The stars of the blue fleet.

The sea swallow - Glaucus atlanticus can be identified by the two dark stripes with a silver strip inside them on the top, while the Glaucus bennettae has two dark stripes that are have a blue strip inside them, instead of silver (as seen in this obs) .

G.atlanticus is larger, up to about 30mm while G.bennettae is smaller - about 12-15mm.

If you ever come across a washup take a close look at different Glaucus, as once you spot the difference they are easy to tell apart, even at a quick glance.