Adults are of the order of 3 cm long. The skin is thin and fairly transparent, often blackish, but brown and pale forms also occur. The mucus is colourless. Thus the external appearance is similar to that of various other
Deroceras species, such as
D. invadens,
D. panormitanum and
D. sturanyi. An indicative but not consistent distinguishing character is a ″pinched″ appearance of the tip of the tail in
D. cecconii. Like with other
Deroceras species, reliable identification requires dissecting the animal to reveal the penis. The most prominent features of the penis are the penial glands, which sit on a substantial trunk, and the penial lobe (a blind-ended pocket). Between these structures lies an indented saddle near where the vas deferens inserts. This indentation to the profile of the penis distinguishes
D. cecconii from
D. golcheri. The penial caecum is often just a swelling, much less prominent than that of
D. invadens or
D. panormitanum. ==Distribution and ecology==