All dragonflies belong to the order Odonata, as do their close cousins, the damselflies. Because there are distinct differences between dragonflies and damselflies, taxonomists divide the order into two suborders. The suborder Anisoptera includes only the dragonflies.
So what makes a dragonfly a dragonfly, as opposed to a damselfly? Let's start with the eyes. In the dragonflies, the eyes are quite large, so large in fact they make up the bulk of the head. The eyes often meet at the top of the head, or come close to it.
Next, look at the dragonfly's body. Dragonflies tend to be stocky. When resting, a dragonfly holds its wings open horizontally. The hind wings appear broader at their bases than the fore wings.
Male dragonflies will typically have a single pair of cerci at their hind ends, as well as a single appendage projecting from the underside of the tenth abdominal segment (called an epiproct). Female dragonflies often bear vestigial or nonfunctional ovipositors.
Dragonfly nymphs (sometimes called larvae, or naiads) are entirely aquatic. Like their parents, larval dragonflies generally have stocky bodies. They breathe through gills located in their rectums (there's an interesting bit of insect trivia for you), and can propel themselves forward by expelling water from the anus. They also bear five short, spiky appendages at the hind end, giving the nymph a rather pointed appearance.
All dragonflies are predaceous throughout their life cycles. Adult dragonflies hunt other insects, including smaller dragonflies and damselflies. Some dragonflies capture prey in flight, while others will glean meals from vegetation. Naiads eat other aquatic insects, and will also catch and consume tadpoles and small fish.
Dragonflies undergo simple, or incomplete, metamorphosis, with just three stages to the life cycle: egg, larva or nymph, and adult. Mating in dragonflies is a fairly acrobatic achievement, and which sometimes begins with the male scooping out his competitor's sperm and flinging it aside.
Once mated, the female dragonfly deposits her eggs in or near the water. Depending on the species, the eggs may take anywhere from a few days to over a month to hatch. Some species overwinter as eggs, delaying the start of the larval stage until the following spring.
The aquatic nymphs will molt and grow repeatedly, a dozen times or more. In the tropics, this stage may last only a month. In temperate areas, the larval stage can be considerably longer, and even last for several years.
When the adult is ready to emerge, the larva climbs out of the water and fixes itself to a stem or other substrate. It sheds its exoskeleton one final time, and the adult emerges, looking pale and delicate in its teneral stage. The castoff skin that usually remains affixed to the substrate is called the exuvia.
Special Adaptations and Behaviors:
Dragonflies operate each of their four wings independently, which enables them to perform sophisticated aerial moves. Observe dragonflies patrolling around a pond, and you'll see that they can take off vertically, hover, and even fly backwards.
The dragonfly's large, compound eyes each consist of about 30,000 individual lenses (called ommatidia). Most of their brainpower goes to processing visual information. A dragonfly's range of vision is nearly a full 360°; the only place it can't see well is directly behind it. With such keen eyesight and skillful maneuverability in the air, dragonflies can be tricky to catch – just ask anyone who has ever tried to net one!
Families in the Suborder Anisoptera:
- Petaluridae – petaltails, graybacks
- Gomphidae – clubtails
- Aeshnidae – darners
- Cordulegastridae – spiketails, biddies
- Corduliidae – cruisers, emeralds, green-eyed skimmers
- Libellulidae – skimmers
Range and Distribution:
Dragonflies live throughout the world, wherever aquatic habitats exist to support their life cycle. Members of the suborder Anisoptera number roughly 2,800 worldwide, with over 75% of these species living in the tropics. About 300 species of true dragonflies inhabit the U.S. mainland and Canada.
- Borror and DeLong's Introduction to the Study of Insects, 7th edition, by Charles A. Triplehorn and Norman F. Johnson
- Suborder Anisoptera - Dragonflies, BugGuide.Net, accessed November 23, 2012
- Anisoptera, University of Wisconsin BioWeb, accessed November 23, 2012
- Dragonflies and Damselflies, Odonata, University of Florida, accessed November 23, 2012
- Dragonflies and Damselflies of the West, by Dennis Paulson