You're probably referring to the Terrestrial Amphipods or "Lawn Shrimp," (Crustacea: Amphipoda). Amphipods comprise an order of crustacea, shrimp-like in form, which contains mostly marine and freshwater forms. While some species are terrestrial, they still require moist habitats. These terrestrial species are sometimes referred to as "lawn shrimp" due to their similarity to true shrimp.

Many amphipods, like those called beach fleas, live on the beach, where they occur under stones or decaying vegetation. Most amphipods are scavengers. Some amphipods are found living under ivy used as ground cover. Here they jumped about like fleas and were difficult to capture. Amphipods are present in soft ground up to a depth of 13 mm. Leaf mold beneath shrubbery also offers a suitable habitat for terrestrial amphipods.

Terrestrial amphipods live on the surface (top 1/2 inch) of mulch and moist ground. After rains, large numbers of amphipods can migrate into garages or under the doors of houses. There they soon die. Amphipods do not have a waxy layer on their exoskeleton as do insects. They lose or gain moisture from their environment. Too much of a water loss results in desiccation while too rapid a gain is also lethal. This is why they migrate out of rain-soaked soil to drier areas where they usually end up dying anyway. Most species are active at night.

Terrestrial amphipods require a moist environment and die quickly in the drier environment found within structures or on drying pavements. Once dead, they turn a reddish color. They may be safely swept or vacuumed up and discarded. There are no labeled insecticidal recommendations for control. Spot treatment of doorways with a residual contact insecticide will accelerate amphipod mortality in homes.