Ephemera danica Müller, 1764

The Mayfly, Green Drake, Black Drake, Grey Drake

Nymphs: Large cylindrical nymphs with two tusk-like projections from the head and thick, feathery gills that are held over the back.

Adults: Large flies with three tails and large hind wings.  The sub-imago has grey wings, with a yellowish green tinge and heavy venation.  The body is creamy yellow with distinctive brown markings on the body segments.  The tails are dark grey black and the legs are a creamy olive colour with black markings.  In general, the colours of the wings, tails and legs are darker, and the body paler in the male.

The imago has transparent wings, which have heavy brown venation and several dark patches.  The body is creamy white with the last three segments brownish.  The tails are extremely long and almost black in colour.

Nymphs: The British Ephemeridae can be separated by the markings on the upper surface of their body.  The markings in Ephemera danica are neither dark triangles nor a series of lines.  On segments 7 to 9 the markings are elongated and obvious, however on the remaining segments the markings are indistinct.  In E. vulgata there are dark triangular markings on all the body segments, although in some cases these are indistinct on the first and last segments.  In E. lineata the markings consist of a series of six stripes on segments 7 to 9.  On the remaining segments the markings are indistinct.

Adults: Adults of the British Ephemeridae can also be separated by the markings on the upper surface of their body.  In Ephemera danica these markings are distinct on most segments but are often absent or faint on segments 1 to 5.  In E. vulgata the markings are present on all segments and are dark and triangular or elongated, often with a pair of longitudinal stripes.  E. lineata also has markings on each body segment.  On segments 5 to 9 these consist of a series of 6 longitudinal stripes, while on other segments they are square, triangular or angular.

Nymphs of this species live in lakes and fast flowing rivers and streams with a sandy or gravely bed.  The nymphs dig into the gravel to form a tubular burrow and they use their gills to force the water through this burrow.  The nymphs feed by filtering or collecting fine particulate organic detritus from the water column.  Ephemera danica usually has a two year life cycle, however recent work has shown that in the warmer waters of Southern England it is able to complete its life cycle in one year.  The main flight period is towards the end of May, however adults are often present between April and November.

Emergence of the adults takes place during daylight on the surface of the water or occasionally on a stick, stone or plant stem partially or entirely out of the water.. The males of this species can be found swarming throughout the day, and often swarming continues until dusk.  Once mated, the female flies upstream and descends to the surface of the water to release a few eggs by dipping the tip of her abdomen onto the surface at intervals, or by actually settling on the surface for short periods.  After several visits to the water the egg supply of upto 8300 eggs is finished and the spent female falls on to the surface.

Ephemera danica is the most common of the British Ephemeridae.  It can be found in unpolluted rivers and lakes throughout the British Isles.