Genus: Arion

Common name: Roundback slugs 
​Family: Arionidae

Arion distinctus, brown slug

Origin: Europe
Diet: Live and dead plants. These are some of the most significant crop and garden pests.
Other species in Oregon: Arion intermedius, A. circumscriptus circumscriptus and A. circumscriptus silvaticus
Where in Oregon: Widespread.
Similar exotics to look for: There are at least 19 additional species of Arion in Europe. They are difficult to differentiate from species already present in Oregon. Dissection and inspection of internal organs is required to confirm identification. 

Arion distinctus, brown slug

Arion hortensis, garden Arion

Arion rufus, red or chocolate Arion, contracted

Arion rufus, red or chocolate arion

Arion subfuscus, dusky slug

Quick key to Arion species (lengths refer to mature specimens)
1. Sole yellow, less than 40mm long.

    A. less than 20mm long.....A. intermedius
    B. more than 20mm long....A. distinctus and hortensis

2. Sole is not primarily yellow, length variable. 
    A. With dorsal stripes
          i. Clear mucus, no vertical stripes on foot fringe, less than 45mm....A. circumscriptus
          ii. Yellow mucus, vertical stripes on foot fringe, adults 50mm or greater....A. subfuscus
    B. No dorsal stripes and large (60mm+)....A. rufus

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