There are two sets of retractable tentacles on their heads: the upper pair have light-sensing organs, and the lower two are used to smell.

Arion fasciatus is about six centimeters in length when moving, secreting a clear mucus which is also left behind it and visible as a trail.

It then takes up to a day to get mucus production up to pre-attack levels, leaving the slug susceptible to other predators.

[3] Although the orange-banded arion will eat fresh vegetable matter, a study on the eating habits of Arion fasciatus found that it does not find fresh leaves as palatable as old dead leaves, likely because these slugs generally feed under the leaf litter layer.

Also that evidence of interspecific hybridization in places where these predominantly self-fertilizing slugs apparently outcross contradicted their status as biological species.