Mesomerodon Ohaus, 1905

Seidel, Matthias, Jameson, Mary L. & Stone, Rachel L., 2017, A new cryptic species and review of the east-Andean leaf chafer genus Mesomerodon Ohaus, 1905 (Coleoptera, Scarabaeidae, Rutelinae), ZooKeys 671, pp. 61-85 : 63-65

Genus Mesomerodon Ohaus, 1905 Figs 1-6, 7-8, 9-17, 18, 19-21, 22-24, 25-27

Mesomerodon Ohaus, 1905: 319. Type species M. spinipenne Ohaus, 1905: 320-321 (by monotypy).

Description.

Length from apex of clypeus to apex of pygidium 17.0-20.0 mm (♂) and 19.0-24.0 mm (♀); width at mid-elytra 10.0-12.0 mm (♂) and 11.0-14.0 mm (♀). Color: Dorsal surface tan to ochre (cream or whitish when alive) with or without weak green reflections, ventral surface castaneous with weak metallic green or red reflections; specimens tend to darken with age. Form (Figs 1-8): Elongate oval, widest at mid-elytra, pygidium exposed beyond apices of elytra; apices rounded with one short spine or tubercle near apex (males) or swelling (females). Head: Disc of frons and clypeus in lateral view flat, clypeus with margins and apex weakly reflexed; length of clypeus to frons (ratio) 0.5-0.6: 1.0. Frons and clypeus moderately densely punctate, punctures small to moderate in size. Frontoclypeal suture weakly impressed, incomplete at middle. Eye canthus flattened, not weakly cariniform. Interocular width 2.5-3.8 transverse eye diameters. Clypeus rounded, with apex and lateral margin weakly reflexed, lacking bead; frontal view flat with short tawny setae, length (at middle) about 1/10 length of frons, disc moderately densely punctate, lacking setae. Mandible (Fig. 9) broadly rounded externally with 2 interior, acute teeth; molar region broad; mandibular apex always exposed. Labrum (Fig. 11) with apex emarginate medially, surface moderately densely punctate, punctures moderate in size, setose (setae moderately long and thick, tawny). Maxilla (Fig. 10) with 6 teeth; galea not fused, with moderately long setae. Mentum (Fig. 12) subrectangular in shape, broadest at middle, apex emarginated. Antenna with 10 antennomeres and 3-segmented club; club subequal in length to antennomeres 2-7 combined. Pronotum: Widest at base, apical angles acute, basal angles obtuse. Dorsal surface moderately densely punctate; punctures small and moderate in size. Bead complete anteriorly, laterally, and basally; setose basolaterally (setae moderately long, tawny or white). Scutellum: Parabolic, wider than long; base declivous at elytral base; dorsal surface as in pronotum. Wing (Fig. 14): Dense, thick setae present anterior to RA3+4 to apex; ScA with dense, thick setae near fold and with weak precostal pegs from near fold to base; AA1+2 shorter than AA3+4. Elytra: Surface punctate with weakly impressed striae; finely, densely rugose at apex. Punctures sparse to moderately dense, small to moderate in size, lacking setae. Sutural stria indicated by a row of punctures from base of scutellum to apex. Epipleuron from base to metacoxa with shelf and associated setae; beaded. Apex of elytra (Fig. 17) weakly rounded; elytral callus with well-defined tubercle or spine; sutural apex spiniform. Elytral sutural length about 10 times length of scutellum. Propygidium: Hidden beneath elytra. Pygidium: Subtriangular, about twice as wide as long at middle; finely, densely rugopunctate. Margins beaded with sparse, moderately long setae; setae tawny. Apex rounded. Venter (Figs 2-3): Prosternal process elongate-oval, projecting anteroventrally at about 35° with respect to ventral plane; apex produced to level of protrochanter, rounded; surface posteriorly protuberant in basal 1/4, with setaceous punctures; setae long, dense, tawny. Mesometaventral process produced anteriorly to prosternal process; apex acuminate, rounded; ventral surface weakly recurved toward apex in lateral view, lacking setae apically, with moderately dense setae basally (setae moderately dense, moderately long, tawny). Abdominal ventrites 1-4 subequal in length in male and female, ventrite 5 about 1.5-2 times the length of ventrite 4, ventrite 6 subequal in length to ventrite 4 (male) or 1.3 times length of ventrite 4 (female). Abdominal ventrites 1-4 subequal in length in male and female, ventrite 5 about 1.5-2 times the length of ventrite 4, ventrite 6 subequal in length to ventrite 4 (male) or 1.3 times length of ventrite 4 (female). Last ventrite with widest width to median length in males about 5.5:1 and in females about 4.3:1; surface smooth (male) or rugose (female). Last ventrite (male) subequal in length to ventrite 5, quadrate at subapex, subapical corners not produced, surface moderately densely punctate; region from subapex to apex less sclerotized, surface smooth. Last ventrite (female) subequal to ventrite 4, apex trapezoidal, surface rugose. In lateral view, male ventrites flat, female ventrites weakly convex. Legs: Protibia with 3 external teeth subequally separated in apical half; spur present, subapical; inner base lacking protibial notch. Protarsomere 5 of male a little longer than tarsomeres 1-4 and with well-defined ventromedial emargination (Fig. 15). Modified foreclaw of male 1.5-2 times width of unmodified claw, inner subapical tooth present, small. Foreclaws of female simple, internal claw as wide as outer claw. Unguitractor plate laterally flattened, weakly exposed beyond tarsomere 5; apex with 2 short setae. Protarsomere 2 (male) with or without striated region at ventral apex; lacking in female. Mesofemur with acute process projecting posteriorly on posterior margin (male) (Fig. 16). Mesotibia with sides subparallel, apex weakly divergent or parallel; external edge with 2 weak carinae (female), less pronounced in male; inner apex with 2 spurs; apex with 12-15 spinulae. Meso- and metatarsomere 4 apicomedially with 1 outer spinose seta and 1 inner stout spinose seta (male and female). Meso- and metatarsomere 5 with weak, triangular interomedial tooth or swelling. Outer claw of meso- and metatarsal claws slightly longer than inner claw; outer mesotarsal claw twice as wide as inner claw in males, subequal in width in females; metatarsal claws subequal in width; claws simple. Metatibia with sides subparallel, weakly divergent towards apex; external edge with 1-2 weak carinae (slightly more robust in female); inner apex with 2 spurs; inner apex with 14-26 short, stout spinulae. Metacoxal corner (female) weakly produced or square. Spiculum gastrale (Fig. 13): Weakly Y-shaped (arms ~30 degree angle), lacking associated sclerites and setae. Male genitalia (Figs 22-24): Parameres less than twice length of phallobase. Parameres fused dorsoventrally (not laterally), asymmetrical; diagnostic, species specific (Figs 22a, 23a, 24a). Ventral sclerite of phallobase asymmetrical or symmetrical, elongate (produced to apex of phallobase), apex subquadrate; diagnostic, species specific (Figs 22c, 23c, 24c). Female genitalia: Gonocoxites subtriangular to subquadrate with sparse setae; not diagnostic for species.

