Cybister explanatus LeConte, 1852

Miller, Kelly B., 2013, Review of the Genus Cybister Curtis, 1827 (Coleoptera: Dytiscidae: Dytiscinae: Cybistrini) in North America, The Coleopterists Bulletin 67 (4), pp. 401-410 : 407-408

Cybister explanatus LeConte, 1852 Figs. 5, 7 , 16–19

Cybister explanatus LeConte, 1852: 202 . Crotch 1873: 399; Sharp 1882a: 734, 1887: 759; Zimmermann 1920: 259; Blackwelder 1944: 80; Leech 1948: 413; Hatch 1953: 239; Leech and Chandler 1956: 323.

Cybister explanatus laevicollis Zimmermann, 1919: 243 . Zimmermann 1920: 259 (as a variety of C. explanatus ); Blackwelder 1944: 80 (as variety of C. explanatus ).

Cybister explanatus fusculus Zimmermann, 1919: 242 . Zimmermann 1920: 259 (as a variety of C. explanatus ); Blackwelder 1944: 80 (as a variety of C. explanatus );

Cybister (Nealocomerus) explanatus . Brinck 1945: 13.

Cybister (Nealocomerus) fusculus . Brinck 1945: 13.

Diagnosis. Cybister explanatus is easily distinguished externally from all other New World

Cybister by the acuminate and sharply pointed anteroventral angle of the metafemur in both sexes ( Fig. 7 ). Males have the median lobe (Figs. 16, 17) more slender, apically broadly rounded in ventral aspect, and lacking the large marginal lobes that enclose the ventral sclerite in C. fimbriolatus and C. occidentalis (Figs. 8–10, 12–14). The male lateral lobe is more slender and straplike apically (Fig. 18) compared with the other two species (Figs. 11, 15). Many specimens have some yellow coloration along the anterior and posterior margins of the pronotum. Females usually have sexual sculpturing (fields of fine scratches) on the pronotum, bases of the elytra, and, often, on the base of the head. This sculpturing is variable in the species, with many specimens having no sculpturing on the elytra.

Measurements. TL = 24.5–29.0 mm, GW = 13.0–15.2 mm, TL/GW = 1.8–1.9.

Variation. This species tends to be less variable than others throughout its range, with most variation in size and coloration. The lateral stripe of yellow on the pronotum and elytron is somewhat variable in width and length. The dorsal coloration varies from green to brown-black. The ventral coloration varies from brown to black, some of which appears to be due to how teneral is the specimen.

Taxonomy. LeConte (1851–1852) seemingly had multiple specimens of C. explanatus since he indicated multiple collecting localities. There are three specimens in the LeConte Collection (MCZC) attributed to C. explanatus . One is a male of C. fimbriolatus labeled with “Cala” (determined as C. ellipticus by J. Zimmerman). Another is a male C. explanatus labeled “Cal” and the last is a female C. explanatus labeled “[gold disc]/ T. explanatus S.D. & Sac. Lec. [handwritten]/ Type 6088 [red label].” The red type label on the female was added later, and does not constitute a lectotype designation, and no lectotype has yet been selected. Of the three specimens, the C. fimbriolatus specimen is not a suitable lectotype since it would upset stability of the name. Also, LeConte described C. ellipticus (= C. fimbriolatus , see above) in the same paper as C. explanatus , and this specimen is more likely to be part of the syntype series for that species (he would have been unlikely to misidentify it). Of the two other specimens, the female has the gold disc characteristic of specimens collected in California by LeConte and included in the same paper (LeConte 1851–1852). Although LeConte did describe species labeled “Cal,” these are generally in later papers. Although a male lectotype might be preferable given that most diving beetle classification is based on males, the female specimen is unambiguously C. explanatus (it has acuminate posterolateral angles of the metafemur), and the male specimen, though also clearly C. explanatus , may not be part of the syntype series. Fort these reasons, the female specimen is selected as the lectotype to promote stability of the concept of this species. The type locality for the species was listed as San Diego and the Colorado and Sacramento Rivers (LeConte 1851–1852). The lectotype does not help clarify this since it is just labeled as both “S.D. & Sac.”

The types for C. explanatus laevicollis Zimmermann nor C. explanatus fusculus Zimmermann (presumably in the Zoologische Staatssammlung, Munich) were not examined. Their type localities are each “ Mexico.” Each of these were described as “varieties” or subspecies of C. explanatus based on variation in color and microsculpture ( Zimmermann 1919), though Brinck (1945) recognized C. fusculus at the species rank. However, the variation of each of these fits well within the range of variation of C. explanatus throughout its distribution, and their status as species or subspecies is abandoned here.

Distribution. Cybister explanatus occurs from extreme southern Oregon through California and western Nevada and through western Mexico to Chiapas ( Fig. 19 ).

Natural history. Cybister explanatus specimens are found in a variety of habitats, but mainly larger ponds and lakes. They often come to lights. Specimens likely attributable to this species (curiously, without the heads) were found in prehistoric human coprolites in a Nevada cave ( Roust 1967), indicating their historical use as human food, this seemingly despite the prothoracic chemical defensive compounds produced by these dytiscids. Many other components (e.g., fish, mussels) of the coprolites were found to be characteristic of the Humboldt Sink ( Roust 1967), suggesting the aquatic beetles probably also came from that site.

Figures

Figs. 1–7. Cybister species. 1) C. fimbriolatus, male, base of left metacoxa, metatrochanter, and metafemur showing position of stridulatory ridges on the metacoxa; 2) C. fimbriolatus, California; 3) C. fimbriolatus, Florida; 4) C. occidentalis, Bahamas; 5) C. explanatus, California; 6) C. fimbriolatus, right metatrochanter and metacoxa; 7) C. explanatus, right metatrochanter and metacoxa.

Fig. 19. Distribution of Cybister species in North America and Mexico.