Differences
There are several morphological differences between insects and crustaceans. Insects have six legs and three sections of the body. Crustaceans, on the other hand, have more than six limbs and only two body sections.
Life Cycles
Crustaceans are almost strictly aquatic creatures, though different varieties may live in saltwater versus freshwater environments. Insects, on the other hand, may require water for their larval stages but live on or above the water as adults.
Crustaceans on Land
There are a few examples of crustaceans that live on land. Such crustaceans are commonly confused with insects or other "bugs." The most widely known example of such a crustacean is the common pillbug.
Aquariums
Crustaceans such as shrimp can provide a critical service in an aquarium environment by cleaning up waste products, uneaten food or eating algae. However, they may also be tempting prey for fish. Likewise, certain larval stages of insects, such as mosquitoes, can serve as treats for fish such as the domestic betta.