|Emperor Dragonfly, Blue Emperor
|Anax imperator (Leach 1815), The
Emperor Dragonfly or Blue Emperor, is a large and powerful species of hawker
dragonfly of the family Aeshnidae, averaging 78 millimetres (3.1 in) in
length. It is found mainly in Europe and nearby Africa and Asia. As it
wanders widely it can also be encountered elsewhere.
The combination of green thorax and
blue abdomen distinguishes males (and some females) from most other dragonflies.
The all-green, non-metallic patterning of females is characteristic. Among
remaining species, the blue-eyed hawker (Aeshna affinis), green hawker
(Aeshna viridis) and southern hawker (Aeshna cyanea) have a predominantly
black, banded abdomen with blue and/or green spots, lack a dorsal stripe
and have distinct black markings on the thorax. Lesser emperor (Anax parthenope)
has a brown thorax and a yellow ring at the base of the second abdominal
segment that persists into adulthood. Occasional individuals of vagrant
emperor (A. ephippiger) may have blue abdomens, but never have blue eyes
and the thorax is brown. Common green darner (Anax junius) may be encountered
as an occasional vagrant from the Americas in Western Europe; this species
lacks blue eyes, never has blue markings on the thorax, and has a distinct
front marking resembling a bulls-eye, with a circular, black central mark
surrounded by a blue ring.
|Adult body about 73-78-82 mm long
(the males somewhat the longer). Average wingspan 106 mm; hindwings 45-51
The male has a strikingly sky blue
abdomen which when in flight has a slightly downward curve, giving a bent
appearance, a useful feature for separating the restless Emperor from the
other hawker type dragonflies whose abdomens generally appear straight.
The blue green eyes & green thorax make this a very handsome insect.
The female is generally green in
appearance with dark markings along the centre of the abdomen. They oviposit
alone into floating vegetation or submerged plant stems, a feature which
helps to separate this species from the Lesser Emperor (Anax parthenope)
which generally oviposits in tandem.
The eyes dorsally broadly contiguous,
or dorsally narrowly apposed to dorsally broadly contiguous (in the female);
brown and green, or green. Legs brownish black and yellow (the undersides
of the fore-femora yellow). Thoracic antehumeral stripes absent. The wings
spread more or less horizontally in repose; dissimilar in shape and venation;
sessile; unpatterned and clear. The inner wing venation blackish. Discoidal
cell divided longitudinally into a conspicuous triangle and supra-triangle.
Antenodal veins in the forewings about 16-20 (fewer in the hindwings);
incorporating two conspicuously stronger primaries, and those in the costal
and subcostal spaces unaligned. Pterostigma narrow-linear; black, or dark
Abdomen linear from a conspicuously
swollen base (briefly constricted behind the swelling in the male); 49-51
mm long (female), or 53-61 mm long (male); predominantly blue (male), or
green (female); predominantly longitudinally lined (with a continuous,
median black line which is crossed by a short band on each segment of the
male, and on the anterior segments in the female); without mid-dorsal spots.
The male abdomen without auricles on segment 2; with a single inferior
The larvaes stout, the body expanded
in the middle; when mature, 45-56 mm long. The eyes large; approaching
one another closely at a point on the top of the head; markedly flattened
dorsally, their posteror margins forming a transverse straight line in
dorsal view (this and the more rounded outline of the head readily distinguishing
this species from the Aeschnas). The head in dorsal view not markedly narrowing
from immediately behind the eyes. The postocular lobes curving smoothly
to the back of the head from immediately behind the eyes. The antennae
7 segmented. The mask narrowed gradually to the hinge; with a flat prementum;
with a short slit-like median cleft. Legs shorter than the abdomen; fore-
and middle tarsi 3-segmented. The abdomen terminating in five short spine-like
appendages; gizzard with 4-8 folds.
This powerful dragonfly is one of
the largest species in Europe. The male is highly territorial, and difficult
to approach. The species lives by larger ponds, gravel pits, and slow rivers.
