Slugs are considered gastropods, not insects. Although both gastropods and insects are invertebrates, or animals lacking backbones, the two groups are not closely related. Slugs have more in common with clams and oysters than with insects, as both slugs and clams are mollusks.
Gastropods and insects belong to entirely separate phylums. A phylum is a biological grouping of organisms that is very general, second only to kingdom in size. Kingdoms, phylums, and lower order classifications like genus and species represent genetic relationships between living things. Animals within the same phylum are more closely related to each other than to the animals outside it. For example: reptiles, amphibians, birds, mammals and the majority of fish all belong to the same phylum, Chordata. This means a slug is less related to a ladybug than a cat is to a shark.