Spanish Slug

The size of the adult slug is 60–140 mm. The colour ranges from yellow to black, but is most commonly brick-red, dirty orange, or brown. The tentacles are darker. Adult A. vulgaris may not differ in external appearance from Arion rufus, and so reliable identification requires dissection to examine the genitalia. In contrast, the juveniles of these species can be distinguished because only A. vulgaris has longitudinal bands. However, juveniles of other large Arion species such as Arion flagellus also have longitudinal bands. Reproductive system: The atrium is small. The adjacent part of the oviduct is dilated and muscular, with the same diameter as the atrium and containing a longitudinal ligula. This distinguishes A. vulgaris from Arion ater s.l., in which the oviduct is thinner and the atrium is larger and contains the ligula. Arion flagellus also has a ligula in the dilated part of the oviduct, but the ligula does not reach as far towards the atrium as in A. vulgaris. The spermatheca is spherical, its diameter twice that of the oviduct.

The native distribution of Arion vulgaris is not exactly known. Genetic evidence (the higher incidence of rare alleles) suggests an origin in France or Spain, contrary to earlier genetic analyses that did not adequately sample these regions. In Britain the slug was first recorded in 1954, which is not an indication of it being native there. It is presumed that the specimen illustrated in Moquin-Tandon's original 1855 description was from France.

Non-indigenous distribution. The non-indigenous distribution of Arion vulgaris includes almost the whole of Western and Central Europe and extends to various parts of Southern, Northern and Eastern Europe. The first confirmed Asian records were in 2022 from Armenia and from the Pacific coast of Russia. A 2017 report from the Asian part of Turkey was not based on dissection and is liable to be A. ater s.l. Arion vulgaris is now also recorded from Canada (2009) and Mexico. Reports of "A. lusitanicus" from Madeira may be copied from an 1895 report, in which case they need confirmation; similarly, a 1975 report from the Azores could be of the true A. lusitanicus. …

The habitat of Arion vulgaris includes all agricultural ecosystems, as well as natural environments such as river and lake margins, forest edges, forests in valleys or meadows. In Switzerland it has been found up to 1700 m in altitude. It is a serious agricultural and horticultural pest in large parts of Europe, eating a cosmopolitan range of growing plant parts as well as decaying vegetation. Opportunistically it eats carrion, including squashed conspecifics. It is active mostly during the night and in wet weather during the day. Densities can reach 50 individuals per m2 or locally even higher. Whilst a slug can crawl several metres within a night, long-distance dispersal is believed to be on vegetables, on horticultural seedlings, and on plant debris disposed of as waste. The species has an annual life cycle with mating starting in July and eggs first laid some weeks later in late summer. Clutches are laid on the soil surface or in crevices up to 10 cm underground, with an average clutch size of about 70 eggs; an adult slug typically lays about 400 eggs in its lifetime. The eggs hatch from autumn to spring. Neither eggs nor slugs can survive temperatures below ―3 °C, so overwinter survival depends on hiding under shelters. Adults normally die off in autumn before winter frosts. In the Czech Republic, population density varies considerably from year to year. …