They frequently fly high up into the sky in search of prey, which includes
butterflies, Four-spotted Chasers and tadpoles; small prey is eaten on
the wing. The females lay the eggs into plants such as pondweed, and always
Reproduction and Development
|This is a species of still and occasionally
slow-flowing waters, most often encountered around larger, well-vegetated
waterbodies. It is known to colonise newly-formed pond, and is able to
tolerate brackish conditions. Larvae inhabit pondweed.
In contrast to other European emperors
(genus Anax), females lay eggs alone, ovipositing directly into floating
vegetation, including deadwood, away from shore. This exposed situation
makes animals highly visible compared with related species.
The eggs are laid in surface water-plants,
and develop in the region of highest temperature and light intensity. They
hatch in about 3 weeks. Ovi-position lasts for about 8 weeks each year,
usually extending from early June to early August. The protracted period
of oviposition and the direct development of the eggs combine to endow
the hatching larval population with a wide temporal variation.
Duration of the larval stage is usually
about 2 years, but exceptionally it may be completed in 1 year. Larval
growth is restricted to about 6 months each year, from May to October.
A few precocious larvae hatching from eggs laid early achieve the penultimate
or final instars by the first autumn, and emerge as adults the following
spring. The remainder, usually composing more than 90% of an emergence-group,
attains an average length of about 24 mm (range 7-42 mm) before growth
ceases in autumn. Growth is resumed in May, and the final larval instar
is entered in August, approximately 12 months after hatching. The next
9 months are spent in the final instar without visible growth, and emergence
takes place the following spring between mid-May and mid-July, about 23
months after hatching.
Yearly emergence is restricted to
a period of 40-50 days, usually extending from late May to early July,
and about 90% of an annual population of adults emerge during the first
10 days of the emergence period. Over the whole emergence period males
tend to emerge earlier than females. Emerging as an adult on reeds or rushes
around the margins of their chosen water body, the emergence took place
in the middle night, and newly emerged adults fly shortly before sunrise.
For the maiden flight they fly away from water and may fly several hundred
metres before alighting. This orientation is perhaps an adaptation directed
towards protecting teneral adults from the aggression of sexually mature
males. Adults fly again during the first day, and remain away from water
for the next 2 weeks. The cast skin or exuvia can be found long after this
spectacular transformation takes place and is an exact replica of the larvae
which can be identified to species level. Collecting these exuviae can
give an excellent indication of the true population size at a site and
is conclusive proof of successful breeding.
Following emergence, maturation takes
between a week and 12 days in males, 13-16 days in females. During this
maturation period they fly often, feed, shun water and show no sexual response
to other dragonflies. After a minimum of 9 days of adult life, the first
males begin to exhibit sexual behaviour over water; females return to water
to oviposit from 2 to 3 days later. Copulation occurs promptly upon arrival
of a female, and is followed almost immediately by oviposition. Both sexes
probably return to the water frequently after the first mating. Males are
capable of copulating more than once. The expectation of life at emergence
of those adults destined to survive the maturation period is probably about
4 weeks, and the maximum life-span approximately 8 1/2 weeks. Predators
of mature adults are few.
Flight season highly dependent on
location, with North African populations flying from March until December.
In northern Europe, animals are most abundant from June to August.
|The species appears in Europe, Western
Asia, Africa & the Middle East, ranging including all North Africa
and Europe north to Denmark on the mainland, and through southern Britain
and Ireland Populations become scarcer and more scattered in the north
of this range and inland from the coast in Ireland. In East Africa. the
species occurs south as far as Zimbabwe and Madagascar.
|Red List status as Least Concern.
This common and familiar dragonfly in Europe is nevertheless a comparatively
recent colonist from Africa, and still rapidly expanding its range northwards.
Species on Stamps
With courteous to Mr. Richard
Lewington for the Dragonfly Illustration
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