Geographic Range
Shadow darners are very common and are in all provinces and territories of Canada,
as well as 42 states in the U.S. This species is not found in Utah, Arizona, Wyoming,
Texas, Louisiana, Florida, Hawaii and Alaska.
There are two described sub-species:
Aeshna umbrosa umbrosa
, which is found in the eastern part of North America, and
Aeshna umbrosa occidentalis
, which is found in the western part of North America. They appear to differ only
in geographic range.
Habitat
Most commonly found in areas with standing water or slow-moving streams with shadowy
areas, shadow darners usually inhabit lakes, ponds, boggy meadows, marshes, and mountain
lakes in forests. They are also occasionally found in clearings or along roads, particularly
when hunting. At higher elevations, this species has more diverse phenotypes.
- Habitat Regions
- temperate
- terrestrial
- freshwater
- Aquatic Biomes
- lakes and ponds
- rivers and streams
- temporary pools
- Other Habitat Features
- riparian
Physical Description
Both males and females have pale gray-green faces with two antennae, a strong toothed
jaw, and three large, bright eyes, or ocelli, which vary from blue-gray to brown and
form a triangle. Surrounding these ocelli are the compound eyes, which are larger
and darker in color. Unlike other species of
dragonflies
, shadow darners do not have a dark black stripe across the face. The head is approximately
7.4 to 8.4 mm long. The total length of a shadow darner is generally 6.5 to 7.8 cm.
There are two pairs of large wings spanning approximately 8.5 to 10 cm. The forewings
are slightly narrower than the hindwings, which are approximately 4.2 to 4.7 cm long.
The body is a powerful, brown thorax and a slender abdomen, with six spined legs and
strong claws. In males, there are yellow-green stripes on the side of the thorax and
blue stripes on the top. The abdomen has paired blue spots, usually on 9 out of its
10 segments. Females can have the same coloring as males, known as the blue form,
or in rare cases, sometimes green spots (known as the green form), or a combination
of green and blue spots, on the abdomen. Colors vary geographically in some cases:
the Vancouver Island population has no abdominal spots, while populations living in
colder climates generally have darker spots. Adults that have recently emerged from
their larval shell will have pale, unpigmented bodies until the colors develop.
At the end of the abdomen, males have hooked anal appendages called cerci, while females'
are unhooked. Females are also lacking the tuft of hair at the tip of the ovipositor
sheath. Shadow darners are very similar to lance-tipped darners (
Aeshna constricta
), but can be differentiated by looking at the cerci. The cerci of shadow darners
are much brighter. Often shadow darners are found flying around with
paddle-tailed darners
and are therefore often confused with this other species; however, paddle-tailed
darners have the black stripe that shadow darners are lacking across their faces.
Shadow darners also have paired spots on the ventral side of the abdomen, but paddle-tailed
darners do not.
- Other Physical Features
- ectothermic
- bilateral symmetry
- polymorphic
- Sexual Dimorphism
- sexes colored or patterned differently
Development
When the eggs are deposited at the end of the season in a cold climate, they enter
diapause to survive the winter until they can hatch the following spring. Otherwise,
the eggs generally hatch between 5 days and 2 months after oviposition, depending
on the water temperature. The eggs hatch, releasing a very large, greenish-brown naiad,
which is approximately 3.8 to 4.4 cm (1.5 to 1.75 inches) long. The naiad is wingless,
faded in color, and lives in the water. When it is nearing the end of its larval life,
the naiad climbs out of the water and switches from breathing through gills to breathing
through spiracles in the thorax.
After a significant period of time in larval form, the naiad matures into the larger
adult form, reaching up to 8.9 cm (3.5 inches) in length. At this time, it bursts
its cuticle shell by swallowing water, and emerges from its naiad skin. During this
process, the dragonfly is very vulnerable to predation, because it is soft and unable
to fly until the new wings harden. For this reason, this process normally occurs at
night. The adult dragonfly only lives for a few weeks.
- Development - Life Cycle
- metamorphosis
- diapause
Reproduction
Unlike some species of
dragonflies
, male shadow darners do not court females. Male and female shadow darners mate in
flight using the tandem position, in which the male's head is at the female's tail
and vice versa. The male transfers his sperm from the primary to the secondary genitalia,
known as the hamulus, in preparation for copulation, and also removes any sperm inside
the female from prior mates. The male then holds the female with his legs and cerci,
while the female reaches underneath the male, forming the wheel position, and removes
a sperm packet with which the eggs are fertilized. After mating, the male persuades
the female to oviposit in his territory. During this process, the male guards the
female to ensure that another male does not steal her before she finishes ovipositing.
