This family contains a single genus, Pleocoma, and includes 26 species that are distributed from southern Washington to northern Baja California, Mexico. As the generic name implies ("pleos" from the Greek, meaning full or abundant, and "kome," Greek for hair), adult rain beetles are densely pubescent ventrally, on the appendages, and at the margins of the elytra and thorax.
Length: males 16.5-29.0 mm; females 19.5-44.5 mm. Body form robust, broadly oval in outline, strongly convex dorsally, dorsum glossy, venter densely clothed with long, fine hairs. Color reddish-brown to piceous-black with setae golden, reddish, chocolate-brown, or black. Head not deflexed or retractile. Clypeal process deeply bifurcated, outer angles produced, and acute in males, broadly bilobed in females; vertex with a conical tubercle or erect horn medially. Antennae 11-segmented, scape subconical, antennomere 2 short and moniliform, antennomere 3 elongated and subcylindrical (sometimes angulated anteriorly), antennomeres 4-11 variable (moniliform, angulated anteriorly, or variously produced to form lamellae); antennal club of male elongate and comprised of 4 to 8 full lamellae; antennal club of female with 4-8 lamellae, lamellae short, stout in comparison with male. Eyes with exocone or duocone ommatidia, partially divided by canthus. Mouthparts partially fused, reduced; labrum connate with clypeus. Mandibles non-functional; esophageal opening closed by a membrane. Maxillary palpi 4-segmented, elongate; labial palpi shorter, 4-segmented. Pronotum broad, evenly convex or depressed anteromedially, widest at or behind middle, lacking tubercles, horns or sulci, but some species with low, transverse, median ridge. Mesothoracic spiracles with opening elliptical, with 2 intersegmentalia on each side. Scutellum exposed, subtriangular, narrowly to broadly rounded apically, lightly to densely pubescent. Elytra convex, with costal striae lacking, feebly indicated, coriaceous, and/or strongly indicated; lateral margins rounded to sutural angle, sutural margins contiguous to apices. Pygidium exposed in both sexes. Legs with procoxae large, conical and prominent, procoxal cavities open; mesocoxae contiguous, prominent; protibiae strongly toothed on outer margin; meso- and metatibiae strongly ridged externally; tarsi simple and subcylindrical, elongate, 5-5-5, tarsomeres 1-4 subequal in length, tarsomere 5 longer than preceding 2 together; unguitractor plate produced beyond apex of tarsomere 5, with 2-3 setae. Abdomen with 8 functional spiracles in pleural membrane. Male genitalia simple, trilobed, internal sac small, unarmed, setose. Female genitalia unsclerotized or with few small, separated sclerites; ovipositor with styli; ovariole numbers 14-25 per ovary. Wings with M-Cu loop, 2 apical detached veins. Karyotype 9+Xyp. References: Ritcher 1969a, Ritcher 1969b; Stemwedel 1973; Yadav et al. 1979; Ritcher and Baker 1974; Scholtz 1990; Browne and Scholtz 1995; Hovore 1977a, Hovore 1977b.
The genus Pleocoma has had a turbulent classification history. Previous authors placed Pleocoma in the subfamily Geotrupidae, the subfamily Melolonthinae, in its own subfamily (Pleocominae), or, in the current usage, its own family. The genera Acoma and Benedictia (currently in the subfamily Melolonthinae) were often treated in conjunction with Pleocoma (e.g., Cazier 1953). Browne and Scholtz (1995) consider the Pleocomidae to be a sister group to the Bolboceratinae (Geotrupidae) based primarily upon a series of minor structural synapomorphies. There are many basic structural dissimilarities between Pleocoma and the bolboceratines, however, and their respective larval characters, biologies, andgeographical distributions are completely discordant. This suggests that any such putative relationship must have had an ancient point of divergence. Clearly, Pleocoma is a monophyletic and taxonomically isolated genus, and the relationship of the Pleocomidae to other scarabaeoids remains to be fully resolved. The various forms of Pleocoma have been treated as species or subspecies with most taxa differing from one another by quantitative characters. Modern collecting methods have revealed considerable intraspecific variation in some taxa, resulting in synonymies and status changes. The only "revision" for the group was by Davis (1935). This work is out-dated and contains fewer than half of the present valid taxa. Linsley (1946) provided a provisional key to species. Currently there are 26 described species, 6 subspecies, and several additional new taxa awaiting description. Checklist: Smith 2001.
The genus Pleocoma is found from southern Washington through most of montane Oregon, southward through the Sierra Nevada and coast ranges of California, and into extreme northern Baja California Norte. The putative record of Pleocoma from Alaska is not considered valid. Regional work: Hatch 1971.
