Lab critters are animals that are used in scientific research. The most common lab critters are mice, fleas, and white rats. Mice are small rodents that are often used in medical research due to their similarity to humans. Fleas are small insects that are often used in studies of insect behavior. White rats are a type of rat that is bred specifically for laboratory use. All three of these lab critters are important for scientific research and have been used in many studies.