The ground colour in tristella varies considerably and the one shown below is a particularly dark specimen. The light streak is creamy-coloured, narrows at one-third and branches into four fairly obvious 'fingers' at two-thirds, while the termen (end of wing) has a fairly rounded appearance.
|Agriphila tristella, Westcott 9th August|
The ground colour in selasella is fairly consistently a sandy brown colour. The light streak is white with a noticeable thin black line on the costal (lower) side, doesn't narrow at one-third, the branching into 'fingers' at two-thirds is less obvious and the termen is more substantial and squared in appearance.
|Agriphila selasella, Westcott 9th August|
The facial cone in tristella is normally more substantial too but that's not a particularly reliable feature because it can become worn quite quickly.