As I've observed it in
Lehmannia valentiana, copulation begins with the eversion of a genital appendage which I believe to be the penial appendix, perhaps a caecum containing the sperm masses to be exchanged. Please understand that this is a
hypothesis, as I don't have definitive evidence to prove it. What evidence I do have is this: the function of the sarcobelum, or excitatory organ, in slugs is to stimulate and to secrete. According to Reise, in
Deroceras the slugs extrude their sarcobela at the beginning of courtship and use them to stroke or touch the partner's body. This occurs before the slugs extrude their penes to exchange sperm. The slugs of
Lehmannia valentiana do evert a particular part of the genital apparatus before the penis, but they do not use it in the way
Deroceras uses the sarcobelum. There is no stroking involved; in fact, the appendices hardly touch. Also, the fingered penial gland should emerge after the penes, but in
Lehmannia valentiana this is absent. I believe this is because
L. valentiana has a much simpler genital apparatus than
Deroceras, which is multiformed and complex. Compare these diagrams:
The genitalia of
Lehmannia valentiana:
 |
| © R. Forsyth, modified by K. Weigel, University of Florida |
And that of
Deroceras reticulatum:
 |
© M. Horsak et al., Wikipedia
a - atrium
p - penis
s - sarcobelum
mr - penis retractor muscle
gp - penial gland
bc - bursa copulatrix
ov - oviduct |
As well as that of
Deroceras caucasicum:
 |
| © Wiktor 2000 modified by K. Weigel, University of Florida |
What Reise calls the longer and shorter side pockets in
D. panormitanum (shown below), can also be referred to as the penial caecum and penial lobe. The caecum contains the sperm mass, which some slugs, such as
D. invadens, deposit onto their partner's sarcobelum. The usual order of eversion is 1. caecum, 2. lobe, and 3. gland, but depending on species there can be some overlap or different order.
 |
| Deroceras panormitanum genital tract, © 1992, 2006 Heike Reise |
While
Lehmannia valentiana does not appear to have a penial gland attached to the penis in between two pockets (caecum and lobe), I believe that the slugs of this species do evert the penial appendix instead of a sarcobelum. Below you'll find several sets of photos of this particular appendage. In order to make the proper comparison, you'll need to look at least a few of these videos made by Reise and her team:
http://www.senckenberg.de/files/content/forschung/abteilung/zoologie/malakologie/malak/reise/ss.htm
Now that you have an idea of what mating looks like in another genus, here are some shots of the unknown appendage in
Lehmannia valentiana. The
Fact Sheet for this species says: "The appendix on the penis of this species is somewhat tubular or the apex may appear expanded."
Couple 2:
Couple 6:
Couple 11:
Couple 17:
Couple 20:
Couple 24:
Couple 25:
Couple 26:
In the next post I will highlight the shape of the appendage in comparison to that of the appendix shown in the diagram of the genitalia.
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