Slug standing up

'Look, a slug standing up' called my 7 year old daughter. I went over by her and watched. Indeed, a Spanish Slug, Arion vulgaris, reared up as high as it could go in between some pots. It waved its body from side to side, and after some time it bent over itself turning round, touching its mucus pore with its head. I had seen this behaviour previously described in Rowson and colleagues' FSC guide 'Slugs of Britain and Ireland':

'Arionids are sometimes seen rearing up off the ground, as if sniffing the air' 

It is a curious behaviour of unclear function, are they really 'sniffing' the air? could they be sensing humidity in the air, assessing when to retreat to a refuge, or looking/smelling for food? Although slugs have very simple eyes, this rearing and moving side to side could allow them to assess where shadows are indicating plants to climb. Alternatively, they could be detecting or emitting pheromones to find or attract potential mates.

An impressive slug

A showery day, I come across this enormous Arion slug crossing the garden path. It must be close to 15 cm. Identification to species level is difficult to impossible as they are distinguishable by examination of internal genitalia characters - or molecular genetic analysis, and hybrids are common. British large Arion slugs are made of several species complexes, Arion ater/rufus A. lusitanicus and A. flagellus. Both groups are very polymorphic slugs, from almost white to black, including all shades of brown yellow red and orange. I have never found the the black morph in the garden, but a good representation of the others abound. The polymorphism of these slugs in the U.K., not only in colour, but also in genital characters, might stem from common hybridisation between several subspecies that colonised the British Isles from Europe, postglacially but also in more recent human introductions. Distribution changes might also come about due to expansions as a result of climate change. In addition, the slugs - except for A. flagellus - are able to self-fertilise and form relatively homogeneous populations, which might give the impression of being a separate species.

 Arion slugs had deep tubercles on their backs, and no keel. At the end tip is the mucus gland, producing an extremely sticky mucus. Unlike the yellow slug, which is strictly nocturnal, Arion slugs are active all around the day, provided that it is damp. When disturbed they show a characteristic behaviour: they contract their bodies into an almost round shape and retract their tentacles inside their mantle. Inspired by a post in Myrmecos, I set the photo vertically. I had to use a white plate - instead of bowl - and allow the slug to relax and start to crawl out. 

A very yellow specimen

A disturbed Arion

A selection of colours found in the garden

The black morph of Arion sp.

References

Noble, L. R. Jones, C. S. (1996). A molecular and ecological investigation of the large arionid slugs of North-West Europe: the potential for new pests The ecology of agricultural pests: biochemical approaches Ed. William O. C. Symondson, Systematics Association, special volume. Clarendon Press., 53, 93-132
Evans, N. (1986). An investigation of the status of the terrestrial slugs Arion ater ater (L.) and Arion ater rufus (L.) (Mollusca, Gastropoda, Pulmonata) in Britain Zoologica Scripta, 15 (4), 313-322 DOI: 10.1111/j.1463-6409.1986.tb00232.x
UPDATE 30/08/11: reference added and Arion ater in a photo caption changed to Arion sp.