Courtship begins with one slug following the other's slime trail, circling for an hour or more. The pair then climbs a wall, branch or overhang, secretes a common mucus strand and drops, hanging suspended and coiled around one another.
Suspended, both animals evert their penes, which are long, pale and often translucent blue, from openings behind the head. The organs entwine into a knot and sperm is exchanged in both directions, since each animal is a simultaneous hermaphrodite carrying both sets of gametes. Afterwards one or both slugs may descend the strand or eat it as they go.
Why the aerial position evolved is not established. Suggested reasons include reduced predation while the animals are immobile and vulnerable, and mechanical advantage in disentangling the very long organs, but neither has been tested well. What is clear is that the arrangement is species-specific and reliable enough to be filmed repeatedly, and that the slug's later eggs may be fertilised by sperm stored from more than one partner.