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Deroceras reticulatum
Maybe a 1/2 inch in length.
Thanks, Apple. I'm assuming that they were introduced to Rock Island as well. The man who own the island in the 1920s to 1940s had a cultivated Japanese garden. It is highly likely they were either introduced then or even earlier.
The wiki sites say they are not documented in North America outside of Michigan but I found a resource from Cornell that talks all about how they were introduced in N. America in the 1800's. See site: http://nysipm.cornell.edu/factsheets/fie...