24-Aug The Secret Life of Slugs

Another garden slug that is usually met with alarm is the large black slug Arion ater which, when moving, can reach 18 cms. In length yet contract to a hemisphere if touched. Within its species complex it can vary dramatically in colour from grey to yellow, white, brown, purple and even a fluorescent red. In London we just tend to see jet black, brown or bright orange specimens. Sometimes after rain large numbers of this slug seem to suddenly appear on heathland.

Other common slugs include the dusky, brown-banded, keeled and Budapest slug. The dusky slug Arion subfuscus (60 mm.) is usually yellow with a transparent yellow sole. If handled its mucus can stain your fingers. The brown-banded slug A. circumscriptus (35 mm.) has helpfully got dark bands along its sides as well as a dull opaque foot.

The keeled slug Tandonia sowerbyi (70 mm.) is brown with a long raised keel along its back which makes it look like a Cornish pasty when it contracts. The Budapest slug T. budapestensis (60 mm.) is very similar but with an additional dark band along its sole.

Two slugs that are much more likely to be found in wetter conditions are the grey field slug and marsh slug. The grey field slug Deroceras agreste (40 mm.) is a distinctive oatmeal colour and the marsh slug D. laeve (20 mm.) is a glossy brown slug which can even be found in water. It is thought it may even be distributed by running water. It is unlikely you will see the worm slug Brettgerilla pallens (40 mm.) as it spends most of its time underground looking for earthworms which it feeds upon. It looks very like a worm itself except for the raised keel along the length of its back.

Other slugs have more varied features which may help to identify them. The yellow slug Limax flavus (90 mm.) is a large greyish or greenish yellow slug with attractive steely blue tentacles. It has a marked preference for damp cellars. The hedgehog slug Arion intermedius (20 mm.) when it contracts has a dimpled surface making it look a little like a hedgehog. It is unusual being found in conifer woods and heathland. The Sicilian slug Deroceras panormitanum (30 mm.) is so called as it was first described in Palermo which doesn’t particularly help to identify it. Howevcer, its watery transparent almost gelatinous look usually marks it out, especially in green houses which it occasionally wanders into.

Quite different and significantly more interesting are the tree slugs which can climb into the canopy of trees. The tree slug Lehmannia marginata (75 mm.) can even move from branch to branch using a mucus thread not unlike the way a spider uses silk. Using the same technique it can even abseil back down to the ground. It is usually grey, slightly transparent and it also has a gelatinous look but can be blue-green or even spotted. Sometimes it can be confused with the juveniles of an even more impressive tree slug, the leopard slug Limax maximus which some people have even described as beautiful.

It is usually greyish-yellow with attractive leopard-like markings. On the move it can reach an impressive 16 cms. in length. It often spends the day in crevices on the trunk of a tree and then ascends the tree at night. When it pairs, which is usually in the winter months, both slugs produce an intertwined hanging mucus thread which can be up to a metre and a half in length. At the end of this thread in mid air the two hermaphrodite slugs then spiral together and with blue penises almost the length of their bodies exchange sperm. This whole process can take several hours and they may still be hanging there together in the early morning. Once their unusual courtship is finished they both re-ascend the mucus thread and go their separate ways.

Slugs are generally found in gardens, under stones, among rubbish or just discovered eating plants. Bookham common and Hampstead heath are good places to look for the various species mentioned here. Both have ten different species recorded with Bookham common even providing the chance of seeing a leopard slug.