evolutionarily stable, as both sex roles are required for successful reproduction. Because sperm digestion in land slugs reduces the likelihood of a given set of sperm fertilising the partner's eggs, increased sperm investment is selected for. This can lead to a co-evolutionary cycle in which the amount of sperm digested and male allocation both increase until eventually, male and female gametes are equally invested in. The larger and more costly ejaculates become, the more reciprocal sperm transfer is favoured, as each slug receives compensation for its investment. Reciprocity, either simultaneous or serial, appears to alleviate the sexual role conflict, and is a feature of most slug matings. Finding a mate As land gastropods such as land snails and land slugs have no sense of hearing and very limited vision, they rely heavily on chemoception, primarily olfaction, and tactile perception. Mate location is also greatly mediated by airborne chemicals. Slugs often raise their head at the detection of pheromones in the air, and then turn to orient themselves and crawl towards the perceived source. Many slugs also exhibit some degree of mucus-trail-following in order to locate a mate. Trail following may be an alternative or complementary strategy to airborne cues, and can constitute a major part of precourtship behaviour. Using mucus trails to find a mate involves the discrimination between conspecific and heterospecific trails. It is therefore advantageous to incorporate species specific cues into trails to facilitate mate finding. In some species, such as banana slugs (Ariolimax), pheromones are secreted together with the mucus to attract mates. It may even be possible for a follower to perform a quality assessment of a potential mate based on its mucus trail. The mucus may reveal information on body size or parasite infection, giving an insight into fecundity. A larger size suggests a slug is highly fertile, whereas parasitism could mean it suffers from decreased egg production or even sterility. Trail following is a complex behavioural pattern which usually involves the active participation of both slugs. The follower keeps very close to the leader once they are in proximity. In species with pronounced trail following, such as Deroceras panormitanum, the leader flattens its tail laterally and waves it from side to side in front of the follower. It is not yet known whether tail waving is a visual cue or whether it wafts chemical attractants in the direction of the follower. Sometimes, the tail