The family contains more than 220 described species placed in 35 genera, including those described in the following chapters, but this figure is likely to increase as indicated by the large number of undescribed taxa that have been detected in recent years. The Hybosoridae is a heterogeneous group of relatively small scarabs, with adults usually ranging in size from 3-7 mm. The majority of the species are brown or black. The antennae are 9-10 segmented, with a 3-segmented club. Little is known about the biology of hybosorid species. While these are useful to diagnose most species of the Hybosoridae, many exceptions are found.
Length 5.0-7.0 mm. Shape oval, dorsal surface convex. Color light brown to black, shining. Head not deflexed. Antennae 10-segmented with 3-segmented, opposable club (last 2 segments tomentose), first segment of club hollowed to receive segments 2 and 3, basal segment of antenna expanded apically. Eyes with eucone ommatidia, divided by reduced canthus. Clypeus lacking tubercle or horn. Labrum truncate, produced beyond apex of clypeus, prominent. Mandibles produced beyond apex of labrum, prominent, external edge rounded, apex pointed. Maxillae with 4-segmented palpi; labium with 4-segmented palpi. Pronotum convex, base wider than elytral base. Elytra convex, surface polished, glabrous. Pygidium concealed by elytra. Scutellum exposed, triangular. Legs with anterior coxae conical, contiguous; mesocoxae contiguous; protibiae tridentate on outer margin, apex with one spur; meso- and metatibia with prominent, oblique ridge, apex with 2 spurs; spurs mesad, adjacent (not separated by basal metatarsal segment); tarsi 5-5-5; claws equal in size, simple; empodium short, not extending beyond fifth tarsal segment, with 2 setae. Abdomen with 6 free sternites (first sternite obscured by hind coxae except at lateral edges); 8 functional abdominal spiracles, situated in pleural membrane (spiracles 1-7) and in tergite (spiracle 8). Wings well developed, with 2 apical detached veins. Male genitalia variable. References: Cooper 1983, Scholtz 1990.
The Hybosoridae is considered as a family within the Scarabaeoidea or as a subfamily of the family Scarabaeidae. In this volume, we follow Gardner (1935), Paulian (1939), and Lawrence and Newton (1995) and consider the group a family. Hybosorid larvae are distinct and are the primary reason that the group is treated as a family. Larvae possess prothoracic and mesothoracic stridulatory structures and three truncate lobes at the apex of the labrum, both of which are unique to this group. In adults, the form of the antenna (10-segmented with a 3-segmented club, the first segment hollowed to receive segments two and three) is unique. The Hybosoridae is hypothesized to be intermediate between the Ochodaeidae and Ceratocanthidae (Scholtz et al. 1988) or intermediate between the Trogidae and Ceratocanthidae (Howden and Gill 1988). Aside from revisions of some genera and catalogs, little systematics work has been conducted on this group.
The Hybosoridae contains 29 genera world-wide and about 200 species (Allsopp 1984; Martinez 1994). Species are widely distributed in the tropics. Fifteen genera and about 65 species occur in the New World. Phaeochrous emarginatus Laporte, a species that occurs in Africa, Australia, and India, was once reported from California, but the species is not established in California and the record was probably in error (Evans, pers. comm. 1998).
Subfamilies of Hybosoridae
Little is known about the biology of hybosorid species. Adults feed on dung, carrion, fungi, or rotting wood, and some are found in ant and termite nests. Some species, particularly in the Australian genus Liparochrous Erichson and those of the subfamily Ceratocanthidae, are capable of rolling up their body to form a compact ball. Larvae are known for only six species in five genera, and they were collected in decomposed plant material or under bark. Larvae and adults of some species are known to stridulate.
Form scarabaeoiform (C-shaped, cylindrical). Color creamy-white or yellow (except at caudal end which may be darkened by accumulated feces). Cranium heavily sclerotized, brown to dark brown. Antennae 3 or 4-segmented (if 3-segmented, then third and fourth segments fused). Frontoclypeal suture distinct. Labrum at apex with 3 truncate lobes. Epipharynx with row of setae on each chaetoparia, a blunt tooth in the haptomeral region, and united tormae. Maxilla with galea and lacinia distinctly separate; maxillary stridulatory area consisting of a row of conical teeth; maxillary palp 3- or 4-segmented. Abdominal segments 1-6 with 3 annuli, each with 1 or more transverse rows of short setae. Spiracles cribriform, with closing apparatus. Venter of last abdominal segment with raster consisting of 2 curved rows of many short setae that converge caudally. Legs 4-segmented, well developed, with stridulatory apparatus on pro- and mesothoracic legs, each with a well-developed claw. References: Ritcher 1966; Scholtz 1990.
ALLSOPP, P. G. 1984. Checklist of the Hybosorinae (Coleoptera: Scarabaeidae). Coleopterists Bulletin 38: 105-117.
COOPER, J. B. 1983. A review of the Nearctic genera of the family Scarabaeidae (exclusive of the subfamilies Scarabaeinae and Geotrupinae) (Coleoptera), with an evaluation of computer generated keys. Doctoral Thesis, Department of Biology, Carleton University, Ottawa, Ontario, Canada. 1,121 pp.
GARDNER, J. C. M. 1935. Immature stages of Indian Coleoptera (16) (Scarabaeidea). Indian Forest Rec. (New Series) Ent. 1:1-33.
HOWDEN, H. F. and B. D. GILL. 1988. A fourth genus of unusually modified Ceratocanthinae (Coleoptera: Scarabaeidae) from South Africa. Canadian Journal of Zoology 66: 2077-2079.
LAWRENCE, J. F. and A. F. NEWTON, JR. 1995. Families and subfamilies of Coleoptera (with selected genera, notes, and references and data on family-group names), pp. 779-1006. In J. Pakaluk and S. A. Slipinski (eds.), Biology, Phylogeny, and Classification of Coleoptera. Papers Celebrating the 80th Birthday of Roy A. Crowson. Muzeum i Instytut Zoologii PAN, Warszawa, Poland.
MARTINEZ, A. 1994. Notas sobre Hybosorinae (Coleoptera: Scarabaeidae), II. Elytron 8: 223-239.
PAULIAN, R. 1939. Les caractères larvaires des Geotrupidae (Col.) et leur importance pour la position systématique du groupe. Bulletin de la Société Zoologique de France 64: 351-360.
RITCHER, P. O. 1966. White Grubs and Their Allies: A Study of North American Scarabaeioid Larvae. Oregon State University Press, Corvallis. 219 pp.
SCHOLTZ, C. H. 1990. Phylogenetic trends in the Scarabaeoidea (Coleoptera). Journal of Natural History 24: 1027-1066.
SCHOLTZ, C. H., D. D'HOTMAN, A. V. EVANS, and A. NEL. 1988. Phylogeny and systematics of the Ochodaeidae (Insecta: Coleoptera: Scarabaeidae). Journal of the Entomological Society of South Africa 51: 207-240.