As suggested by the bat’s name, its fur is uniformly dark brown and glossy on the back and upper parts with slightly paler, greyish fur underneath. Wing membranes are dark brown on a typical wingspan of 22–27 cm (8.7–10.6 in). Ears are small and black with a short, rounded tragus. Adult bats are typically 6–10 cm (2.4–3.9 in) long and weigh 5–14 grams (0.2–0.5 oz). Females tend to be larger than males. The fore and hind limbs have five metapodials. The skull of the brown bat lacks a sagittal crest. Its rostrum is shortened and has upslope profile of the forehead. Its braincase flattened and sub-circular when observed dorsally. The bat has 38 teeth all of which, including molars, are relatively sharp, as is typical for an insectivore, and canines are prominent to enable grasping hard-bodied insects in flight.

The little brown bat can be distinguished from the Indiana bat by the absence of a keel on the calcar and long hairs on the hind feet that stretch longer than the toes. Compared to the long-legged bat, the brown bat has a shorter tibia and lesser amounts of ventral wing hair in addition to the lack of a keel on the calcar. Source

Little brown bats, Myotis lucifugus, are abundant in southern Alaska, Canada, across the United States from the Pacific to Atlantic coasts, and the higher elevation forested regions of Mexico. Although little brown bats are not found in northern Canada, individuals have been observed in Iceland and Kamchatka. Those outlying records are presumed to have been the result of accidental ship transportation by humans. Little brown bats are also absent from much of Florida, the southern Great Plains regions of the U.S., southern California, and parts of coast Virginia and the Carolinas. Source

The swampy habitat for the Little Brown Bat offers them access to plenty of insects. They also have access to water for drinking. Wasps and moths are the majority of their diet in such an environment. They can consume up to half of their body weight each night when it comes to their eating habits.

Most of the time the Little Brown Bat will consume its meals while in flight. They have very sharp molars. They have canines that are shaped in a manner that allows them to hold onto their prey while they are in flight. They will also use the tip of their wing to capture food.

They are known to eat insects off of the fur of other types of animals too. Humans observing such behavior often mistake these actions. They assume the bats are biting those animals but that isn’t the case. The use of echolocation allows them to easily find their prey in the dark.

When a Little Brown Bat doesn’t find enough food to eat on a given day, it will have to slow its body rate while sleeping during the day. This process allows it to conserve energy. Otherwise it may become too weak to be able to fly about and look for food the next night. Usually though they have no trouble finding enough food each night. Source