Until 2006, I had never studied what I had assumed to be Small Skippers (T. sylvestris) closely enough to check if certain specimens were in fact lineolus. The key differentiators are:

1) the underside of the tip of the antennal club (sylvestris is orange, lineolus black) - but this is not always clear-cut

2) the upf black border, well defined in sylvestris, slightly diffuse in lineolus

3) the length of the sex-brand in the male, longer and stronger in sylvestris than lineolus, and sometimes almost undetectable in lineolus. The lineolus sex brand is broken at v2 and does not reach v3, according to T&L.

In the new European taxonomy, this species now has the scientific name Thymelicus lineola.

observations

I originally labelled this as sylvestris and a male based on body shape even though I cannot see any sex brand - sometimes they get lost in the creases, especially the weaker sex brand of lineolus. The black upf border is strong but rather undefined at the inner edge, suggesting lineolus, and the underside of the antennal club, not very clear from this angle, looks as if it could be orange even though it is largely black on the upperside, slightly extending slightly to the underside tip. Compare with 5441 where the entire antenna is orange (=sylvestris). I now believe it more likely to be lineolus, the key factors being the antennal tip being perhaps more black at the tip than orange, and, perhaps most significantly, the very weak sex brand. Thanks to Olli Vesikko for the comments that led to a review of this photo.

the diffuse upf margins and weak sex brand plus the just-visible dark underside of the antennal club, strongly indicate lineolus. Also the sex brand is broken at v2 and clearly does not reach v3.

a female based on body shape. Although the undersides of the antennae can't be seen, they look sufficiently dark to strongly indicate lineolus. However, the upf back borders are quite neat and not fusing into the orange, which might hint at sylvestris. The sex brand clues to the ID of the male are not available in the female.

another dubious underside of an antennal club, and on balance I'll go for lineolus as the tip seems black. The sylvestris I have seen have clearly orange antennal tips, but I'm not sure how consistent this is.