|
pdf
version
Natural History
The Blue Darners:
dragonflies of the genus Aeshna in British Columbia
by Rob Cannings
This
article was first published in Cordillera, volume 3, Number 1,
Summer 1996. Cordillera is available for subscription for $15.00
(FBCN members) or $20.00 (non-members). Please write the editor at Box
625, Kamloops, British Columbia, V2C 5L7.
Throughout the British Columbia summer
the skimmers (Libellula) flash by with their spotted wings and darting
flight, scarlet meadow-hawks (Sympetrum) catch our eye, and myriads of
bluets (Enallagma) crowd around the canoe on the lake. But the most conspicuous
dragonflies of all are the big blue darners, tirelessly hawking after
their insect prey, often in great numbers, filling sunny marshes and still
evenings along lakeshores everywhere. These darners make up the genus
Aeshna, represented by 13 species in British Columbia, about one-sixth
of our damselflies and dragonflies.
Darners probably got their name from the old
superstition that they sew up the lips of naughty boys with their long
slender abdomens -- the devil's darning needles. The origin of the name
Aeshna, coined by the famous Danish entomologist Fabricius in the 18th
century, is also lost in history. One intriguing explanation is that it's
a printer's error for the Greek Aechma, a spear (Cannings & Stuart 1977).
Figure
1: A Canadan Darner (male) showing the hunting equipment of a darner--powerful
thorax and wings (for fast and agile flight); large compound eyes (for
tracking moving prey); and spiny legs (for capturing prey in flight).
The colour and shape of the lateral thoracic stripes are important clues
for species identification.
Even the smallest of our darners is over
five centimetres long, and all are striped on the thorax and spotted on
the abdomen with blue, green or yellow. Males almost always have blue
abdominal spots. Females vary considerably, and in most species there
are two or even three colour forms, one like the male, and others more
green or yellow. For species identification, the colour of the face is
useful, especially in males, and so is the colour of the horizontal line
across the face. The shape and colour of the stripes on the sides of the
thorax (lateral thoracic stripes) are critical (Figure 1). The shape of
the terminal abdominal appendages in both the male and female, and the
structure of the ovipositor (egg-laying organ) in the female are also
important.
Figure
2:
The larva of a Variable Darner stalking prey. Like adults, darner larvae
have large compound eyes and are visually-oriented predators.
The immature stages of darners (larvae)
live in water and, like other species of dragonflies, may require rather
specific habitats (Cannings & Cannings 1994). Some of these are outlined
in the species list. The larvae are predators of aquatic invertebrates
and are just as voracious as the adults are in the air. Larvae are smooth
and slender, mostly brown, mottled and banded with darker shades, and
climb through aquatic vegetation, stalking their prey (Figure 2). They
breathe by pumping water in and out of the anus, extracting oxygen with
gills lining the rectum. By building water pressure in the rectum and
expelling water violently, they can propel themselves like a jet, legs
folded against the body, to escape predators.
Figure
3:
The emergence of a Variable Darner (male) on the edge of a pond in the
Chilcotin grasslands. The dragonfly larva climbed out of the water on
a grass stem and the adult is extracting itself from the larval skin.
Dragonflies are especially vulnerable to predation during emergence; they
are exposed, weak, and cannot fly.
Larvae may moult up to 15 times as they grow,
and may live several years (Walker 1958). Probably most of our species
spend at least two years as larvae and, in the north and high in the mountains
where water is cold and the growth rate is slow, a six-year span is likely
not unusual. When growth is complete, the larvae climb out of the water
on vertical supports and transform into adults (Figure 3). Even the cast
larval skins clinging to emergent vegetation can be identified to species
and can provide clear evidence of larval habitat preferences.
Figure
4:
A Canada Darner feeding on a meadowhawk dragonfly (Sympetrum sp.).
Darners are opportunistic predators, feeding on any flying insects they
can capture and subdue.
In the warmer parts of the province, darners
can be on the wing from late April (California Darner) to early November
(Paddle-tailed and Shadow darners) but, in general, the greatest diversity
of species and peak abundance of adults is in August. Especially when
young, adult darners may venture several kilometers from their emergence
site, hunting and maturing before returning to water to breed. They feed
throughout the day (Figure 4), but most species also hunt well into the
evening to take advantage of the abundance of aquatic insects swarming;
sometimes darners hunt in twilight so dim that it is difficult to see
them flying by. At night or in bad weather they fly up into the trees
and hang from a protected leaf or branch, or cling to a tree trunk.
Figure
5:
Paddle-tailed Darner male. Male Darners often hover in their territories
when searching for females. Hovering probably advertizes the presence
of a male in his territory and allows him to detect females and competing
males more readily than if he were in forward flight.
