A bit of slug romance

As their snail relatives, slugs are hermaphrodites, each individual producing eggs and sperm. Although some species regularly self-fertilise, in many others, individuals  - given the chance - will trade sperm with other individuals they encounter. I came across this pair of courting slugs under a fallen apple an evening last week. They are Deroceras panormitanum (thanks to Fauna, from WAB for the ID). At the right time of the year, and when meeting a potential partner, these slugs crawl in circles, producing and eating chemicals mixed in their respective mucus trails, before mating happens. The outcomes of courtship and mating are diverse: the slugs are able to mate repeatedly and use sperm from several donors to fertilise their eggs, or they might decide not to give sperm to its partner, or digest the sperm obtained and self-fertilise. This means that, despite being hermaphrodite, a slug can decide if to act only as male (digesting the partners sperm but providing its own), as female (refusing to give sperm, maybe preferring to save it for a better partner) or as both. This can create conflict between the partners - each with their own interests - and has been hypothesized to spur an "arms race" - or "genitals race" if you wish, and has led to the evolution of a range of bizarre copulatory structures and complex mating courtships.
 Heike Reise has reviewed and described the behaviour of these diverse genus of slugs and summarized courtship and mating as follows:

(i) Precourtship phase: the partners encounter and investigate each other.

(ii) Courtship phase: both partners have their sarcobelum protruded from the genital opening and assume a position with their genital pores facing each other, forming a circle or yin-yang configuration. 

(iii) Copulation phase: the slugs evert their penes, entwine them, and mutually transfer the ejaculates from penis to penis (there is no intromission). 

(iv) Withdrawal phase: the penes are retracted together with the attached sperm masses.

After coming across a potential partner or trail, some behaviour follows. This species has a flattened tail, which is enlarged during courtship and waved from side to side. These movements increase the chances of contact with the partner's tentacles. If the leading individual is ready to mate, it eventually turns round and starts following the second individual, forming a circle. Both individuals then evert their sarcobellums - as seen in the top shot - a solid, mobile structure in the genitalia which produces chemicals, that are exchanged during their circling behaviour. The sarcobelum is very active during the courtship, tracking closely the partner tail end and stroking it, at the same time that smears the partner with chemical secretions. In this species, courtship can last over an hour, followed by a relatively quick copulation (a few minutes), in which the slugs genital openings become very close, actual penises - bluish, transparent masses - are everted and entwined with the partners and sperm are exchanged externally. These slugs also have a penial, multifingered gland, that they also evert after insemination and lay over their partner's back, transferring some secretions. The function of this gland has been suggested to be equivalent to the snails' dart, the smeared chemicals on the partner might increase the chances of paternity. The slugs above were in the courtship stage, but unfortunately, by the time I went out again to check on them, they had disappeared.

References
Heike Reise (2007). A review of mating behavior in slugs of the genus Deroceras (Pulmonata: Agriolimacidae) American Malacological Bulletin, 23, 137-156
Benke M, Reise H, Montagne-Wajer K, & Koene JM (2010). Cutaneous application of an accessory-gland secretion after sperm exchange in a terrestrial slug (Mollusca: Pulmonata). Zoology (Jena, Germany), 113 (2), 118-24 PMID: 20202803