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Well I can not argue with your identification, it is a slug. The question is, of course, which one? Slugs are nasty things as they are just so variable in colour and being soft and flexable they rarely have any body features to use for identification. Therefore, it is often jizz you have to go by.
This has all the features of a common slug (Arion ater), chubby body, uniform ridges down back and large, flat saddle at the head end. However, the rich brown colour and darker antenae suggest it could be the introduced Spanish slug (Arion lusitanicus). The trouble is this species readily hybridises with other slugs, including A. ater. I beleive if you boil one the mucus will turn the water yellow; however, I am not suggesting you do this!
If it is a Spanish slug they are a pest species, not only eating crops but also other slugs.
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Very interesting, I once went to a talk on slug identification and they came to a similar conclusion to the one you have suggested, that with Arion you can almost get to a full ID easily but being sure between the last one or two species is very tricky.
I had not heard of Spanish slug (Arion lusitanicus)
What do you think my iSpot is ?
Do I need to reassess the identification?
Wildlife of Assynt
See comment added to http://www.ispot.org.uk/node/7305
Entomologist and biological recorder
Below is some very useful information that Roy Anderson of the Conchological Society provided via their egroup some months ago - I hope he won't mind me reproducing it here. He confirms the difficulty in being certain of these three species, but provides some helpful hints. For photography it is very useful to have a shot of the underside as well as the upperside, and if you can do the touch test as well that all helps.
Information on distinguising Arion ater, Arion rufus and Arion vulgaris (= Arion lusitanicus)
From Roy Anderson, on the conchology-uk egroup (http://tech.groups.yahoo.com/group/conchology-uk/), 21 April 2009:
First of all - Arion lusitanicus as a name has been superseded by the name Arion vulgaris as lusitanicus refers only to an endemic Portuguese species and not the common pest species of central Europe and the British Isles.
Distinguishing Arion vulgaris from related species - Arion rufus, Arion ater - is fraught with difficulty. So try the following approach:
1) Try to stick to mature specimens - the large Arions mature between June and September
2) Try the touch test - does the animal, when in a contracted state, weave from side to side if stroked firmly from head to tail? If yes, you almost certainly have Arion ater (I have yet to find an instance of Arion vulgaris making the slightest movement when stroked).
3) The colour of the foot fringe (that striped line along the margin of the body) can be important. Is it the same dull colour as the rest of the upper surface i.e. brown, black, grey or a dull yellowish? If yes, you are dealing either with Arion ater or Arion vulgaris.
4) If the foot fringe contrasts strongly in colour with the upper surface i.e. bright orange or red against a body colour of brown or black you are probably dealing with Arion rufus. Arion rufus is further distinguished by a reddish pigment both on the sides below the mantle and, if you turn the animal over, on the sole near the head end. It also occasionally responds to the touch test.
5) If the animal is dark with dull foot fringe and doesn't respond to the touch test you may have Arion vulgaris. Black vulgaris usually have foot fringes of a dirty yellow or orange-brown colour rather than black. Black foot fringes invariably mean ater.
6) Finally, if your animal is brown with dull foot fringe i.e. ater or vulgaris, turn it over to examine the sole. If the sole is a distinctly darker colour than the upper body i.e. dark grey or black against a brown body, then you probably have vulgaris. If it is the same or a lighter shade then you probably have ater. Heathland ater are black with black foot fringe and either a black or dark grey sole.
Sorry, but it is as complicated as that. The only way to be certain is to catch a pair in cop. and examine the spermatophore, or dissect the genital canal. But that is beyond the scope of this message.
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