- Mating System
- polygynandrous (promiscuous)
Adults become sexually mature approximately 2 to 3 weeks after leaving the larval
stage, but this can take longer in more severe climates. Breeding occurs from late
April through the end of November. Since their sexually active lifespan is so short,
shadow darners mate as often as possible (generally every 1 to 5 days) for the duration
of their adult lives. After the male and female mate, the female fertilizes the eggs
and oviposits in the late afternoon and early evening, usually into aquatic plants
or wet, decaying wood. She is able to cut a hole in the plant with a chitinous blade
that is part of her ovipositor. The number of eggs in each clutch varies depending
on the climate and amount of sunlight available, but is usually around 500.
- Key Reproductive Features
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- fertilization
- oviparous
The male chooses a territory that is a prime site for oviposition to ensure successful
hatching of eggs. He then guards the site from other males, thereby protecting any
eggs that may have been deposited there. The female protects the eggs inside her until
it is time for oviposition. To deposit them safely, she uses her chitinous blade to
cut holes in plants, making a nest-like arrangement to protect the eggs until they
hatch.
- Parental Investment
-
pre-hatching/birth
-
provisioning
- female
-
protecting
- male
- female
-
provisioning
Lifespan/Longevity
In harsh environments, shadow darners experience diapause, which allows them to live
for up to 7 months in the egg. In moderate climates, however, shadow darners generally
hatch after about a week. After hatching, shadow darners begin their larval stage
as naiads. The majority of their life (from approximately 2 months in warmer climates,
up to several years in colder climates) is spent as a naiad. After leaving the larval
stage, the shadow darner lives for approximately 2 weeks as an adult; however, this
can be extended if the dragonfly survives more difficult environmental conditions
in earlier stages (for example, a harsh Canadian winter). The longest known adult
lifespan of any species of dragonfly is 77 days.
Behavior
Shadow darners are very active, and are able to fly forwards, sideways and backwards
since their wings beat independently of each other in figure-of-eight patterns. During
the day or in bad weather, shadow darners often roost by perching on tree trunks or
hanging for plants, sometimes in dense groups. They adapt well to different temperatures,
and are known for surviving in unusually cold temperatures. In hot weather, they dip
themselves into the water to cool down, while in cold weather they bask in the sun
on a rock or tree, or whir their wings to warm up. Naiads are able to swim and propel
themselves forward by spraying water out of the end of their abdomens.
When on patrol, males often defend their territory against other males of the same
species, while investigating intruders of other species. They generally deal with
intruders in four stages: 1. approach, 2. threaten, 3. fight, and 4. chase. The defended
territory is used for ovipositing, but adults often hunt outside their territory.
- Key Behaviors
- flies
- crepuscular
- motile
- territorial
Home Range
Shadow darners' territory generally encompasses a shaded area near still or slowly
flowing water, but the size depends on the density of males in the area. When more
males are present, it is more difficult to hold territory, so each territory tends
to shrink in size.
Communication and Perception
Shadow darners rely heavily on seeing through their large, compound eyes which are
designed to detect movement. They contain more ommatidia, or light-sensitive, photoreceptor
cell clusters, than the eyes of any other insect. This allows them to see in color,
and to detect ultraviolet and polarized light. They also have 3 ocelli located between
the compound eyes, which are used to monitor the horizon and their orientation during
flight. Shadow darners normally identify mates visually by recognizing colors, sizes,
or shapes.
Since most of their prey are fast moving and easily visible, shadow darners do not
need long antennae to reach out and search for prey. Males use their shortened antennae,
legs and cerci to grasp a female that they are interested in mating. The male's cerci
must successfully link to the back of the female's head, or the match won't work (perhaps
because the organisms are of different species or of the same sex). A female who is
disinterested in mating can reject a male by bending her abdomen down, thereby preventing
the male's cerci from making a successful connection.
During the larval stage, shadow darners still rely on sight and touch. However, since
most live in the water in boggy or swamp-like conditions, they rely more heavily on
touch and therefore have longer antennae and smaller eyes.
- Perception Channels
- visual
- ultraviolet
- polarized light
- tactile
- chemical
Food Habits
By hawking for prey, shadow darners eat up to 20% of their body weight every day.
They hunt primarily at dusk, as they are most active in the shade, but at any elevation
and generally out in the open. Sometimes they hunt in a swarm with other shadow darners.
They also use the spines on their legs to form a basket with which they can catch
their prey. They usually eat any smaller insect, but especially
mosquitoes
,
midges
, and other
dragonflies
, as well as
moths
,
locusts
, and
beetles
. They do not, however, eat the wings of their prey, and so they pull these off before
starting to eat. Naiads mostly eat larvae of aquatic insects, but sometimes supplement
their diet with
freshwater shrimp
,
tadpoles
and
small fish
.
