They are not insects but mollusks, like oysters and clams, and are often described as snails without shells.
Slugs move along by secreting a path of mucus.
Hatching from masses found in damp areas of the garden, slugs are one of the first pests of spring. They begin hatching when temperatures are consistently greater than 40 degrees F. Young slugs resemble the adults and begin feeding as soon as they have hatched, creating large holes in the leaves, fruit and crowns of plants. They prefer damp, cool locations and will hide in soil crevices, earthworm holes or under leaves, boards or other garden debris during the day. Wet weather in spring and early summer results in an increase in the slug population. If the weather is dry, they will die or burrow deep into the soil until conditions improve.