The two most widespread members, Scarites subterraneus and S. vicinus, are shiny black ground beetles with large mandibles, head with two large depressions, elytra grooved and separated from a smooth pronotum by a narrow "waist" (peduncle). Besides differences in antennomere shape as noted in the taxonomic key below, these two species can usually be distinguished by size: most subterraneus body length < 20 mm and elytron < 10 mm compared to most vicinus body length > 20 mm and elytron > 10 mm. Elytron length is the more reliable separator. Earlier reported distinctions based on various external morphologies (protibial denticles, mandibles, body proportions) are unstable and therefore unreliable. Assessment of antennomere shape is unfortunately not reliable in most BugGuide photographs which usually do not capture the required perpendicular viewing angle of the broad faces of the antennomeres. An oblique angle will make the antennomeres appear artificially more elongated.
Distinguishing sex externally in Scarites
is not obvious primarily because the protarsi do not exhibit the sexual dimorphisms typical of other carabid genera. Therefore, other external morphologic clues are needed. Lindroth (1961) observed that in S. subterraneus
the females show four setae-bearing punctures (“anal setae”) equally spaced along posterior border of last sternite while in males the two median punctures are more widely spaced. Summarized below are unpublished observations by Peter W. Messer [PWM version 18.07.31]
regarding the external sexing of Scarites vicinus
. The first six observations are subjective and each will fail in some cases. The last one is objective with precise measurements done best in the lab rather than in the field. It is anticipated that these observations also apply to closely related S. subterraneus
and S. quadriceps
1) The spacing of anal setae noted above does not reliably distinguish the sexes in all cases.
2) In males the forebody, especially head, is proportionately larger compared to the elytra.
3) Mandibles tend to be slightly longer and more toothy in most males.
4) Area immediately behind eye tends to be more bulging in most males.
5) Area immediately before eye tends to be more bowed outwardly in most males.
6) Viewed ventrally, the abdominal apical border tends to be more evenly rounded (almost semicircular) in most males as compared to a slightly more tapered abdomen in females.
7) The most reliable external separator discovered so far is a comparison of the maximum distance across head that includes the eyes (width H) with the distance across the shoulder at the level of the prominent humeral teeth (width S). Careful measurements reveal the following ratio H/S for separating the sexes:
♀ ≤ 0.96 ≤ ♂.
In either sex of Scarites it is rare to observe in dry mounted specimens a naturally exposed apical portion of the genitalia protruding from the abdominal posterior end. However, for many other carabid groups the genitalia are frequently exposed. In carabid females the genitalia appear as a symmetrical pair of small dark styli while in males a single penile structure is pointed to the anatomic left.
When genitalia remain hidden, the sex can be absolutely determined by gently probing for either the actual genitalia or the sexually dimorphic internal plates that surround the genitalia. The entire specimen needs to be relaxed in at least 10 minutes of hot water before probing can begin. In either sex these surrounding plates occur just inside the apical opening of relaxed abdomens. The ventral-most plates are a pair of opposing "genital sternites" and a dorsally situated single plate is the "genital tergite". In females the pair of identical genital sternites are symmetrically split down the middle with their adjacent apices right angled. The corresponding male genital sternite pair is asymmetrically split in which the apices are well-rounded. The left plate is most rounded and it ventrally overlaps the right plate.
The genitalia and sexually dimorphic genital sternites described above apply to other carabid genera.
KEY TO NORTH AMERICAN (NORTH OF MEXICO) SPECIES OF SCARITES - version 2014.XII.11b.
The following is a key after Bousquet & Skelley (2010) and Nichols (1986) with consolidations/modifications/expansions by Peter W. Messer. This provisonal taxonomy of North American Scarites awaits refinement from future morphological scrutiny and DNA sequence analysis. Users of this key will find that it supersedes outdated popular keys by Downie & Arnett (1996) and Ciegler (2000).
The most challenging species separation is between S. quadriceps and recently resurrected S. vicinus in the quadriceps group. Body size and relative lengths of metasterna as specified in the key are currently the only reliable ways to externally distinguish the latter two species. Nothing has been published on genitalia comparison. Easy and reliable methods to determine sex externally (as started above) will hopefully encourage investigators to begin serious comparative work on the genitalia in this group.
Because antennomeres are not cylindrical, couplet #1 below requires that the examiner turn each antennomere until its maximal width comes into view. Routine photographs of entire beetles usually do not capture the viewing angle desired for assessing antennomere shape.
Abbreviations: ABL = apparent body length (mandible tip to elytron apex); LE = elytron length; LMs/LMc = ratio of metasternum length to metacoxa length through same line as defined by Bousquet.
