Schistomerus is an extinct genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of predaceous diving beetle that is known from one species, Schistomerus californense, which inhabited Lake Barstow during the middle Miocene. It is the most common beetle found in the concretions from the Barstow Formation

Barstow Formation

The Barstow Formation is a series of limestones, conglomerates, sandstones, siltstones and shales exposed in the Mojave Desert near Barstow, California. It is early to middle Miocene in age, and lends its name to the Barstovian North American land mammal age...

. The specimens are typically preserved in three dimensions.

All ontogenetic stages of the beetle are present in the concretions. The larval stages of the beetle are most commonly represented, and the mature larvae range from 4.5 - 5.5 mm long. Only 3 adults have been recovered.

Due to the preservational conditions of the formation, a number of individuals were preserved with intact internal anatomy. The tracheal

Invertebrate trachea

The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....

system as well as parts of the organism's nervous system have been described.