Description

The horned passalus beetle, Odontotaenius disjunctus, is a large, widespread, deadwood inhabiting (saproxylic) beetle that feeds on decaying wood. It is found throughout the eastern United States. Adults are 3 cm to 4 cm in body length, parallel sided, with grooved wing covers (modified forewings called elytra). The surface of the body is smooth and shining, giving rise to one of several common names, patent leather beetle. Other colloquial names include Betsy bug and bess beetle. The thorax is rounded on the sides, and the head is broad, bearing a stout, forward projecting horn. The antennae are elbowed and the last three large asymmetrical segments form a loose club. The edges of the undersurface of the body are densely covered with patches of yellowish brown hairs (pubescence). The legs are thick, shiny black and armed with short spines and spurs.

Larvae are large, fleshy grubs, up to 6 cm long when fully developed. The head capsule is large, rounded and yellowish brown. Larvae possess two pairs of walking legs, with the third pair of legs reduced and modified into noise and vibration producing “stridulatory organs.” Pupae are light yellow or cream colored and resemble pale, contracted versions of the adults. Eggs are oval, white, and 3 mm to 4 mm in length.

Two species of the genus Odontotaenius occur in the U.S., but only the horned passalus is widespread. The other species, Odontotaenius floridanus, occurs in Florida. It is similar in appearance but differs in possessing a much smaller head horn. About 500 additional species occur in other parts of the world, with more diversity in tropical regions.

Life Cycle

The life cycle of the horned passalus beetle, as with all beetles, involves four stages of development, egg, larva, pupa and adult. It begins with the deposition of eggs within galleries inside fallen limbs of deciduous trees. Eggs are manipulated by adults using their mandibles. This involves careful placement in a prepared nest that optimizes survival. Females also lay nonviable eggs that are used as food for the growing larvae. Larvae hatch after two weeks during warmer weather or up to six weeks at lower temperatures. The larvae feed on decaying wood as well as food prepared by the adults. They require a dose of specialized gut symbionts. Larvae obtain this initial culture from the adults, either by mouth or from feces (proctodeal feeding). This helps the larvae digest cellulose and convert it to nutrients. The larvae undergo three growth stages (instars) before transforming into pupae. Pupation times vary depending on environmental factors. Both larvae and adults occupy the same complex of galleries within large rotting logs and communicate using stridulatory organs, possibly through vibrational cues. The exact nature of the communication is unclear. Horned passalus beetles are quasi-social, in addition to exhibiting parental care. Their colonies comprise eggs, larvae, pupae and adults. This suite of behaviors is highly unusual for beetles and insects in general. Adults primarily rely on walking for dispersal, but possess functional wings and will occasionally fly, especially during mating season.

Ecological Significance

Horned passalus beetles contribute to forest ecosystem health by breaking down deadwood through direct consumption, tunneling and feces (frass) production. This accelerates further decomposition by fungi and bacteria and provides living spaces for other wood inhabiting organisms. The symbiotic relationships between the beetles and their gut symbionts, particularly yeasts, have been extensively studied by scientists at LSU and colleagues elsewhere. These studies serve as research models for future work of other wood feeding insects, most or all of which rely on gut microbes to convert cellulose to useable nutrients.

References

Bar‐Shmuel, N., A. Behar, and M. Segoli. 2020. What do we know about biological nitrogen fixation in insects? Evidence and implications for the insect and the ecosystem. Insect Science 27 392–403.

Jackson, H.B., K.A. Baum, T. Robert, and J.T. Cronin. 2009. Habitat-specific movement and edge-mediated behavior of the saproxylic insect Odontotaenius disjunctus (Coleoptera: Passalidae). Environmental Entomology 38: 1411–1422.

Nardi, J.B., C.M. Bee, L.A. Miller, N.H. Nguyen, S.O. Suh, and M. Blackwell. 2006. Communities of microbes that inhabit the changing hindgut landscape of a subsocial beetle. Arthropod Structure and Development 35: 57-68.

Schwarz, M., G. Tokuda, H. Osaki, and A. Mikaelyan. 2023. Reevaluating symbiotic digestion in cockroaches: unveiling the hindgut’s contribution to digestion in wood-feeding Panesthiinae (Blaberidae). Insects 14: 768. (accessed 30 January 2026).

Shattuck, A., C. Martnez-Mejia, F. Smith, and A. K. Davis. 2022. Observations and photographs of eggs of the horned passalus beetle, Odontotaenius disjunctus (Illiger) (Coleoptera: Passalidae). The Coleopterists Bulletin 76: 350-353.

Whitaker, M., T. Procter, and F. M. Fontanella. 2021. Phylogeography and demographic expansion in the widely distributed horned passalus beetle, Odontotaenius disjunctus (Coleoptera: Passalidae). Mitochondrial DNA Part A 32: 85–97.