The Dragonflies
(Insecta: Odonata)
of the Columbia Basin, British Columbia:
Field Surveys, Collections Development and Public Education
Introduction to the Dragonflies (Odonata)
Although the insect order Odonata (Greek for
toothed jaws) contains both the groups of insects known in English
as the dragonflies and damselflies, following common usage we also
use the name "dragonflies" to refer to the whole order. The term
"odonates" is also sometimes used. The Odonata is a small order
of insects of about 5,000 named species and 23 families worldwide.
It is predominantly tropical in distribution and is not as diverse
at higher latitudes.
Dragonflies are large and abundant insects and, because
of this, the order forms one of the predominant groups in standing
freshwater communities in the Columbia Basin. In the western mountains,
species are less abundant in running water than they are in standing
water habitats. Dragonflies live around most types of
|
Female Amphiagrion
abbreviatum.
Photo: Rob Cannings
& Brent Cooke. |
fresh water. Certain kinds prefer lakeshores, others
are found only along streams, or around springs and in peatlands.
Ponds and marshes rich in aquatic vegetation support the most species.
Male
Libellula
quadrimaculata.
Photo: G. Doerksen.
|
The Odonata and their ancestors are some of the
most ancient of insects. They have many primitive features, but also
possess many specializations that reflect their aerial and predatory
lifestyle. The order is divided into three suborders: the Zygoptera
(damselflies), the Anisoptera (dragonflies) and the Anisozygoptera,
a small group of two species from Asia that is intermediate in appearance
between the other two suborders. Damselflies
are slimmer, often smaller, and usually fly more slowly than dragonflies.
At rest their equal-sized wings are usually held together above the
body. Zygoptera means "joined wings". Dragonflies
are robust, often fast-flying, with the hindwings broader than the
forewings: when perched they hold their wings out away from the body.
Anisoptera means "unequal wings".
Anax
junius.
Larva from above
showing extended labium.
Photo: George Doerksen. |
The aquatic larvae are predacious and are armed
with an enormous hinged labium, sort of a lower lip, which is used
as an extendible grasping organ for capturing prey. Larvae are voracious,
eating small aquatic insects, crustaceans and even fish. Larvae can
be placed in three categories according to their feeding behaviour.
Climbers (Zygoptera,
Aeshnidae) are streamlined
stalkers that live in submerged vegetation. Sprawlers (Macromiidae,
Corduliidae and Libellulidae) lie in ambush on the bottom mud and
detritus. Burrowers (Gomphidae, Cordulegastridae) cover themselves
with sand and mud and await their prey. Larvae moult 10 to 15 times
as they grow. When fully grown, the larva crawls out of the water
up a plant stalk or some other support. The skin on its back splits
open and the adult dragonfly squeezes out. The newly emerged dragonfly
pumps blood into the wing veins and the wings expand. Gradually the
body hardens, and after an hour or so the dragonfly can fly. It leaves
the empty larval skin (exuviae) clinging to the plant.
Male Aeshna interrupta emerging
from exuviae.
Photos show different stages of emergence.
 |
 |
 |
Adults are aerial, visually oriented predators:
they are large, strong-flying insects with large eyes, strong mandibles
and spiny legs. Their prey is a wide range of flying insects, which
are usually captured in flight. Adults are often colourfully patterned,
and exhibit a wide variety of readily observed behaviour. Mature
males patrol the breeding
habitats, aggressively searching for mates, and may, like birds,
defend a territory against other males of the species. These territories
limit aggression and prevent undue disturbance of egg-laying females.
Sometimes in crowded situations group territories with dominance
hierarchies are established.
When he is ready to mate, a male grasps a female
by the front of the thorax (damselflies) or by the top of the head
(dragonflies) with the appendages at the tip of the abdomen. The
female loops the end of her abdomen up to the base of the male's
abdomen where the sperm is stored and transferred. The Odonata are
the only insects that mate in this wheel position.
The female lays the eggs once they are fertilised.
All damselflies
and some dragonflies (mainly the Aeshnidae)
have a knifelike egg-laying structure called an ovipositor, at the
tip of the abdomen. They lay their eggs in plant tissue of various
sorts. Competition for mates is usually fierce, and male aggression
can prevent females from laying their eggs. Females ovipositing
alone are usually secretive. In many species, the male often
retains his hold on the female while she lays her eggs,
| 
Aeshna
palmata
female laying eggs in dense stands of emergent plants. Photo:
George Doerksen. |
guarding her from other males who may attempt to
mate with her. Some damselflies crawl
below the water surface to escape the attentions of males, remaining
there for over an hour to lay their eggs. They can take a film of
air down with them, trapped in the hairs on their body. The large
stream-dwelling Cordulegaster has a spikelike ovipositor that
drives eggs into the mud and sand of the streambed. Other species
lacking ovipositors
usually just dip the tip of the abdomen into the water and wash the
eggs off, and the eggs sink to the bottom.
Damselflies, and many dragonflies, develop rapidly. For many species
in the Kootenays the life cycle takes about a year. Lestes
and some Sympetrum species overwinter as eggs, hatch in the
spring and emerge as adults in the summer. Others overwinter as
larvae and emerge the following spring or summer, although probably
in some species under certain conditions, the larvae overwinter
two years. However, in the larger dragonflies, such as Aeshna
or Somatochlora, the short summers of high altitudes in the
region often mean that four or five years are spent in the larval
stage. Adult dragonflies in the Columbia Basin live for about one
to two months.
|
Anax
junius.
Larva from above 
