Odonata (dragonflies and damselflies) spend up to two years on average as nymphs in the water, where the largest are phenomenal predators thanks to their size and telescopic jaw. They are one of the best indicators of water quality since some of them simply cannot cope with pollution. Better control of river water in the last two decades, along with the creation of ponds to replace some of those lost in earlier decades, has assisted.
The Emperor (Anax imperator, length 80mm) is the largest of the group and one readily seen when the male patrols his territory, mostly in May and June. Mating occurs in the ‘copulation wheel’ formation used generally by Odonata, and some species can fly while paired together in this position. Egg laying takes place on vegetation below the surface.
Emergence into the adult form occurs at night and tends to be synchronised, with one site in Surrey producing 76 larval cases (exuviae) in a single morning.
The Brown Hawker (Aeshna grandis, length 75mm) looks brilliant in flight in the sun thanks to the amber-coloured wings. They are common in various habitats from June to October. The related Migrant Hawker (Aeshna mixta, length 70mm) emerges slightly later and continues just as long. This species was uncommon in the 1940s but boosted by annual migrations from Europe it has increased to the point of becoming a common breeder.
The Broad-bodied Chaser (Libellula depressa, length 60mm) is a coloniser of newly-made habitats such as gravel pits and is seen from May to August. The male, shown on the opening page of the Gardens section here, has powder blue markings on the abdomen, the female is brown. From the same group and seen in June and July, the Four-spotted Chaser (Libellula quadrimaculata, length 55mm) resides in a wide range of wetlands and perches regularly on emergent vegetation.
As with all dragonflies, which are hunters using their barbed legs to catch and hold prey which sometimes is eaten on the wing, the eyes are crucial. They are large and compound, holding up to 30,000 facets and providing all-round vision to pick up the slightest movement.
The Black-tailed Skimmer (Orthetrum cancellatum, length 65mm) has always been very much a southern species though now it seems to be expanding its range northwards and westwards. The males are very active but when not flying around rapidly they persistently perch, often on bare ground. Consequently exposed areas, preferably on the margins of a pond or river, suit them perfectly.
One of the earlier dragonflies to emerge, Black-tailed Skimmers can be seen from May to early July. The female is essentially yellow and black, with no blue.
Two species which are found mainly in acidic conditions are the Black Darter (Sympetrum danae, length 35mm) and the Small Red damselfly (Ceriagrion tenellum, length 28mm). The former is around from July to October and the latter, which is nationally notable, from June to September.
The type of habitat that suits the Small Red Damselfly can also suit the insectivorous Oblong-leaved Sundew (Drosera intermedia, to 20cm), which is much less common than the round-leaved version. Any damselfly or smaller insect landing on the sticky red hairs on each leaf stands no charge of escaping and is broken down and absorbed by the plant. Its success is variable from year to year, depending on water levels when the majority of damselflies emerge.
One of the commonest damselflies along slow-flowing rivers is the Banded Agrion or Demoiselle (Calopteryx splendens, length 40mm), with hundreds visible on occasions. Although like other damselflies they settle in daylight with their wings always closed (unlike dragonflies), they roost gregariously in waterside vegetation often with the wings open.
The Beautiful Demoiselle (Calopteryx virgo) is larger than its relative but not so widespread. Suited by fast-flowing watercourses, including those with pebbles or sand as the base, they are commonly found in heathland.
The White-legged damselfly (Platycnemis pennipes, length 35mm) is a riverine specialist which has been enjoying something of a comeback thanks to efforts at improving water quality – it is one of the least capable of all damselflies at dealing with pollution. One example of a return is along the River Mole, a tributary of the Thames, which faces major impacts from Gatwick Airport and fair-sized centres of population with the accompanying sewage treatment works.
Quite why storm conditions can still lead to outpourings of raw sewage into our rivers, as happened in the Thames in 2005, is at best anomalous and at worst a disgrace. Joseph Bazalgette, the genius who sorted out London’s sewerage in the 19th century, would be turning in his grave.
Whatever the finances of water companies – and they appear to be strong – preventing raw sewage getting into our rivers is a matter of public health, and one which should be treated as a priority above all others. Since it directly affects the public, and has the potential to cause disease, public funding should be involved if necessary.
Females of the Blue-tailed have the thorax marked in various colours, including red, and the species is one of only a handful to be breeding in central London. The Emerald, which sometimes breaks the golden rule by holding its wings slightly open when settled, is found mainly among thick vegetation close to still water.
Images © Jeremy Early. All rights reserved.