Slugs in the genus Arion have the pneumostome (respiratory pore) in the front part of the mantle and a round back without a keel.

As an adult it lacks the lateral bands found in many smaller congeners, but in some forms the sides are paler than the back.

The body colour is often reddish, sometimes vividly so, but orange, yellowish, brown and greyish forms are also usual, and some individuals may be black.

[7][8] However, in much of its range A. rufus has declined dramatically over the last decades due to replacement by the externally similar Arion vulgaris.

The eggs hatch over a broad period from late autumn to spring and studies find great variation in growth rate.

The life cycle is predominantly annual; but possibly a few late hatchlings overwinter as juveniles and mature only 18 months or so after hatching.

When the leading slug doubles back, they may form a wheel configuration circling clockwise, with each nibbling the other.

[28] The genitalia take some minutes to engage and evert, the epiphallus (spermatophore producing organ) connecting to the bursa trunk of the other, mutually.

Then the genital atria evert, and swell rapidly (1–2 min) to form a large, white, spherical mass between the bodies.

The slugs remain like this for 90 minutes or longer, during which time a spermatophore is manufactured, filled with sperm, and in part passed over to the partner, hidden by the enveloping atria.

Eventually one partner starts to become active, the configuration consequently rotates, the atria contract, and the genitalia separate.