Nigrobaetis digitatus (Bengtsson, 1912)

the Scarce 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 digitatus and N. niger 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.

Adults: Nigrobaetis digitatus and N. niger 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. digitatus and N. niger 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.  Little is known about the lifecycle of this species however adults have been found between May and October.  Recent work has shown that there appears to be two distinct peaks in the flight period - one in the spring and another in the autumn.  This may suggest that there are two generations per year - a slow growing winter generation and a much faster summer

generation.

Nigrobaetis digitatus is a highly localised species with records from a small number of watercourses in Wales, southern England and a single watercourse in Scotland. A voucher specimen is required for all records.