- Primary Diet
- carnivore
- Animal Foods
- amphibians
- fish
- insects
- aquatic crustaceans
Predation
Adult shadow darners are extremely agile in the air, and so it's difficult for predators
to catch them. However, birds such as
American kestrels
,
Swainson's hawks
,
merlins
, and
purple martins
specialize in catching dragonflies, and consequently have keen eyesight and can fly
fast enough to catch them. Shadow darners are also, on occasion, eaten by large insects
such as
robber flies
. Females can also be at risk during oviposition for predation by amphibians, like
frogs
and
newts
. Greenish-brown coloring of naids helps camouflage them from predators. However,
when emerging from their larval skin, they are undefended, unable to fly, and vulnerable
to predation by birds.
- Anti-predator Adaptations
- cryptic
Ecosystem Roles
Shadow darners are predators and prey as both larvae and adults. During reproduction,
shadow darner females lay their eggs inside the stems or leaves of plants to shield
them from harsh weather. Many parasitic organisms, such as
mites
,
parasitic worms
and
protozoans
attach themselves to shadow darners during their emergence from the larval stage.
- parasitic worms ( Helminthes )
- protozoans ( Protozoa )
- mites ( Acari )
Economic Importance for Humans: Positive
Shadow darners, like other
dragonflies
, play a critical role in pest control, particularly in crops and water storage, where
much damage is caused by
mosquitoes
. For example, mosquito larvae often inhabit rice fields or water containers, where
they pose particular threat to both the crop and to humans, as they carry numerous
fatal diseases. By adding a few shadow darner larvae, the mosquitoes can be controlled,
and even eliminated entirely. Shadow darners also eat other pests and disease vectors,
including
locusts
,
moths
,
sandflies
and wood-boring
beetles
.
Researchers have also noted that a reduction in the population of shadow darners is
often a sign of water contamination, and can therefore be used as a warning sign for
conservationists.
- Positive Impacts
- source of medicine or drug
- research and education
- controls pest population
Economic Importance for Humans: Negative
Many humans incorrectly believe that shadow darners are venomous, and are therefore
scared of them. However, all
dragonflies
can sometimes carry parasites, and when eaten raw could potentially transmit these
parasites to a human. Dragonflies are rarely eaten in North America, and so this is
an unusual occurrence in the case of shadow darners.
- Negative Impacts
- injures humans
Conservation Status
Shadow darners are not threatened or endangered. They are classified as a species of "least concern" by the IUCN Red List of Threatened Species.
Other Comments
As a species of
dragonfly
, shadow darners are often connected with their relatives in China and Japan as inspiration
in folklore or haiku. Dragonflies, in general, are often considered symbols of grace,
courage, luck or happiness, and associated with spirits or gods. In Asia, it is thought
that they also carry medical properties that combat syphilis, asthma, fever, and other
illnesses.
Additional Links
Contributors
Heather Sanders (author), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects, Catherine Kent (editor), Special Projects.
- Nearctic
-
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- temperate
-
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
- terrestrial
-
Living on the ground.
- freshwater
-
mainly lives in water that is not salty.
- taiga
-
Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.
- forest
-
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
- mountains
-
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
- marsh
-
marshes are wetland areas often dominated by grasses and reeds.
- swamp
-
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
- bog
-
a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.
- riparian
-
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
- ectothermic
-
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
- bilateral symmetry
-
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
- polymorphic
-
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
- metamorphosis
-
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
- diapause
-
a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.
- polygynandrous
-
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
- seasonal breeding
-
breeding is confined to a particular season
- sexual
-
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- fertilization
-
union of egg and spermatozoan
- internal fertilization
-
fertilization takes place within the female's body
- oviparous
-
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
- crepuscular
-
active at dawn and dusk
- motile
-
having the capacity to move from one place to another.
- territorial
-
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- visual
-
uses sight to communicate
- polarized light
-
light waves that are oriented in particular direction. For example, light reflected off of water has waves vibrating horizontally. Some animals, such as bees, can detect which way light is polarized and use that information. People cannot, unless they use special equipment.
- tactile
-
uses touch to communicate
- chemical
-
uses smells or other chemicals to communicate
- cryptic
-
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
- drug
-
a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease
- causes disease in humans
-
an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).
- carnivore
-
an animal that mainly eats meat
- insectivore
-
An animal that eats mainly insects or spiders.
References
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Dunkle, S. 2000. Dragonflies through binoculars: A field guide to dragonflies of North America . Oxford: Oxford University Press.
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