Sight and flight are the hallmarks of the darners
(Figure 5). Each of the large compound eyes consists of up to 30,000 tiny
eyes, and each of these contributes to a mosaic image, integrated by the
brain, that is extremely sensitive to movement. Flying insects are easily
detected. Although darners patrol at about 1-2 m/sec, they can accelerate
after prey to more than 15 m/sec (54 km/h) (Miller 1987).
Figure
6:
A pair of California Darners mating. The male, at the top, grasps his
mate by the back of her head. The female links the tip of her abdomen
with the sperm-transmitting organs at the base of the male's abdomen.
The life expectancy of adults is eight to ten
weeks, and during much of this time reproduction is the main occupation.
In their territories along the shore, males search diligently for any
females that have come to the water to lay eggs. After mating (Figure
6), the females are left alone to deposit their eggs in plant stems, floating
leaves, or waterlogged wood (Figure 7).
Figure
7:
A Paddle-tailed Darner female laying eggs in waterlogged wood at the edge
of a lake in the coastal forest. The eggs are inserted in the wet wood
by the sawing action of the ovipositor at the tip of the female's abdomen.
The ovipositor, a structure at the end of the
female abdomen, saws into plant tissue and inserts the eggs. This crucial
process can be constantly disrupted by the attempts of other males to
mate with the egg-laying female. Unlike many other dragonflies, darner
males, after they have mated, do not guard females from the advances of
other males and, in most species, laying females are secretive and inconspicuous.
Female Black-tipped Darners, however, are coloured like males and fly
around behaving like them, presumably making it more difficult for males
to recognize their sex, thus reducing the harassment the females suffer
while laying eggs.
Getting to Know
Them
Azure Darner (Aeshna septentrionalis)
The most boreal of the boreal darners, the Azure Darner is rarely seen
in British Columbia, although far to the north in the northern Yukon it
is the most widespread large dragonfly. It is recorded on Herschel Island
in the Beaufort Sea, about as far north as dragonflies live in North America.
In British Columbia the species has been found only at Atlin and Tweedsmuir
Park, but it occurs at least as far south as Banff National Park in the
Alberta Rocky Mountains. Its breeding habitat is characterized by aquatic
moss, although in the Yukon adult males patrol a wide range of waters
from bog pools to marl-bottomed lakes (Cannings et al. 1991). The recorded
flight period is 10 June to 28 August, but some individuals probably survive
well into September in favourable seasons.
Black-tipped Darner (Aeshna tuberculifera)
Recorded only from east-central Vancouver Island, the North-shore mountains
near Vancouver, and Wells Gray Park in the central Interior, this long,
slender, blue species is seldom encountered in British Columbia. Although
in eastern Canada the Black-tipped Darner inhabits a wide variety of wetlands
including cat-tail marshes, in the West it prefers peat-margined ponds
and lakes, habitats typical of the Canada Darner. The known flight period
in the Cordillera ranges from 16 July to 3 September. This species and
the Lance-tipped Darner are the only species of Aeshna in the province
that regularly lay their eggs above the water surface in emergent vegetation.
Blue-eyed Darner (Aeshna multicolor)
This brilliant blue darner with southern affinities is common, especially
in midsummer, around most kinds of lowland standing waters south of 51o
N latitude, although it is locally common around McBride, in the upper
Fraser Valley at almost 54oN. It is typical of warm waters covered with
the floating leaves of water lilies (Nuphar), pondweed (Potamogeton) and
other aquatic plants. The Blue-eyed Darner has been recorded in the province
from 18 May to 16 October.
California Darner (Aeshna californica)
In its range across the southern third of the province (it occurs north
to the Cariboo-Chilcotin), the California Darner is remarkable for its
springtime flight season. In the south it may appear as early as the last
week of April, emerging with our earliest dragonflies, and it disappears
by early August, just when many darner species are reaching their peak
abundance. This small darner inhabits a wide range of lowland waters from
the alkaline ponds of the Thompson-Okanagan and Chilcotin to forest lakes
on Vancouver Island.
Canada Darner (Aeshna canadensis)
In British Columbia the Canada Darner has not been found north of the
Prince George area, although there is a single record from the southern
Yukon. Its usual haunts are ponds and lakes with flooded or peaty margins,
bog ponds and fens, especially where floating vegetation or logs occur.
Beaver dam ponds are ideal. The species is particularly common on the
south coast and in the wetter parts of the southern Interior where these
habitats are common. The recorded flight period is 19 June to 9 October.
