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.
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