Nigrobaetis niger (Linnaeus, 1761)

the Southern Iron Blue

Nymphs: Streamlined nymphs with plate-like gills, the first pair (nearest the head) are absent.  Tails with a median black band.

Adults: Small flies with two tails and small oval hindwings.  The sub-imago has dull yellow green eyes in the female and dull red brown eyes in the male.  In both sexes the wings are a dull grey blue colour and the body is dark brown which is sometimes tinged with olive.  The legs are pale to dark olive brown, whilst the tails are dark grey.  The imago has transparent wings and pale grey legs and tails.  The body in the female is dark claret brown, whilst in the male it is translucent white, with the last three segments being dark orange brown. 

Nymphs: Nigrobaetis digitatus and N. niger are both narrow-bodied nymphs which have a black band across their tails.  Nigrobaetis niger and N. digitatus are the only Baetidae with only 6 pairs of plate-like gills.  Separation of nymphs of N. digitatus and N. niger can be achieved by examination of the last gill.  In N. digitatus the hind edge of the gill is slightly concave resulting in a different shape when compared to other Baetidae species, including N. niger.

Adults: Nigrobaetis niger and N. digitatus can be distinguished from other adult Baetidae by the presence of only two veins on the oval shaped hind wing, the second of which is forked.  The only other Baetidae species with a forked vein is Alainites muticus, however this species has a third vein which runs along the lower edge of the wing.  It is often difficult to see the third vein, and care must be taken to double check whether it is present or not.  Separation of adults of N. niger and N. digitatus is not reliable at present.

Nymphs of this species typically crawl amongst in-stream vegetation in riffle areas of rivers and streams or swim in short, darting bursts swims amongst the substrate.  They feed by scraping algae from submerged stones and other structures, or by gathering or collecting fine particulate organic detritus from the sediment. 

There are two generations per year - a slow growing winter generation and a much faster summer generation.  This results in a fairly long flight period, with adults being present between April and October.  Recent work on the River Test, Hampshire has found that the summer generation is considerably reduced, if not absent.

Emergence of the adults is thought to occur at the surface of the water during daylight hours.  The males of this species can be found swarming throughout the day, and often swarming continues until dusk.

Once mated, the female fly either pulls herself under the water surface to lay her 1200 eggs directly on a partly submerged stone.  In some cases she will fly to the river, where she descends to the surface of the water and releases her eggs in several batches by dipping the tip of her abdomen onto the water surface.

Nigrobaetis niger is a widespread, though localised species found in running waters throughout the British Isles with the exception of Ireland.  As a result, a voucher specimen is required for records from Ireland.