OBSERVATIONS ON THE PAIRING OF LIMAX MAXIMUS L.
By LIONEL E. ADAMS, B. A.
(With Plate III).
(Read before the Society, December 8, 1897).
DURING the months of last July and August I devoted considerable
time to the observation of the pairing of this species, and as few
conchologists seem to be aware of the extraordinary manner in which
this takes place, the following account may be of interest.
I may mention that my figures (kindly drawn by my friend, Mr.
J. W. Taylor) are all from carefully-measured sketches on the spot,
and that, though I have made drawings of a dozen or more ex-
amples, in no case did the measurements vary materially.
The size of the organs is remarkable, but is accounted for by the
fact of their being congested with the sexual secretions. Dissection
of these slugs during the pairing season shows that the albumen gland
and the common duct occupy the greatest part of the interior.
How these slugs find each other I cannot say. It is certainly not
by sight, as (so far as my experience goes) pairing is only carried on
at night, between 10 p.m. and 2-3° a.m., though perhaps later. Nor
do they follow each other by their mucous tracks; yet I have
seen an individual make straight for another on the side of a brick
wall from the distance of six or seven feet. Most likely the power of
scent mentioned in a note of mine in this journal (vide antea, p. 24),
is the explanation. This nocturnal habit is most fortunate for the
preservation of the species, as otherwise their enemies would find
them in an exposed and helpless condition.
When the pursuer overtakes the pursued, each touches with its tentacles the tentacles of the other, after the manner of ants. Then begins a circular procession,l each with its mouth at the other's tail, and this procession lasts from half an hour to two hours and a half. Careful observation leads me to suppose that during this performance each is eating the external mucus from the other, for a purpose which will "presently appear. The circle now grows more contracted, the slugs overlapping and showing evident excitement, the mantles flapping before and behind. Then, suddenly, the slugs intertwine fiercely, and launch themselves into space, heads downwards, but suspended by a thick strand of mucus, for the distance of 15-18 inches. This mucus thread, which they seem to have been col- lecting for the purpose, is of a yellowish brown colour, and does not seem to mingle with the mucus on their bodies. The fall is generally as rapid as if there were no support, but is gently checked at the finish. On one occasion, however, the fall was very gradual, and during the descent the couple were busily eating more mucus from each other's bodies. The thread appears to come from their mouths, and runs along the centre of the footsole of each, joining into a single thread where their tails intertwine. The upper part of the thread is joined to the circular patch of mucus left on the wall where their previous gyrations have taken place. I have seen a couple suspended in space from a projecting beam in an outhouse, and also from the leaves of a currant bush, and also from the branch of a yew tree, and once from a glass pane of a greenhouse; but a perpendicular wall or tree-trunk is the usual situation.
Directly the descent is accomplished, an organ is protruded from the genital orifice of each. This organ, cylindrical at first, quickly assumes a club-shape of from 1.5-1.75 inches in length (PI. III., fig. I), but presently a frilled edge appears along one side (fig. 2) as if Un- rolled, and in a second or two the unrolling is complete, and the organs assume the shape of fig. 3. The organ along the inner curve of the figure is thick and rounded, while the edge of the whole outer curve is thin and sharp. The unrolled organs now commence to intertwine (fig. 4), finally closing round each other so as to form a knot (fig.. 5), of which it is easy to count the whorls. The two upper whorls of the knot thus formed now spread out in the form of a mushroom or umbrella, leaving the lower portion of the knot as before (fig. 9). Fig. 7 gives the view of this from above. Sometimes the two out- spread whorls do not overlap as in fig. 9, but curve in different directions as shown in fig. 8, and sometimes theyare nearly horizontal, but separate as in fig. 6. The colour of the extruded organs is milk white, flecked here and there with dead white. During 5-10 minutes the slugs hang motionless with the tentacles contracted and flabby, while the two upper outspread whorls keep revolving upon one another ; and in this extraordinary manner the mutual act is consummated. This ended, the process above described is reversed; the organs un- wind, roll up and finally disappear .
It now usually happens that one slug drops to the ground and lies there for sometimes 15 minutes motionless and apparantly exhausted, and finally crawls away; while the other doubles down its head and picks up the supporting thread in its mouth from the centre of its own footsoleand climbs up to the point of support, eating the thread as it goes. Frequently the animal will then descend the thread (which usually adheres to the wall) and eat more of it, and on one occasion I saw an individual ascend the thread again and consume all that was left. It often happens that the thread is left untouched and it may be seen for days on the side of a wall or tree trunk. On the occasions when the pair were suspended from the projecting beam neither of them dropped, as if aware that the distance (6 to 7 feet) was too great for safety, but first one ascended the thread while the other remained stationary, and then the other followed, leaving the thread dangling. After an absence of 10 minutes I found that the thread had dis- appeared. It certainly had not dropped, so I concluded one or both had returned and eaten it. What power of sight these creatures possess I c-annot say, but the strong light of a bull's-eye lantern flashed suddenly upon them and close to them never causes them to withdraw their tentacles or affect their composure in any way, and this not only when they are in a state of sexual excitement, but when they are roving about solitary or feeding. Being anxious to determine the identity of the organs extruded, I severed the parts in question from a paired couple and submitted them and the animals to Mr. W. M. Webb, F..L.S., for dissection and to him I am indebted for the following interesting particulars. "One of the severed structures resolved itself into the' half- evaginated pents, bearing at its posterior end the retractor muscle cut off short and a considerable portion of the vas deferens. The second peni.)' was completely cxtruded, showing the peculiar frill on one side of its tube, down the interior of which tube the vas deferens was traced, though the retractor muscle was not discovered;" Mr. Webb also kindly furnished me with the annexed rough dia- gram to illustrate his further remarks, which are as follows : " A glance at the unravelled genitalia brings out the fact that there is practically no vagina, the pents, oviduct and spermathecal duct meeting almost at the same point, just within the genital opening. The shortness of the spermathecal duct and the position of its opening also point to the filling of the spermatheca from the outside of the animal; and one would say that the use of the penis is to convey the sperm cells to and not into the female orifice, its frill being deve- loped for the.purpose of holding them during the act of passing them over to another individual."
The only detailed description of this interesting method of pairing is in Ferussac's scarce work, II Histoire naturelle generale et par- ticuliere des Mollusques Terrestres et Fluviatiles," published in 1819, and this description the author gives at second-hand from M. Werlich. The details tally very fairly with my own observations, but not altogether, and this discrepancy inclines me to think that some other, though closely-allied, species was under observation. Werlich speaks of the extrusion of the organ before the suspension of the animals, which I have never seen, nor do I think it likely to happen, for this reason; if the organs were extruded previous to the mutual suspension, they would inevitably become covered with grit and dust, which would seriously interfere with their functions; (the case is very different with Arion ater, Agriolimax agrestis, &c., which copulate on the ground, and extrude a much smaller organ, and one which is erectile. In the case of these latter, actual penetration takes place, which is not the case with Limax maximus). Ferussac's figures, too, are quite unlike anything that I have observed, and I fancy he drew them himself from Werlich's description, as I cannot think that Wer- lich after having witnessed the operation could have drawn figures so much at variance with the reality. He says, "Nous avous fait copier les figures qui accompagnent cettenote, PI. IV. A., de notre Histoire generale, afin de faciliter l'intelligence du recit. tres interessant de Mr. Werlich." It is just possible that the suspension of L. marginalus Muller (L. arborum, B. Ch.) from the branches of trees is for the same pur- pose as that of L. maximus.