Lake Darner (Aeshna eremita)
The largest of the Canadian blue darners, the Lake Darner is a common
inhabitant of the forest lakes of the province's mountains and plateaus.
It is dominant in sparsely vegetated or wooded lakes and ponds, but is
also found at deep fens and sedge- or spikerush-bordered ponds. Its recorded
flight period is 19 June to 14 October.
Lance-tipped Darner
(Aeshna constricta)
The Lance-tipped Darner is one of the three species in the province with
flattened, spine-tipped superior appendages in the male. Unlike its two
close relatives, the Paddle-tailed and Shadow darners, it is not well
known in British Columbia, having been found only infrequently in the
warm valleys of the Thompson-Okanagan and Kootenay regions around waters
dominated by cat-tails and bulrushes. This species has a southern transcontinental
distribution; our records are from August, although it probably flies
from late July to early October.
Paddle-tailed Darner (Aeshna palmata)
Perhaps the most common dragonfly of the northwestern Cordillera, the
Paddle-tailed Darner seems to fly everywhere in the southern half of British
Columbia. It is more typical of forest lakes and ponds than of peatland
pools, although in the south it occurs in all these habitats. In northern
regions, however, it is more restricted to warm, low elevation lakes and
ponds and is rare in the abundant muskeg habitats. Even though the recorded
flight period for British Columbia is 22 May to 1 November, adults are
usually not seen until July.
Sedge Darner (Aeshna juncea)
The most common of the boreal Aeshna species, the Sedge Darner is found
in virtually every sedge pothole and marsh in the forested regions of
the province's mountains and northlands. Although it is most abundant
in ponds and marshes, it is also found in meandering streams and a variety
of mossy fens, as long as sedges are present. Although a number of our
darners have close relatives in Eurasia, the Sedge and Subarctic darners
are the only ones that range westward across Siberia to Europe. The recorded
flight period is 26 June to 2 September, but in favourable years adults
undoubtedly survive well into September.
Shadow Darner (Aeshna
umbrosa)
The Shadow Darner is fond of the shade as well as the sunshine, and is
one species that is just as much associated with slowly moving streams
as with still waters. It is typical of forest lakes and beaver ponds and
associated channels, but also occurs in bogs and fens. The species is
common in southern British Columbia, but rarer in the north; it ranges
as far north as the southern edge of the Yukon. Throughout most of its
wide range, the Shadow Darner has small green abdominal spots in the male,
but many of the specimens from southern British Columbia show large blue
spots. Females often lay eggs in wet, decaying wood, such as logs and
stumps in water. Flight records range from early July to the end of the
first week of November.
Subarctic Darner (Aeshna subarctica)
Closely related to the Sedge Darner, the Subarctic Darner has a range
similar to it in British Columbia, although it is much less common. It
is restricted to Sphagnum bogs and deep fens that are dominated by aquatic
moss. The moss need not be Sphagnum and the water not necessarily very
acidic; other aquatic mosses such as Drepanocladus and Scorpidium are
more commonly associated with this dragonfly (Cannings & Cannings 1994).
Eggs are laid directly into floating moss. The recorded flight period
is limited by few data; it ranges from 20 July to 29 September.
Variable Darner (Aeshna interrupta)
This darner, the dominant one of the Prairie Provinces, is also the commonest
one in the rangeland ponds of the drier plateaus and valleys of southern
British Columbia. In other habitats, all over the province, adults are
found primarily at sedgy ponds or lakes, but can also be common around
peatland pools, especially along the coast. The recorded flight period
is 27 May to 22 October. Most emergence is in July. The common name comes
from the shape of the lateral thoracic stripes of the male, which are
usually very thin but complete in specimens from the Interior of the province
but divided into spots (thus, interrupta) in coastal ones. This difference
is not constant, however.
Zigzag Darner (Aeshna sitchensis)
This small species, named for the shape of its front lateral thoracic
stripes, is a common inhabitant of shallow, evenly vegetated sedge or
sedge/moss fens and bogs. In these habitats open water is restricted to
very small puddles, if present at all, and dried algal mats are common.
The Zigzag Darner flies low over the surface of its peatland home, often
landing on the ground, an unusual habit for darners. The recorded flight
period is 10 June 6 to 18 September.
Where They Live
The genus Aeshna is widely distributed throughout the world, and consists
of about 75 species. It is primarily a temperate group, although there
are species in the New and Old world tropics. It is especially dominant
across the northern forests of the world. Defining faunal elements is
a convenient way to unite various British Columbia species having similar
distributions:
1. Boreal -- species occurring in the northern
spruce (Picea) forests. In general, these species range from the Atlantic
Provinces across the northern New England states, Quebec, northern Ontario,
parts of the northern tier of mid-western states, the Prairie Provinces
north of the Great Plains, and northern British Columbia, often ranging
considerably southward in the western United States in the higher mountains
of the Cordillera. These species can be further subdivided into:
a) Full Boreal -- with ranges as described above.
