Saturday, 29 July 2017

Dragonflies 1: The Emperor

A.imperator ovipositing
One of the largest dragonflies in Europe, the Emperor Dragonfly Anax imperator has a range from North Africa to the North Sea coast and most of the south of the British Isles, and reaching east into the Ukraine. 

There are around 30 species of Anax worldwide, mostly in the Old World, but there are some in North America and there is one species native to Hawai’i. The genus belongs to the widespread family Aeschnidae, hawker dragonflies. Hawkers get their name from the behaviour – they catch and usually east their prey in flight and are powerful fliers. Dragonfly genera can usually be identified by behaviour – chasers and darters generally hunt from a perch.
A.imperator male
All dragonflies are predators and as adults feed on smaller insects, usually mosquitoes and flies but the larger species like Emperors will take butterflies or even small species of dragonflies and damselflies. As aquatic naiads they feed on smaller aquatic life and the larger nymphs may even take tadpoles or small fish.

Dragonflies and Damselflies – the Odonata – are one of the oldest lineages of flying insects. The oldest relatives of modern dragonflies are known from the Carboniferous around 300 million years ago, these are the griffenflies, “giant dragonflies” which in some species are believed to have had a wingspan of around 70cm. How they managed to support such a gigantic size with the high metabolic rate of dragonfly lifestyle is not certain, but it probably is related to the higher oxygen levels of the atmosphere during their reign, which at its peak may have reached 35%. This ended with the end-Permian mass extinction during which global environmental changes resulted in a fall of atmospheric oxygen levels to well below the present day level of 20%.
Anax junius
Although dragonflies are thought of as exclusively water-loving, water is only needed for reproduction. Once they have emerged as adults they may hunt over other habitats entirely, hunting around trees in woods, over fields, or preferring moorland heaths depending on the species. As adults they are fairly long lived, and in the several months many species live they can also travel long distances, as especially in tropical reasons they can have regular migrations following the rains. They can also be found as vagrants far outside their normal range – the North America Green Darner Anax junius has been found in the UK on several occasions having crossed the Atlantic in the same way as many birds do.

Emperor dragonflies begin as eggs laid directly into the water by an unaccompanied female. This last is slightly unusual – in many species the male remains attached by his claspers to the female while she lays her eggs in order to guard her from mating again. After an incubation period of three weeks the eggs hatch into the aquatic naiads that feed on aquatic insects and tadpoles for one or two years before climbing out of the water to moult into the winged adults.
Teneral A.imperator
On leaving the water the naiads may travel as far as 30m from their pool and 5m up a tree before shedding for the last time and appearing as an adult. With these large dragonflies even after emerging they take one or two weeks to attain the full colours and becoming able to reproduce. Males are very territorial and spend most of their lives in flight during the day, even feeding on the wing. 
Male A.imperator patrolling the pool where the female at the head of the post was egg laying

(Green Darner, Emperor emerging photos from wikipedia, others are my photos)

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