According to Heike Reise and other malacologists, the first phase of the mating process is precourtship. She divides the precopulatory period into precourtship and courtship because the latter phase involves so much activity in Deroceras (stroking with sarcobela, lunging, biting, circling). However, based on my observation of Lehmannia valentiana, there is no such courtship phase in this species as she describes it in Deroceras. Therefore, regarding Lehmannia valentiana, I will re-divide the process into a very brief precourtship (touching cephalic tentacles, nibbling body mucus), and a longer courtship (trail following, mouthing, and circling).
The precopulatory period is tentative, in that potential pairings often break up, with either one or both of the slugs ending contact and losing interest. Similarly, couples may skip one or more of the actions listed above, and some even forgo courtship altogether, jumping straight to the copulatory phase. Also, actions occurring during courtship are not necessarily confined to that phase; for instance, trail following and nibbling can occur in the withdrawal phase after copulation.
Currently I don't have any photos of the investigative touching of tentacles that usually precedes trail following. However, I have observed this to take place on many occasions, and frequently nothing comes of the encounter. The reason is unclear. Sometimes the withdrawal is mutual, and other times one slug seems to prefer not to pursue mating with the other. Reise believes that in Deroceras the precourtship phase is likely a mutual testing of each slug's readiness to mate or desirability as a partner. She also thinks the behavior during this period might serve as a means of identifying the other slug as one of the same species. As slugs cannot see well at all, gathering sensory information from touch and chemical signals is crucial to their (limited) understanding of their environment.
In Lehmannia valentiana, if investigation is successful, one slug will then become the leader, leaving the other to follow. As Reise notes regarding Deroceras, while some malacologists describe this behavior as a chase conducted to catch up with a potential mate, she has observed that there is more to it than that. I tend to agree, though I haven't been able to determine whether in Lehmannia valentiana the slug that becomes the leader was the one that initiated the contact. While the lead slug moves in a particular direction, the follower keeps close on its tail, touching it with its oral tentacles. Again, I've seen trail following break off, where the leader keeps going but the follower stops, either losing track or losing interest. Tentative trail following occurs when the follower hangs back and must move along the actual mucus trail, out of touch with the leader's tail. This can be a distraction in itself if the follower decides that eating the mucus is preferable to mating. Or, owing to slugs' poor eyesight, the follower may simply be unable to find the leader if too much distance separates them. Therefore, successful trail following involves the follower's staying right up against the leader's tail, using the sense of touch or perhaps a chemical attractant to maintain the bond. This "slug train" can then remain intact until the leader settles on a mating site.
Once the leader determines the mating site, it will turn back towards the follower's tail, and the two slugs will begin to circle each other. As I said before, while Reise defines actual courtship as beginning when the slugs evert their excitatory organs, or sarcobela, for Lehmannia valentiana I believe it begins with trail following and continues up to eversion, not of the sarcobela, but perhaps of the penial appendices, or of the penial caeca which contain the sperm.
In the next post I will provide photos for the precopulatory period. For the above discussion, I consulted this article:
http://www.senckenberg.de/files/content/forschung/abteilung/zoologie/malakologie/malak/reise/reisepub/amb2_rev.pdf
**Note: the above discussion includes original research, so if you reference any of this information please remember to use the proper citation.
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