Actually, neither an insect nor an arachnid. Millipedes belong to "diplopoda". More info about millipedes and centipedes here: http://insects.about.com/od/identifyaninsect/a/centiormilli.htm One thing I found interested on this website was that millipedes don't bite, but centipedes, do. From now on I'm shaking out the gardening boots I keep in the garage, before I put them on!