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Status and distribution
Aeshna isosceles (Muller) Norfolk hawker
Description A. isosceles is one of the two predominantly brown hawker dragonflies resident
in Britain. It can be distinguished easily from the much commoner A. grandis by
the clear untinted wings (although there is a small, and diagnostic, ambercoloured
area at the base of the hind wing), and the green eyes. The coloration
of the thorax and abdomen of A. isosceles is a paler brown than A. grandis and,
in mature males, the abdominal coloration is rich amber-brown. Both sexes
possess two short yellow bands on either side of the thorax, and a yellow mark
on the dorsal surface of abdominal segment 2. This mark superficially
resembles a narrow triangle - hence the name isosceles. There is a thin black
mid-dorsal line along the length of abdominal segments 3-9, thickening on
segments 8-9, and also a number of thin transverse bars.
Habitat A. isosceles breeds in unpolluted grazing marsh dykes which contain the
aquatic plant water-soldier in areas where the water table is maintained at a
high level. Other plants often present are frogbit, pondweeds and greater
bladderwort. The reason why A. isosceles is confined to dykes with watersoldier
in Britain, while it is not so confined on the continent, is unknown
(Leyshon & Moore 1993).
Breeding biology The males defend rather small territories. The highest steady density is about
9 per 100 m of water's edge. After copulation near water, the females oviposit
into the submerged tissues of aquatic plants, favouring water-soldier, an
indicator species of unpolluted water, On occasions, female A. isosceles have
been observed to select the white flower-head on which to settle during
oviposition. This projects about 2 cm above the water surface and provides
sufficient support for the dragonfly as it lays its eggs into the flower stalk and
submerged leaves. Egg laying lasts for 4-5 minutes, often with only a brief
period of flight of 10-15 seconds' duration before the process is repeated. The
larvae probably take two years to develop. Emergence usually occurs on watersoldier,
but also on bankside vegetation.
Flight periods A. isosceles has a very short flying period, being on the wing from early June to
the end ofJuly. It is often seen with Coenagrion pulchellum, Brachytron pratense
and Libellula quadrimaculata.
A. isosceles was taken by J C Dale at Whittlesey Mere, in Cambridgeshire, on 22
June 1818, and at Horning, Norfolk, by J Sparshall on 5 August 1824 (Dale 1901).
It was recorded again at Whittlesey Mere in June 1845, a female being taken
and subsequently illustrated in British Libellulihae by W F Evans (Evans 1845),
but this mere was drained in 1850. There is an undated 19th century record
from Swaffham Fen, Cambridgeshire (Imms 1938), and an unlocalised record
from the Fens in 1893 (Lucas 1900). Today, it is largely restricted to the
Broadland area of Norfolk and Suffolk. The eutrophication of the rivers and
broads from the leaching of agricultural fertilizers and from sewage effluent has
degraded much of Broadland. Disturbance and pollution from pleasure boats
have contributed to the problem. A. isosceles is now restricted to a few grazing
marshes which are relatively isolated from polluted water. But here the
pressures continue with the conversion of pasture to arable farming. This results
in the loss of traditional benign dyke management techniques, the lowering of
the water table and the reduction of water quality owing to nutrient enrichment
from agricultural runoff. Breeding has been proved recently from the dyke
systems associated with seven grazing marshes, and is suspected at six others.
Important new records from north Suffolk are given by Mendel (1992). The
species occurs on several nature reserves and is given special protection under
the Wildlife and Countryside Act 1981. It was found regularly at a site on Jersey
in the 1940s (Le Quesne 1946), but is now extinct there due to habitat changes.
European and world A. isosceles is widely distributed in Mediterranean areas, including North
distribution Africa, and in central Europe. It is absent from Scandinavia apart from Gotland.
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