1. Antennomeres 8–10 longer than wide (length/width of 10th antennomere = 1.1–1.3) with segments 5–7 usually more elongated. ABL at least 20 mm (LE at least 10 mm) but with a small variant in regions west of Mississippi River and only rarely an individual with LE < 10 mm east of Mississippi River. ==> 2 [quadriceps group]
– Antennomeres 8–10 wider than long or subquadrate (length/width of 10th antennomere = 0.8–1.0), without obvious further elongation of segments 5–7 (moniliform). ABL ≤ 20.5 mm (LE ≤ 10 mm). ==> 4 [subterraneus group]
–– QUADRICEPS GROUP ––
2. Metasternum visibly same length compared to that of metacoxa (LMs/LMc = 0.98–1.02) and nearly same length compared to that of mesocoxa. ABL 20.5 mm - 26.0 mm. LE 10 - 13.0 mm. Common and widespread throughout North America in which many northern specimens are mislabeled "S. quadriceps". Bousquet (2012): AL, AR, IA, IL, IN, KS, KY, LA, MN, MO, MS, ND, NE, OH, OK, ON, TN, TX, WI. ==> S. vicinus Chaudoir
– Metasternum proportionally longer compared to metacoxa (LMs / LMc = 1.10–1.30) and clearly much longer than mesocoxa as seen in this example
. ABL > 27.0 mm OR ABL ≤ 20.5 mm. ==> 3
3. ABL > 27.0 mm. LE at least 14.0 mm. Uncommon in north-central USA with somewhat increasing occurrences southward and eastward from Kansas to the Atlantic and Gulf Coast states. Bousquet (2012): AL, FL, GA, LA, MD, MS, NC, NJ, SC, TX. ==> S. quadriceps Chaudoir
– ABL ≤ 20.5 mm. LE < 11.0 mm. West of the Mississippi River (KS, LA, NM, OK, TX). Elytra with intervals flat and striae often less impressed laterally. ==> S. lissopterus Chaudoir sensu Nichols (1986).
–– SUBTERRANEUS GROUP ––
4. Setigerous puncture present on proximal elytral interval #3. Elytral striae impunctate. Metasternum proportionally more elongate (LMs / LMc = 1.10–1.40). Membranous wings fully developed. Widely distributed in the United States and also in southern Ontario. ==> S. subterraneus Fabricius [synonyms include S. texanus (small Texan variant with weakly impressed striae), S. californicus (CA), and S. patruelis (FL, GA)]
– Setigerous puncture on proximal elytral interval #3 absent (or present in some S. ocalensis). Metasternum proportionally shorter (LMs/LMc = 0.75–1.03). Membranous wings reduced. Range only LA to FL in North America. ==> 5
5. Eyes flat, head width at level of eyes smaller than at level of temples. Pustules at base of elytra more expanded, reaching level of humeral carina apex between intervals 2–4. Elytral interval 7 markedly convex, more or less cariniform, in basal third. Elytral striae impunctate. Known only from sandy soil in Levy Co, FL. ==> S. stenops Bousquet & Skelley
– Eyes slightly to moderately convex, head width at level of eyes subequal or greater than width at level of temples. Pustules at base of elytra less expanded, distinctly not reaching level of humeral carina apex between intervals 2–4. Elytral interval #7 slightly convex or flat in basal third. ==> 6
6. Elytral striae very finely impressed, vanishing at apex, striae with punctulae. Elytra very shiny (disc without microsculpture). Metasternum proportionally more elongate (LMs / LMc = 0.98–1.03). Eyes more convex, head width at level of eyes greater than width at level of temples. Larger body size (usually LE > 7.5 mm). Occurs on sea beaches of FL and LA Gulf Coast. ==> S. marinus Nichols
– Elytra with striae well impressed without punctulae, with distinct microsculpture. Metasternum proportionally shorter (LMs/LMc = 0.76–0.84). Eyes less convex, head width at level of eyes subequal to width at level of temples. Small body size (elytron < 7.5 mm). Known only from peninsular FL (inland and coastal). ==> S. ocalensis Nichols
The seven valid North American species of Scarites including some synonyms according to the catalogue by Bousquet (2012)(2)
S. lissopterus Chaudoir, 1880: p93 - considered a synonym of S. quadriceps earlier by Bousquet & Skelly (2010)
S. marinus Nichols, 1986
S. ocalensis Nichols, 1986
S. quadriceps Chaudoir, 1843: p729 [syn. S. substriatus Haldeman]
S. stenops Bousquet & Skelley, 2010
S. subterraneus Fabricius, 1775 [syn. S. californicus LeConte, S. patruelis LeConte, S. texanus Chaudoir, 1880: p94]
S. vicinus Chaudoir, 1843
Chaudoir, M. de. 1843. Bulletin de la Société impériale des naturalistes de Moscou 23.
Chaudoir, M. de. 1880. Annales de la Societe Entomologique de Belgique 23: 5-130.
Lindroth, C.H. 1961. Opuscula Entomologica, Supplementum 20.
Nichols, S.W. 1986. Proc. Entomol. Soc. Wash 88(2): 257-264.
Bousquet, Y., and P.E. Skelley. 2010. The Coleopterists Bulletin 64(1): 45-49.