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Aquatic Invertebrates

 

Dragonflies

Damselfly nymph of the genus Enallagma.
Damselfly nymph of the genus Enallagma. Notice the three long leaf-shaped gills at the hind end of its body.

Class: Insecta - Insects
Order: Odonata - Odonates (Dragonflies and damselflies)
Family: Libellulidae - Common skimmers

Characteristics of the family:
Nymphs are crawlers that move about slowly in dense vegetation and bottom debris. They are usually long-legged and dull in colour.

Interesting facts:
Dragonfly nymph of the genus Erythemis.
Dragonfly nymph of the genus Erythemis.
Dragonflies are divided into two main groups: dragonflies (Anisoptera) and damselflies (Zygoptera). Adult damselflies usually rest with their wings folded upwards, whereas dragonflies hold their wings horizontally outwards. Damselfly nymphs have three external leaf-like gills at the end of the body, whereas dragonfly nymphs do not. Damselfly adults and nymphs are smaller and more slender than dragonfly adults and nymphs.


Representative genus: Dragonflies of the genus Erythemis (described here in their nymph stage)
Description Nymphs are short and broad with long legs. Unlike many in this family, animals in this genus lack dorsal hooks on the abdomen. Because they are such slow-moving, sedentary animals, they are often fouled, or coated with a layer of algae or sand that persists until the animal moults.
Size 12 mm
Distribution Widely distributed in North America
Habitat Slow-moving water habitats including temporary and permanent ponds, ditches, marshes, swamps and lake shoreline. They crawl along the bottom among vegetation and debris.
Food Feed on crustaceans, molluscs, insects or some worms and leeches. These nymphs use a specialized lower lip to seize prey; the lip rapidly unfolds to extend and refolds to contract. The length of the lip is approximately equal to 20 to 25% of the length of the body. This lower lip holds prey at the mouth while the jaw-like mandibles crush the meal.
Life cycle Eggs are laid on sand or silt close to the water surface. After having carried the female around for up to a week prior to mating (thus males assure themselves a female to mate with when she is ready), the male continues to grasp the female as she lays her eggs in order to protect her from other males. Eggs hatch after one to several weeks or a long diapause (overwintering). Nymphs undergo 11 to 12 stages of development during their development. Once this is complete, they crawl from the water onto emergent vegetation or onto the bank. Life cycle is usually one year.

 

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A Project of the Canadian Museum of Nature
 Images: Christopher Allaway