New World Genus
Pleocoma LeConte 1856
Pleocoma larvae feed externally upon roots and often are found deep within the soil beneath their host plants. Although the duration of the larval stage is not known for most species, some species have nine or more instars and require from 8-13 years to reach adulthood. Pupation occurs in late summer in a simple, elongate cell. After pupation, both sexes dig to the surface and emerge more or less synchronously. Some species emerge at the onset of fall or winter rains while others are active during mid-winter or early spring. Above-ground activity of adults closely corresponds to rainfall or snowmelt (depending upon the species), elevation, and specific weather conditions. Most species are active during or immediately following precipitation. Because of the precipitation-oriented timing of adult activity, the common name for all members of the genus Pleocoma is "rain beetles." Both sexes possess strongly toothed protibiae, and most species also have the clypeus and ocular canthi modified for digging through well-consolidated soils. Only male Pleocoma species are fully winged and capable of flight. Although the adults of most species have crepuscular flights, some fly in late morning and others fly in the night during rain. Males may be strongly attracted to light, particularly early in the flight season. Females generally are much larger than males, more heavy-bodied, and have the hind wings reduced to vestigial stubs. Females release pheromones that attract flying males (often in large numbers). Mating takes place either at the soil surface or within the female's larval burrow. Mated females return to the bottom of their burrow and wait for their eggs to mature (a process that may require several months) before depositing the eggs in a spiral pattern at the lower end of the burrow. Adult Pleocoma lack functional mouthparts or digestive tract, so the period of adult activity is relatively brief (dependent upon timing with conspecifics, temperature, and amount of precipitation during the emergence season). References: Hovore 1972, 1979; Fellin 1975, 1981.
Scarabaeiform (c-shaped, cylindrical). Color creamy white except at caudal end which may be darkened by accumulated feces. Cranium heavily sclerotized, glossy, yellowish or reddish-brown. Mandibles piceous. Antennae 3-segmented, terminal segment minute, penultimate segment with apical sensory area. Frontoclypeal suture distinct. Galea and lacinia distinctly separate. Epipharynx with plegmatia and with prominent chaetoparia and acanthoparia; haptomerum with longitudinal group of heli. Hypopharynx without oncyli. Maxillary palpus 4-segmented. Legs with apical claw bearing 2 basal setae; trochanters and femora of meso- and metathoracic legs with stridulatory organs. Spiracles cribriform, lacking closing apparatus, concavities of respiratory plates oriented ventrad. Terga of abdominal segments 3-7 each with 4 dorsal annulets. Anal opening V or Y-shaped, not surrounded by fleshy lobes. References: Ritcher 1947, 1966; Scholtz 1990.
BROWNE, D. J. and C. H. SCHOLTZ. 1995. Phylogeny of the families of Scarabaeoidea based on characters of the hindwing articulation, hindwing base and wing venation. Systematic Entomology 20: 145-173.
CAZIER, M. A. 1953. A review of the scarab genus Acoma (Coleoptera, Scarabaeidae). American Museum Novitates 1624: 1-13.
DAVIS, A. C. 1935. A revision of the genus Pleocoma. Bulletin of the Southern California Academy of Science 33: 123-130, 34:4-36.
FELLIN, D. G. 1975. Feeding habits of Pleocoma larvae in coniferous forests of western Oregon. Northwest Scientist 49: 71-86.
FELLIN, D. G. 1981. Pleocoma spp. in western Oregon coniferous forests: observations on adult flight habits and on egg and larval biology. Pan-Pacific Entomologist 57: 461-484.
HATCH, M. H. 1971. The beetles of the Pacific Northwest, part 5. University of Washington Publication in Biology, 16: 1-662.
HOVORE, F. T. 1972. Three new sympatric Pleocoma from the southern Sierra Nevada mountains of California. Bulletin of the Southern California Academy of Science 71: 69-80.
HOVORE, F. T. 1977a. New synonymy and status changes in the genus Pleocoma LeConte. Coleopterists Bulletin 31: 229-238.
HOVORE, F. T. 1977b. A review of the taxonomic and distributional relationships of Pleocoma hoppingi Fall and Pleocoma rubiginosa Hovore. Coleopterists Bulletin 31: 319-327.
HOVORE, F. T. 1979. Rain beetles: small things wet and wonderful. Terra Magazine 17: 10-14.
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.
LINSLEY, E. G. 1946. A preliminary key to the species of Pleocoma. Pan-Pacific Entomologist 22: 61-65.
O. 1947. Description of the larva of Pleocoma hirticollis vandykei
Linsley. Pan-Pacific Entomologist 23: 11-20.
RITCHER, P. O. 1966. White Grubs and their Allies. A study of North American Scarabaeioid Larvae. Oregon State University Press, Corvallis. 219 pp.
RITCHER, P. O. 1969a. Spiracles of adult Scarabaeoidea and their phylogenetic significance. I. The abdominal spiracles. Annals of the Entomological Society of America 62: 869-880.
RITCHER, P. O. 1969b. Spiracles of adult Scarabaeoidea and their significance. II. Thoracic spiracles and adjacent sclerites. Annals of the Entomological Society of America 62: 1388-1398.
RITCHER, P. O. and C. W. BAKER. 1974. Ovariole numbers in Scarabaeoidea. Proceedings of the Entomological Society of Washington 76: 480-494.
SCHOLTZ, C. H. 1990. Phylogenetic trends in the Scarabaeoidea. Journal of Natural History 24: 1027-1066.
SMITH, A. B. T. 2001. Checklist of the Scarabaeoidea of the Nearctic Realm (Includes Canada, the continental United States, and the following states of northern Mexico: Baja California, Baja California Sur, Chihuahua, Coahuila de Zaragoza, Durango, Nuevo Leon, Sinaloa, Sonora, Tamaulipas, and Zacatecas).
STEMWEDEL, T. A. 1973. The digestive, reproductive and nervous systems of Pleocoma linsleyi Hovore. Unpublished Thesis, California Polytechnic University, Pomona. 30 pp.
YADAV, J. S., R. K. PILLAI, and KARAMJEET. 1979. Chromosome numbers of Scarabaeidae (Polyphaga: Coleoptera). Coleopterists Bulletin 33: 309-318.