Small spur­wing

Trout & Salmon (UK) - - First Cast - By Craig Ma­cadam, co­or­di­na­tor of the River­fly Record­ing Scheme.

Some­times old names for flies are more evoca­tive than their mod­ern monikers. That’s cer­tainly the case for the Small spur­wing (Cen­trop­tilum lu­te­olum). Its cur­rent name is de­rived from the adult hav­ing a small spur­shaped hind-wing, but it was once called the Pale wa­tery, Lit­tle sky blue dun or Lit­tle am­ber spin­ner. The Small spur­wing is wide­spread and com­mon, found through­out the Bri­tish Isles. It has a more lo­calised distri­bu­tion in Scot­land, where it is more of­ten en­coun­tered in stand­ing rather than run­ning wa­ter. Nymphs swim in short, dart­ing bursts among the sandy sub­strate, or climb wa­ter crow­foot and other wa­ter weeds. Once ma­ture (body: 6-8mm, wing: 5-7mm) they emerge from the sur­face through­out the day be­fore fly­ing to bank­side veg­e­ta­tion and trans­pos­ing to a spin­ner. In sum­mer they of­ten de­lay emer­gence to avoid warmer tem­per­a­tures. Ir­ish en­to­mol­o­gist John Har­ris de­scribed the wings of the dun as be­ing “of the same pe­cu­liar shade of very pale blue as the feath­ers on the up­per sur­faces of the wings of terns and some sea gulls”, and in the cor­rect light you can see what he means. The body of the spin­ner is a beau­ti­ful rus­set on the up­per side, and a very pale yel­low below. Males swarm just above the sur­face in the af­ter­noon and of­ten into twi­light. After mat­ing the fe­males re­turn to the river where they dip the end of their body into the wa­ter and re­lease up to 2,500 eggs, be­fore fall­ing spent on the sur­face. There are usu­ally two gen­er­a­tions a year: one over­win­ters as nymphs and emerges in spring; and a fast­grow­ing sum­mer gen­er­a­tion. This re­sults in a long flight pe­riod, the adults present April to Novem­ber.

Did you know?

Fred­eric Hal­ford de­scribed three species, in­clud­ing Cen­trop­tilum lu­te­olum as Pale wa­ter­ies, how­ever the name is now more nor­mally used to de­scribe Baetis fus­ca­tus.

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