On 5th June I witnessed something quite new to me. At eye level a 25 mm long slug, fully extended, was gently swaying suspended by a thread about one metre long attached to an aluminium bracing strut in the apex of my glasshouse. For the next two hours I watched it perform a series of complicated gyrations until at 11.45 am it severed its connection and dropped to the floor. The temperature was about 17ºC, the sky was heavily overcast with periods of rain and the glasshouse was full of mature, well watered plants ensuring a humid atmosphere.

The gyrations followed a repeating pattern and at the stage where the slug was fully extended, head uppermost, a glistening white globule appeared on the tail (the lower end of the tapering body). This was followed by a series of convolutions as the slug folded its body about its middle so that lower surface of the tail was in contact with that of the head end, often twisting its body into coils and spirals as it did so. As it completed this procedure the globule appeared to be taken into the body of the slug or spread over the lower body surface. As it twisted and turned the attachment of the suspending thread moved from the head to the tail or mid-body of the slug so that it hung horizontally or head down. The whole procedure took about three minutes and throughout the period the creature moved up and down, lengthening and shortening the thread by about 300 mm.

Mating in some species of slugs takes place while suspended from a branch by a thread of mucus. This is best known in the Leopard or Great Grey slug (Limax maximus) and is strikingly illustrated in David Attenborough’s TV series ‘Life in the undergrowth’; two slugs come together (slugs are hermaphrodite) and secrete a thick mucous strand which they use to suspend themselves from a branch where they copulate before separating (Chinery, 1978).There was no indication that the glasshouse slug was involved in sexual behaviour, stimulated by a partner which had dropped off or gone away, as its gyrations were purposeful and long lasting. If anyone can shed any light on what was going on or can identify genus or species from the photographs I would be very interested to hear.

Reference
Chinery, M. (1978). The Natural History of the Garden. Fontana / Collins.