Histeridae is a family of beetles commonly known as clown beetles or hister beetles. This very diverse group of beetles contains 3,900 species found worldwide. They can be easily identified by their shortened elytra that leaves two of the seven tergites exposed, and their geniculate (elbowed) antennae with clubbed ends. These predatory feeders are most active at night and will fake death if they feel threatened. This family of beetles will occupy almost any kind of niche throughout the world. Hister beetles have proved useful during forensic investigations to help in time of death estimation. Also, certain species are used in the control of livestock pests that infest dung and to control houseflies. Because they are predacious and will even eat other hister beetles, they must be isolated when collected.
The hister beetles are easily identified by their shiny elytra which is typically shiny black or metallic green. The two main shapes for this family are oval and flat. The elytra is shorter than the abdomen with typically two of the seven tergites exposed.[2] Hister beetles have specialized heads that can retract into their prothorax and two geniculate (elbowed) antennae with clubbed ends. As predators, hister beetles commonly feed on the egg, larval, and adult stages of other insects. Certain species are also used to control livestock pests that infest dung or to control house flies. Histeridae are most active at night and they will play dead if they feel threatened.[3]
Histeridae was named by Leonard Gyllenhaal. Histeridae has two common names, the “clown” beetle and the “hister” beetle. There have been several theories explaining the origin of these common names.[4] One theory for the “hister” nickname comes from the work of Juvenal, a Roman poet. Juvenal used the word “hister” to mean a dirty, lowly being. Another theory for the origin of this beetle's name stems from the fact that in Latin, “hister” means actor.[5] Many people believe the name is associated with the hister beetles’ ability to imitate death when they are disturbed. Still, some believe this family of beetles was named for its physical characteristics.[citation needed] Clown beetles have flattened legs, which can be compared to a clown's flat shoes or loose-fitting pants.
The elytra and forewings are normally well developed in beetles, but in Histeridae the elytra are shortened and rectangular. The shortened elytra expose the final two of the seven tergites. The head has compound eyes, a mandible mouthpart, and shortened antennae. A distinctive characteristic is that the antennae are elbowed and contain three antennomeres that form a club at the end.[6]
Clown beetles have an open circulatory system within its hemocoel, also known as a body cavity. They have a tube shaped heart that spans the length of the body, and use hemolymph as blood.[7] This blood does not contain any oxygen, but carries nutrients throughout the body. Spiracles are located on the abdomen and are the tracheal system of the beetle. Oxygen is brought into the body by spiracles and tiny sacs exchange the oxygen like a mammal's lung.
Hister beetles are found throughout the world in various habitats. Histeridae have been located in North America, Central America, Europe, Asia, and Australia, but each hister beetle occupies certain niches. The beetles live in dung, carrion, dead vegetation, sandy areas, under tree bark, mammal burrows, and ant/termite colonies. Charactertistics of the hister beetle are dependent upon its habitat. For example, the flat hister beetles are found under bark, while the cylindrical beetles are not. The shape of the beetle will vary from species to species.