Recognition: The very convex and compact shape, deflexed head, transverse front coxae, retractile appendages, and moss associated habit are distinctive of this family.
Description: Body form oval to elongate oval, strongly convex, length approximately 1-10 mm. Integument glabrous or covered with clavate bristles, erect setae, or with a dense coat of decumbent setae, giving the integument a velvety appearance. Color variable, usually of gray, brown or black, or integument iridescent green and copper, or castaneous with a pronounced luster. Head hypognathous, frons convex, narrower than prothorax. Eyes oval or slightly emarginate, situated on sides; partly or completely hidden when head retracted into prothorax. Clypeus reduced to obsolescent. Labrum freely articulating, usually emarginate. Antenna 11-segmented, clavate, capitate, or subfiliform. Mandibles with a variable number of apical teeth, with a deep notch at middle, a blade-like molar area, and a basal brush of setae. Maxillary palpus 3-segmented, labial palpus 2-segmented, terminal segment pear-shaped or securiform. Thorax compact, prothorax tightly fitting agains mesothorax. Pronotum convex, lateral margin finely to strongly carinate, posterior margin smooth or with peg-like crenulae. Hypomeron large, subtriangular, rarely with cural depressions. Prosternum V- or T-shaped on disc, relatively broad between coxae, intercoxal process received into a deep cavity on mesosternum; anterior coxal cavities broadly open behind. Scutellum small to minute, ovoid to subtriangular. Elytra entire, strongly convex, surface finely to coarsely punctate, striate to smooth, epipleural fold variously formed, extending to end of elytron, or usually shorter; venter near apex with broad laminar flange. Mesosternum short, broad in front, narrow behind. Metathorax alate, brachypterous, or apterous. Wing often nearly veinless, apical field long, often subequal in length to veined basal portion; radial cell short, oblique; medial area veins reduced, wedge cell absent. Metasternum much broader and longer than pro- and mesosternum, usually with a median longitudinal suture, transverse suture lacking. Legs with anterior coxae transverse, separate; middle coxae globular to flat, slightly transverse, separate; hind coxae usually transverse, approximate medially, nearly attaining the elytral epipleura laterally; trochanters triangular, large; femora usually somewhat flattened; tibiae slender or stout, often flattened and expanded apically, densely covered with setae or spines; tarsal formula 4-4-4, or 5-5-5, tarsomeres usually increasingly larger from first to 3rd, 4th small, 5th long, 3rd sometimes lobate, remaining tarsomeres simple or with pubescent pads beneath; claws simple. Abdomen with 5 ventrites; ventrites 1-2 connate. Male genitalia of the trilobed type; median lobe with apex flattened, pointed or hook-shaped, and short basal struts, parameres well developed, fused basally, asymmetrical, or lacking; pars basalis usually longer than wide, rectangular, trapezoidal, or subcylindrical. Female genitalia with gonocoxites large; styli apical to subapical, short; baculi absent.
Larva: C-shaped, crescent-shaped, to elateriform; mature length 2-15 mm; terga with rows of setae. Head exserted, hypognathous, with lyriform or V-shaped epicranial suture surrounding frons; stemmata 6 each side; antennae short, three-segmented; clypeus separate from frons, transverse; labrum freely articulating; mandibles symmetrical, single to multidentate, robust, with a large brush of setae at the base of the cutting edge, mola absent; maxilla with palpiform galea, 1-2 segmented; lacinia falciform or truncate, with spiniform mesal margin, palpus 4-segmented; cardo bipartite, fused stipes, setiferous. Labium with ligula, palpus 2-segmented. Thorax with the pronotum as long as the meso- and metathorax combined, legs short, 5-segmented, with bisetose tarsungulus. Abdomen 10-segmented; 8th segment often transversely carinate, or operculate; 10th segment short, rarely with strong ventral hooks; urogomphi absent. Spiracles biforous, present on mesothorax and abdominal segments 1-8, closing apparatus absent.
Ecology: Most of the species are obligate moss feeders as both adults and larvae, though feeding on liverworts, lichens, and grass roots has been recorded. Adults of all bryophagous taxa are surface grazers on host mosses. Some Cytilus and Byrrhus species were reported as incidentally feeding on conifer seedlings, grasses, and clover grown in moist areas with abundant mosses. Larvae of most bryophagous species burrow through gametophyte layers, and underlying friable substrates, and feed at tunnel entrances by partially extending the body from the tunnel and graze on leaves and shoots of mosses. Larvae of Cytilus and Arctobyrrhus do not burrow but are entirely surface active. Amphicyrta species, adults and larvae, feed on succulent leaves and stems of forest and meadow herbs, and deciduous shrubs. They occasionally damage various commercial and home garden vegetables and flowers, and are sometimes abundant in weedy lawns.
Adults of most species and can be found active on their hosts under dim light or at night, or under cover of wood fragments, leaves, edges of stones, etc., during bright daytime hours. Species of Morychus and Cytilus are active in open areas on sunny days. Alate species fly during spring and early summer and are often found on beaches as washup or windblown drift. Most species retract their legs and antennae into ventral recesses and become motionless when disturbed, leading to the common name of pill beetle.
Status of the Classification: The family Byrrhidae is fairly well studied in holarctic regions and a stable taxonomy is available. Taxa in southern Asia and throughout the southern hemisphere remain in need of systematic revision. The North American taxa are relatively well studied and a key to the species was provided by Johnson (1991a) and is available here. El-Moursey (1961) suggested that Syncalyptinae should be regarded as a separate family, Syncalyptidae. However, all subsequent authors have retained the subfamily status of this group.
Diversity and Distribution: There are about 290 species known from all areas. Of these, 35 species are known from nearctic North America. Byrrhids are most common and diverse in the western mountains, the northeastern mountains, and the Great Lakes region. Regional endemism is expressed in the far western regions with Sierraclava restricted to the Californian region; Amphicyrta restricted to California and Oregon; and Lioligus, Lioon, Exomella, Listemus, and Eusomalia restricted to mesic coniferous forests in the Pacific Northwest. A number of undescribed species are known from tropical (Chaetophora) and austral regions (numerous genera). Fossil taxa are known from Oligocene strata.