By Steve Oden
The attack comes swiftly and silently. One moment the small tadpole is hovering over the mud, the next it is grasped by a pair of jaws that seemed to appear out of nowhere. The tadpole has just become a victim of an “odonate”. You probably know them as dragonflies, those colorful aerial acrobats that fly around near bodies of water. However, students experiencing the “EcoDetectives” lesson here at the MathScience Innovation Center come face to face with a very different looking creature.
Scientists classify dragonflies into the order Odonata, a group of insects that also includes the damselflies.To tell the difference between these two you need only observe their wings: at rest, a damselfly’s wings are folded back over its body; a dragonfly’s wings are held out perpendicular to its body. Both fly through the air capturing insects with great efficiency, and you’ll often see them flying very close to the surface of the water. If you could slow things down you would see that during these close passes they dip their abdomens into the water to deposit their eggs. The eggs hatch into critters that look very little like the adults, and anyone dipping a net into the mud in our MSiC pond is likely to capture one of these interesting creatures.
Unlike its colorful, quick, graceful adult counterpart, the odonate “nymph” (or “naiad”) tends to be drab, slow, and downright clunky! To get oxygen, the dragonfly nymph circulates water in and out of its body, while the damselfly nymph has several slender, paddle-like gills protruding from its posterior. Different types are designated as “crawlers”, “sprawlers”, or “burrowers” to indicate their hunting habits. The majority of those collected in the MSiC pond are “sprawlers”. The nymph’s brownish-gray coloration allows it to blend into its muddy habitat, an important adaptation for an animal that not only is prey for others but also is itself a voracious predator. A close-up examination of the head reveals what appears to be a mask but is actually a formidable hunting tool called a “labium”.
The labium is like a lower lip, and it covers a pair of powerful jaws. The labium is attached to a hinged, arm-like structure folded up beneath the nymph’s head. The nymph hangs out in the mud, slowly inching up on its prey. When it is close enough, the nymph lunges and the labium suddenly unfolds and extends forward to grasp the prey. The combination of the nymph’s lunge and the labium extension adds to the striking range by an additional 1/3 the nymph’s body length! And what do they eat? Depending upon the size of the nymph, prey may include animals as large as small minnows and tadpoles.
As deadly and terrifying as this animal appears to be, it is actually quite docile to humans. They do not bite people and during the warmer months of the school year, the squeals and cries of students around the MSiC pond are those of excitement and not terror. While many students are at first reluctant to touch something they think looks like a spider or a cockroach, by the end of the lesson they are competing to be the one who spots them and gets to carry them to the observation table. To achieve their objective of evaluating the pond’s ecological status, the students collect other pond denizens such as diving beetles, water scorpions, and snails. However, the undeniable star is a drab, clunky, slow-moving creature with a diabolical hunting tool – the odonate nymph.
Steve Oden is a MSiC 6-12 educator.