There are 8 species of Deroceras slugs in the Czech Republic (all of them has been recorded in Slovakia too, but the presence of Deroceras invadens has been confirmed in greenhouses only in Slovakia). Note, that Deroceras invadens was formally redescribed in 2011:
Deroceras agreste (Linné, 1758)
Deroceras laeve (O. F. Müller, 1774)
Deroceras invadens Reise, Hutchinson, Schunack & Schlitt, 2011 – [This species was formerly considered as Deroceras panormitanum (Lessona & Pollonera, 1882) in the Central Europe.]
Deroceras praecox Wiktor, 1966
Deroceras reticulatum (O. F. Müller, 1774)
Deroceras rodnae s. lat. Grossu et Lupu, 1965
Deroceras sturanyi (Simroth, 1894)
Deroceras turcicum (Simroth, 1894)
According to the Hutchinson & Reise (2009) the Deroceras rodnae s. lat. has been splitted to the western population: Deroceras juranum Wüthrich, 1993; and to the eastern population: Deroceras rodnae sensu stricto Grossu et Lupu, 1965. The population in Veľká Fatra mountains in Slovakia previously named Deroceras fatrense Mácha, 1981 could be named Deroceras rodnae fatrense Mácha, 1981. That would count as 9 species of Deroceras in the Czech Republic.
Unfortunatelly the only 100% sure identification of these species can be done with the dissection. I will try to summarize to what level such photos of Deroceras can identified, when they were neither collected nor dissected. I have sorted them to groups for this my purpose.
Deroceras with one dark color include:
Deroceras laeve + Deroceras invadens + Deroceras sturanyi
But when the slug is dark brown, it is probably Deroceras laeve, especially when they occur in very wet habitats near water.
According to size:
Deroceras laeve – 30 mm
Deroceras invadens – 20-35 (40) mm
Deroceras sturanyi – 35 mm
Deroceras with one light brown color:
Deroceras invadens + Deroceras sturanyi
Light color with some dark spots:
Deroceras praecox + Deroceras reticulatum + Deroceras rodnae + Deroceras turcicum
Deroceras praecox – 45 mm
Deroceras reticulatum – 60 mm
Deroceras rodnae – 45 mm
Deroceras turcicum – 45 mm
It seems that the only one to be properly identified according to the photo could be Deroceras reticulatum, if it will have typical spots and if it will live on cultivated areas.
Deroceras agreste – 40 mm
I need to improve my own experience if it is or isn’t possible to identify it according to a photo only.
violet Deroceras slugs:
Deroceras rodnae s. lat. + Deroceras turcicum
Some of my photos of undissected slugs. All of them are from various localities in Moravia, the Czech Republic:
It is from group 3), it has typical reticulate pattern and it lives on synantrophic locality: on the cargo train station in Olomouc. I think, that it must be Deroceras reticulatum.
It is from group 1), although it looks like Deroceras leave, its locality is not wet. The locality is more dry than wet. It is in Olomouc city. It is one of these three species (Deroceras laeve + Deroceras invadens + Deroceras sturanyi), but I do not know which one.
Another Deroceras from group 3). Locality: near Pateřín village, between fields, close to the creek.
Some Deroceras from group 2). Locality: near Pateřín village, between fields, close to the creek. It is either Deroceras invadens or Deroceras sturanyi.
(in Czech) Horsák M., Juřičková L., Beran L., Čejka T. & Dvořák L. 2010: Komentovaný seznam měkkýšů zjištěných ve volné přírodě České a Slovenské republiky. [Annotated list of mollusc species recorded outdoors in the Czech and Slovak Republics]. – Malacologica Bohemoslovaca, Suppl. 1: 1–37.
Hutchinson J.M.C. & Reise H. 2009: Mating behaviour clarifies the taxonomy of slug species defined by genital anatomy: the Deroceras rodnae complex in the Sächsische Schweiz and elsewhere. – Mollusca 27(2): 183–200.
Reise H., Hutchinson J.M.C., Schunack S. & Schlitt B. 2011: Deroceras panormitanum and congeners from Malta and Sicily, with a redescription of the widespread pest slug as Deroceras invadens n. sp.. – Folia Malacologica 19(4): 201–233. Download from official page requires free registration. (PDF copy on authors website.)