Lake Darner, Sedge Darner, Subarctic Darner, Zigzag Darner. Two of these,
the Sedge and Subarctic darners, are also found across the forests of
Eurasia.
b) Northern Boreal -- species that are common
near the northern treeline, but which are virtually absent from the northern
contiguous United States and from the southeastern Atlantic Provinces,
and do not extend far south into the Cordillera. Azure Darner.
c) Southern Boreal -- species that are uncommon
north of 60°N, absent near the Arctic treeline (e.g. along the Ontario
coast of Hudson Bay), but extend far down the Cordillera and/or into the
southeastern Atlantic Provinces and New England states. Variable Darner.
2. Transition --
species generally most common in the southern boreal forests and adjacent
montane forests in the West and mixed and deciduous forests in the East.
Black-tipped Darner, Canada Darner, Shadow Darner.
3. Cordilleran -- species confined to the western
mountains and intermontane lowlands. Paddle-tailed Darner.
4. Southern -- species ranging across North America
from extreme southern Canada in the north, southward to at least mid-continent.
Lance-tipped Darner.
5. Southwestern -- species mainly distributed
in southwestern North America, ranging north into southern British Columbia
and south into Mexico and sometimes as far south as Central America. Blue-eyed
Darner, California Darner.
Field Key to Adult
Blue Darners (Aeshna) of British Columbia
Using this diagrammatic, illustrated field
key you can identify live adults of all the species of Aeshna found in
British Columbia. In fact, all the species of the family Aeshnidae known
in North America north of California and west of Manitoba are included.
The Green Darner, Anax junius, is the only species of the family outside
the genus Aeshna that occurs in this area.
Colour and colour patterns, which are difficult
to use in dried specimens, are stressed. Colour is much more variable
in females than in males, and therefore is less useful. Some small structural
details are used, especially to separate females; a 10X hand lens and
a millimeter ruler are critical equipment. So is a net. At first, the
species will have to be learned in the hand, because many of the distinguishing
features can be seen only at close range. Once the species are learned,
many will be recognizable in the field, even at a distance. Captured dragonflies
can easily be handled by reaching into the net and gently holding their
wings together at the base between your thumb and forefinger. After examination
they can be released, unharmed.
At each step of the key you have a choice to
make between two sets of distinguishing characteristics. Read each alternative
set before making your decision. The natural variation found among individuals
of any species may sometimes make the decision difficult, but the characters
have been selected to make the choices as clear as possible. Each choice
will lead, via the arrows, either to a species name or to another set
of characters from which to choose.
FIELD
KEY
Acknowledgements
Dennis Paulson kindly allowed me to use some of the information found
in his "Field Key to Washington Dragonflies". Syd Cannings' extensive
experience with Aeshna species in the field was a big help. Hannah Nadel
reviewed the manuscript and Tara Steigenberger created the electronic
key.
This article was first published in Cordillera,
a magazine for British Columbia naturalists. For subscription information
to Cordillera, please write to: the Editor, Cordillera,
Box 625, Kamloops, BC V2C 5L6
Literature Cited
Cannings, R.A. and K.M. Stuart. 1977. The dragonflies of British Columbia.
British Columbia Provincial Museum, Handbook No. 35. Victoria, B.C. 254
pp. (Out of print).
Cannings, S.G., R.A. Cannings, and R.J. Cannings.
1991. Distribution of the dragonflies (Insecta: Odonata) of the Yukon
Territory, Canada, with notes on ecology and behaviour. Royal British
Columbia Museum Contributions to Natural Science 13: 1-26.
Cannings, S.G. and R.A. Cannings. 1994. The Odonata
of the northern Cordilleran peatlands of North America. Memoirs of the
Entomological Society of Canada 169: 89-110.
Miller. P.L. 1987. Dragonflies. Naturalists'
Handbooks 7. Cambridge University Press, Cambridge, U.K. 84 pp.
Walker, E.M. 1958. The Odonata of Canada and
Alaska. Vol. 2. University of Toronto Press, Toronto. 318 pp. (Out of
print).
Rob Cannings, Curator of Entomology
Royal British Columbia Museum
675 Belleville St.
Victoria, B.C. V8V 1X4
Tel: 250-356-8242
Fax: 250-387-5360
E-mail: RCANNINGS@ROYALBCMUSEUM.BC.CA
|
