[2] The greatest diversity of these colourful small slugs is in the tropical and sub-tropical Indo-Pacific region.

[3] Compared to the closely related philinids and the aglajids, the mantle cavity is small, and the gills are located further forward, in an exposed position.

[4] Gastropterids have parapodia (large outgrowths from the mantle wall) and can swim by flapping these, which has led to them being referred to as "bat-winged slugs".

[5] Siphopteron species have a complex mating behaviour, with each of two slugs attempting to use spines for hypodermic insemination of the other one.

[3] This is the case in Siphopteron quadrispinosum, where each slug attempts to inject prostate fluid into the other having stabbed it with a stylet, part of a two-pronged penis.