Natural history.

Biology for species in the genus is not known. Adults likely feed on plant foliage, but no host has been recorded. Because adults are attracted to lights at night, it is likely that feeding occurs at night. Larvae are not described, but likely feed on compost and/or roots.

Etymology.

From the Greek, “mesos” meaning middle or in the middle, “mero” meaning femur, and “odon” meaning tooth. The name refers to the spinose process on the posterior margin of the mesofemur in males, a synapomorphy for species in the genus. The gender is neuter.

Composition and distribution

(Fig. 18). Three species distributed on western (lowland) Amazonia from Colombia, Ecuador, Peru, and Bolivia. An erroneous record from Brazil ( Ohaus 1905) was repeatedly cited by subsequent authors ( Blackwelder 1944, Ohaus 1934, 1952, Machatschke 1972, Krajcik 2008, Soula 2008, Moore et al. 2017) (see Mesomerodon spinipenne type material). We record the genus from elevations between 150 to 762 m. A record of the genus occurring at 2800 m ( Paucar-Cabrera 2005) is beyond the limits of the genus, and we consider it erroneous. A locality record of M. gilletti from Loja (Ecuador) (Fig. 18 [indicated with question mark]) waits for confirmation through future collecting since a short series of specimens supposedly collected from Loja province (west side of the Andes) seems to be out of the altitudinal and longitudinal reach of the genus. The ranges of two species of Mesomerodon overlap in aseasonal Ecuador in a region known for the highest levels of mammal and plant species diversity ( Hoorn et al. 2010). Rutelinae biodiversity in Ecuador is the highest recorded in South America, with 53 genera and 298 species ( Paucar-Cabrera 2005). Of these, 92 species of Rutelinae (or 36%) are endemic to the country ( Paucar-Cabrera 2005). The distribution of the genus is restricted to low elevations alongside the Andes without extending eastward into the Brazilian Amazon. Mesomerodon exhibits a distributional gap between M. spinipenne and the Ecuadorian species in northern Peru.

Niche modeling.

Within the Andean corridor, the genus level distribution model is congruent with the specimen-based distribution (Fig. 19 vs. Fig. 18). Therefore, the apparent distributional gap between Mesomerodon spinipenne and the Ecuadorian species in northern Peru is unlikely a result of a lack of sampling. The distribution model of the Ecuadorian species (Fig. 20) suggests that either M. barclayi sp. n. or M. gilletti extend into northern Peru. The distribution model for M. spinipenne (Fig. 21) shows a continuous distribution in central and southern Peru with a disconnected population in Bolivia (also corroborated with the specimen-based distribution). Specimen-level data do not corroborate the occurrence of M. spinipenne in western Brazil, northern Colombia, or Venezuela. Collecting may reveal occurrence of the taxon in western Brazil, but we consider it unlikely that the taxon occurs in Colombia or Venezuela because these countries have been well collected.

Diagnosis.

Species in the genus Mesomerodon are distinguished from other pelidnotine leaf chafers based on the acute, spiniform processes on the apical callus of the elytra in males (Fig. 17; shared with Hoplopelidnota ) and an acute process on the posterior margin of the mesofemur in males (Fig. 16; autapomorphic for the genus). The ovate body form, size, and color are similar to some species of Pelidnota (Pelidnota) (e.g., Pelidnota lucida Burmeister, 1844), but the form of the mandible clearly separates the two genera ( Mesomerodon species possess a rounded mandibular apex [Fig. 9] whereas Pelidnota species possess a bidentate, reflexed mandibular apex). Additional characters that assist with diagnosis of Mesomerodon include: external edge of protibia with three teeth; pronotum with bead complete anteriorly, laterally and basally; mesoventrite produced beyond mesometaventral suture (Fig. 3); and male genitalia with highly sclerotized ventral sclerite of the phallobase (Figs 22c, 23c, 24c). See Moore et al. (2017) for a key to genera of pelidnotine scarabs.

Key to Mesomerodon species

Clave para las especies de Mesomerodon