Wading for Bugs: Exploring Streams with the Experts

Part V: Dragonfly Detectives

100 PART V Dragonfly Detectives About Dragonflies (Odonata) Dragonflies and damselflies (Odonata) have an ancient lineage on earth, going back over two hundred fifty million years. Their colorful adult markings, large size, and conspicuous behaviors often afford them a charismatic megafauna status among aquatic invertebrates. Their name derives from the Latin odon meaning toothed; this reflects the large mouthparts of the larvae, which allows them to grab and manipulate prey. Dragonflies and damselflies are predators, both as larvae and adults. Most larvae, often called nymphs or naiads, are tied to permanent water, and the few that inhabit temporary habitats have shorter life cycles. Some larvae live on the wet margins of streams and bogs, demonstrating a quasi-terrestrial lifestyle. While some adults spend all their daylight hours cruising stream edges or perched on vegetation near water, others venture away from water for much of their adult lives. Morphology The simple body form of the nymphs is similar to that of adults, though the head and eyes are smaller. The distinctive characteristic of all odonate nymphs is the lower “lip” or labium. It is equipped with various hooks, spines, and teeth. Because the labium is folded under the front legs, it can be sprung forward to capture prey. Six legs, borne on the three segments of the thorax, are often robust and make these predators fairly agile in the aquatic environment. The abdomen of damselflies is more slender and longer than the form of their dragonfly cousins; damselfly nymphs are also distinguished by three external gills on the posterior ends of their abdomens known as caudal lamellae. Dragonflies have gills that are contained within the abdomen; expanding and contracting their abdomens draws water across the gills. This mechanism can also be used in propulsion—some About Dragonflies 101 dragonfly nymphs will rapidly expel water from their abdomens, producing a water jet that shoots them forward. Adults have huge, multifaceted compound eyes. Each eye has approximately thirty thousand facets. Their freely movable heads further enhance the insects’ ability to see prey, detect avian predators, and search for mates. Wings borne on the tough thorax are held horizontally at rest in dragonflies; in contrast, wings are held together above the body at rest in damselflies. Their wings move in a paddle-like fashion that makes odonates excellent fliers; flight for some dragonfly species has been estimated at speeds between twenty-five to thirty-five kilometers per hour. Adults breathe directly through spiracles in the thorax and abdomen. Life History Odonata are hemimetabolous, which means that nymphs develop gradually, without a pupal stage. The egg stage takes from eight to thirty days depending on the weather. Some species undergo diapause during the winter. After hatching, it takes from one to three years (for some rare species, five or more years) for them to undergo ten to fifteen molts, gradually developing wings in the final instars. Their hinged labium makes them highly successful predators of other invertebrates and young fish. The various families in the order have particular predatory strategies: burrowers that lay and wait for their prey, sprawlers that more actively hunt in sediments or organic debris, and climbers that lurk in vegetation to stalk their prey. The nymphs are non-discriminating predators, and they will eat any other invertebrates in the water. At times, these nymphs may be among the few remaining taxa in a habitat, and as a result they prey upon themselves. When nymphs crawl out of the water to emerge, they cling to aquatic plants while their exoskeletons split and blood pressure fills out their wings. This is a vulnerable time in their life history, and emergence often occurs at night to avoid predators. At first their color is pale, but eventually mature body colors develop, with the male often appearing much more conspicuous than the female. Adult behaviors, as with the nymphs, vary greatly, 102 Dragonfly Detectives from those that cruise constantly in search of prey, to some that perch on vegetation by hanging vertically, to others that perch horizontally and snatch flying prey like flycatchers. Adults generally live from two to four weeks, culminating in mating activities that might include continuous tandem attachment of the male and female, even during egg deposition into the water or on aquatic plants. This behavior is a form of mate guarding, by which males prevent access to the female by males of the same species. Bioindicators Odonates do not require...