Baetis buceratus Eaton, 1870

the Scarce Olive

Nymphs: Streamlined nymphs with plate-like gills, with the first and last pair of a similar size.

Adults: Small flies with two tails and small oval hindwings.  The male sub-imago has dull grey wings, the veins of which are tinged with golden olive.  The body is dark grey to medium olive, with the underside each segment more yellowish.  The eyes are dull red brown.  The female sub-imago has dull grey wings with light brown venation and brown to medium olive bodies.  Their eyes are dull yellow green.

In the imago, both the male and female have transparent wings with light brown veins, grey olive legs and long, off-white tails.  The female has a dark reddish-brown body, whereas the male has a grey-olive body, with the last three segments reddish brown.  In both sexes the eyes are brown.

Nymphs: Baetis buceratus is superficially similar to both B. vernus and B. rhodani.  It can be readily separated from B. rhodani by the absence of spines amongst the hairs on the edge of the gills (a).  B. buceratus can be separated from B. vernus by the presence of a series of 2 to 4 light dots on each body segment (b) in B. buceratus.

Adults: Sub-imagines of Baetis buceratus and B. vernus are almost inseparable.  Imagines can be separated by small differences in the male genitalia.

Nymphs of this species typically live amongst the sand and gravel in the riffle areas of rivers where they swim in short, darting bursts 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 collected between April and October.  It is thought that there are two generations a year, one with overwintering eggs that emerge in the spring and another that grows over the summer and emerges later in the year. Emergence of the adults probably takes place at the surface of the water during daylight hours.

Baetis buceratus is a widespread, though localised species.  It has been recorded from rivers in Wales, the Midlands and South of England, but is absent from Scotland, Northern England and Ireland, and as such a voucher specimen is required for records from these areas.