Approximately 14mm. Six legs. Appeared to have a lighter, almost orange line down each side of the body.
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This is an adult of either Velia caprai or saulii. The image is not clear enough to be absolutely certain.It appears to be a female as there do not seem to be spines on the back of the hind femur possessed by the males. The Females of the two species are easy to distinguish, but males need dissection. The end of the abdomen of caprai is curved upwards; in saulli the abdomen slopes downwards.
Lat/Lng: 55.67985, -3.37552
OS grid ref: NT136437
They were skimming across slack water at the side of a stream edged by grass and moss with overhanging trees.