I had no idea slugs were cannibals.
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For example http://snailsandslugs.wordpress.com/tag/cannibalism/
Many slugs are feeders on detritus rather than green plants. They eat dead slugs and any other dead stuff lying around. I presume the prey in the photo had been killed previously, perhaps by being stepped on.
Of course, there are predatory slugs Testacella spp. that eat earthworms http://bogleech.com/bio-gastropoda.html
Lat/Lng: 54.7148, -1.48712
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