The Passalidae, or bess beetles, are a family of about 500 beetles (Coleoptera) within one of the main clades of the superfamily Scarabaeoidea. Adults are relatively large, ranging 20-70 mm (0.75-2.75 inches) long, roughly cylindrical, shiny black beetles with ridged elytra, strong jaws, and lamellate antennae. Larvae have the typical scarabaeiform larval shape and appearance, but do not exhibit the C-shaped curl of other Scarabaeoidea and possess a pair of highly reduced hind legs, such that the larva appears to have only four legs rather than six. Passalid beetles are found world-wide, mainly in the tropics. In the United States, the family is represented by just two species: the widespread Odontotaenius disjunctus and the similar, less well-known species O. floridanus, which is known to occur only in a small area in Florida and was described in 1994.
The ecology of the Passalidae, especially species in the Old World is not well known. While most feed and live in logs in advanced stages of decay, a few are associated with leaf litter collected by arboreal fern rhizomes or even found in the waste chambers of leaf cutter ants (Atta spp.), which are also rich in decaying plant matter.
Most of our knowledge on passalid life history is inferred from observation and experimentation on O. disjunctus. The Passalidae are unusual among the Coleoptera in that most species, if not all, are presocial, ie. they live in small family groups of multiple long-lived individuals that cooperate in rearing offspring. Both male and female adults excavate galleries in rotting logs, engage in defense against competitors, and feed the offspring pre-chewed wood, which the larvae cannot ingest otherwise. Larvae and recently metamorphosed adults also feed on adult fecal pellets, which aid digestion. Adults cooperate to assist larval offspring or siblings in the construction and repair of a pupal case out of wood and feces. Because the development time from egg to adult occurs in only a few months, while adult lifespan can reach years, a group of sexually mature adults in an established gallery can potentially rear multiple broods of offspring over their lifetime. Both larvae and adults of O. disjunctus can produce sound by stridulation, the former using the extremely reduced hind legs and the latter with their wings. While there is some evidence for the adaptive value of adult stridulation for deterring predation, showing aggression, and courtship, the function of stridulation in communication between adults and larvae within the colony is less clear.
Odontotaenius disjunctus is one of several recorded hosts for the tachnid fly Zelia vertebrata, whose larvae somehow penetrate the galleries to burrow into and feed within the bodies of the well-hidden beetle larvae. Other predators of the Passalidae probably include generalist vertebrates such as birds, rodents, or large mammals that find the beetles by searching for nests in logs and breaking open the galleries. The economic importance of bess beetles is minor and lies primarily in the service they provide as part of the community of organisms that decompose decaying plant matter and return carbon into the soil.
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