Family: Scarabaeidae Subfamily: Scarabaeinae Genus: Onthophagus Species: Onthophagus armatus Blanchard, 1853
Total body length 6.0–10.0 mm (0.24–0.39 in). Body shape oval; may be caked in dung. Color shiny black. Small or medium-sized Onthophagus, over 6 mm. Clypeal apex of major male bisinuate, not strongly reflexed; minor male and female with clypeal apex rounded to feebly sinuate. Head of major male with 2 sinuate, vertically oriented horns, that are connected by a broad ridge; female and minor male with feebly sinuate ridge near base. Ocular canthus not completely dividing eye. Pronotum of male with hump-like process; female lacking process. Pronotum with anterior angles rounded. Front tibia of male slightly slender and elongate, female tibia comparatively stout. Scutellum absent.
Undescribed. For Onthophagus spp. (Ritcher, 1966): Grub C-shaped, hump-backed, cylindrical, and cream-colored. Maxilla with galea and lacinia distinctly separate. Epipharynx with tormae united mesally, anterior phoba present. Antennae 4-segmented, distal segment much reduced. Legs 2-segmented. Prothoracic shield without anteriorly projecting processes. Third abdominal segment bearing a prominent conical, dorsal gibbosity covered with numerous short, stout setae.
Eastern Asia. This species is recorded from much of eastern Asia including India, Burma (Myanmar), Taiwan (Chandra and Gupta, 2013), Indochina, Malaysia, Indonesia, and the Philippines (Cartwright and Gordon, 1971).
None. This species feeds on dung as both an adult and larva. There are no records of this beetle feeding on live plant tissues.
Poorly known: Adults of this nocturnal species are dung tunnelers (Chandra and Gupta, 2013). Females locate dung and create a burrow near or under a dung source. The burrow is provisioned with dung that is formed into brood balls. An egg is deposited within each brood ball, and larval development occurs within.
None. This species recycles dung and is beneficial for ranching and farming in Hawaii. Primarily being a dung feeder, this species has never been recorded damaging crop or ornamental plants. Additionally, this species is not a threat to native dung beetles because none occur in Hawaii or Guam.
Not established or recorded. There are no records of this species from Hawaii.
Established. This species is established on Guam (Bourquin, 2002). Based on in-field observations and specimens examined at the University of Guam, this species is common on the island.
It is unclear when or how this scarab first arrived to Guam, although Cartwright and Gordon (1971) proposed that it may have arrived from the Philippines. It is possible that the species arrived on military ships or aircraft during or after World War 2, though this is speculation.
This scarab could be confused with similar small to medium-sized Onthophagus (more than 6 mm) including Onthophagus binodis, Onthophagus incensus, Onthophagus comperei, Onthophagus cuniculus and Onthophagus laminatus.
Major males of these species can readily be distinguished by examining the head armature (O. armatus with vertical, bisinuate horns versus O. cuniculus with paired tubercles, O. binodis lacking horns or tubercles, O. comperei with sinuate, vertically-oriented, weakly bifurcate horns, O. incensus and O. laminatus both with slightly curved, vertically-oriented, undivided horns).
Females are separated by examining the base of the head (O. armatus with feebly bisinuate ridge versus O. laminatus and O. incensus with a straight, transverse ridge, O. comperei with 2 tubercles, O. binodis and O. cuniculus with a slightly curved ridge), pronotum (O. armatus lacking process versus O. binodis with small lobe-like process, O. cuniculus with bi-lobed process) and anterior pronotal angle (O. armatus with anterior angle rounded versus O. cuniculus and O. laminatus with anterior angle curved outward).
Onthophagus luzonicus Lansberge
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