Ameletus inopinatus (Eaton), 1887

the Upland summer mayfly

Nymphs: Streamlined nymphs with pairs of single plate-like gills.  Rear corners of body segments are drawn out into small spines.  Tails with a median black band.

Adults: Medium to large flies with two tails and large hind wings.  The sub-imago has dark grey wings with yellowish brown veins.  The body is dark brown, with the last few segments often more red in colour.  In the imago the wings are predominately clear with a very faint rusty brown sheen and the cross veins are difficult to see.  The tails are light brown with almost imperceptible lighter rings to each segment.

Nymphs: Ameletus inopinatus is superficially similar to Baetis species, however close examination of the body segments will reveal small spines on the rear corners of each body segment.  Heptageniidae and Ephemerellidae also have spines on the body segments however they never have single plate-like gills along their body.

Adults: Adult Siphlonuridae have two tails and large hind wings in common with the Baetidae and Heptageniidae, however in the Siphlonuiridae there are four free segments in the foot.  In the Baetidae there are only three, while in the Heptageniidae all five segments are free.  Ameletus inopinatus can be separated from Siphlonurus species by the claws on each foot.  In A. inopinatus one of the claws is blunt and round while the other is pointed.  In Siphlonurus species both claws are pointed.

Nymphs of this species typically live in upland streams (above 300 metres), but are also found in some Highland lochs in Scotland.  The nymphs are good swimmers and typically swim in short, darting bursts.  They feed by gathering or collecting fine particulate organic detritus from the sediment. 

There is one generation a year, which usually overwinters as nymphs and emerges between May and early August.  Emergence of the adults typically takes place during daylight hours and males of this species can be found swarming in the afternoon.

Ameletus inopinatus has a limited distribution in Northern England and Scotland.  It is thought that the distribution of this species will shrink as water temperatures increase due to climate change.  A voucher specimen is required for records from all other areas.