Critter Corner No. 27 – Dragonflies by George Sly
If asked to do so, we can often identify an ecosystem simply by seeing a photo of one of its iconic animal inhabitants. Example? A polar bear would surely bring thoughts of the tundra to mind. Seeing a Malayan tapir would just as assuredly elicit images of the steamy southeast Asian rainforest. Similarly, the American bison provokes a mental picture of vast, rolling stretches of tall grass prairie resplendent with big bluestem, compass plant, blazing star, and Indian grass.
So it is that one of Goose Pond FWA’s signature animal groups, the dragonflies, symbolizes the wetland ecosystems of the world. Over 6000 species of dragonflies soar across the swamps, lakes, and marshes of our planet. The only continent without them is, predictably enough, Antarctica. Here, in Indiana, we have around 100 species. At Goose Pond FWA, 21 species of dragonflies and 9 species of damselflies were recorded during the 2011 Biodiversity Survey.
Dragonflies are an ancient group with a fossil record extending back some 250 million years. Curry’s authoritative Dragonflies of Indiana, notes that Indiana was once home to dragonflies having wingspans of two feet. During the periods when much of Indiana was glaciated, there were no dragonflies here at all.
Damselflies are sometimes confused with dragonflies and are closely related. They make up the other major group within the insect Order Odonata. The body structure of damselflies and dragonflies is similar but the former are usually more lightly built. At rest, most dragonflies hold their wings level and project them laterally. Damselflies hold their wings along the abdomen or raised above it.
Dragonflies are excellent fliers. They can fly high, hover, fly backwards, and can attain speeds near 30 mph. Their prey is usually smaller insects such as flies, midges, and mosquitoes. They snatch these from the air midflight. Larger prey such as butterflies and even smaller dragonflies may be eaten too.
In order to be effective aerial predators, dragonflies have evolved excellent eyesight. Their huge compound eyes have thousands of individual units (ommatidia) each with its own retina. The result is vision that is nearly 3600, can form sharp images, and is highly sensitive to a variety of colors/wavelengths (including UV). Smaller simple eyes with only one lens are sensitive to light levels and help mediate the dragonflies’ circadian rhythms.
The reproduction of dragonflies is unusual. Mating pairs are often seen perched or flying in what entomologists refer to as the ’wheel position’. At this time, the male grasps the female by the head using the tip of his abodmen. The female then positions the tip of her abdomen under the male’s. Here she takes up sperm which the male has previously transferred from his primary genitalia (at the tip of the abdomen) to the penis located just behind the thorax.
Egg laying varies by species and the ova may be laid into the water, in vegetation near the water, or even in dry places which may later become flooded. Eggs may be laid singly, a few at a time, or deposited in a mass.
Dragonfly eggs hatch into an aquatic stage called a nymph or naiad. At this stage the dragonfly breathes by means of gills. Most of a dragonflies’ life is spent in this developmental stage. Naiads live in the water for two or three years. Adult dragonflies typically survive for only a few weeks.
The nymphs are voracious predators themselves and feed on other aquatic insects, worms, mosquito larvae, and even small fish and tadpoles. The nymphs capture prey using a highly modified lower jaw, the structure of which is distinctive to their kind. Called the labium, this mouthpart is folded back between the first pair of legs when not in use. The labium is shot out with great speed and its terminal, hooked lobes seize the prey in a death grip. For a small aquatic invertebrate, a dragonfly nymph would be the stuff of nightmares. I can’t help but think of the ghastly extraterrestrial xenomorph in the sci fi movie Alien.
Because dragonflies require clear, oxygenated water their presence is considered a bioindicator of a healthy ecosystem. They also play their own important role as prey for birds and fish. Dragonflies are harmed by loss of freshwater habitats such as ponds, bogs, and ephemeral pools. Factors causing the loss of such wetland habitats include draining for agriculture or development, invasion by non-native species such as Phragmites australis grass, and pollution from both point and nonpoint sources.
A prairie without its bison seems barren. The jungle without tigers has lost its mystique. A coral reef, absent its sharks, is no reef at all. A marsh, its sky unpatrolled by an irridescent dragonfly air force, would be but a shabby imitation of a wetland.
Credits/Resources
Common whitetail photos by the author.
Curry, James R. 2001. Dragonflies of Indiana. Indiana Academy of Sciences. Indianapolis.
Dragonfly nymph photo courtesy Missouri Department of Conservation
Dragonflies. National Geographic. https://www.nationalgeographic.com/animals/invertebrates/facts/dragonflies-insects
Green darner pair by Michelle Von Sutphen at www.knowyourinsects.org