The production of a mucus-thread, as a means of progression through the air, has long been known in the Land-Slugs, and has been observed in seven genera, representing Limacidæ, Arionidæ, and Philomycidæ.
The Spinning-Slug (Limax filans) of authors is a myth, the habit being general, extending possibly to all Slugs of the families named. The animals are occasionally seen descending from trees, fences, rafters in greenhouses, &c, and they are easily induced to descend from small exposed objects on which they may be placed. They crawl from the object, and, when the tail parts company with it, the animal is sustained by a thread, which is left by the body at the tail, and is gradually lengthened. Sometimes the animal thus reaches a new support without a fall, but the faculty is imperfect, the animal often falling either without a thread or after making a short one. Large Slugs, when full-grown, are incapable of this kind of locomotion; but small ones, and the young of large kinds, are occasionally capable of making successful descents of surprising length. Threads measuring 3–7 ft. have several times been observed, and others 8–9 ft. long have been reported.
On the foot of the descending animal wave-like appearances
