m far too impressed by how these little ‘bugs’ manage to terrorize the pond with their remarkable adaptations, instilling fear in tadpoles and other potential victims. In fact, “anax” means “king” or “military leader” in ancient Greek – a name that certainly suits these tyrants ruling over their aquatic kingdoms.