Microchilus beckeri (Martinez. The, 2008)

Jameson, Mary Liz, 2008, Review of the genus Microchilus Blanchard (Coleoptera: Scarabaeidae: Rutelinae: Geniatini), Insecta Mundi 2008 (25), pp. 1-14 : 4-6

Status of Microchilus beckeri

The classification status of M. beckeri is affected by two historical complications: 1) lack of robust discussion regarding shared characters that established generic placement of the species, and 2) synonymy of M. beckeri and L. bucki as alluded to by Martínez (1964).

The proper generic assignment for M. beckeri and L. bucki was an issue not well addressed by Martínez (1964) or Machatschke (1974). Martínez (1964) considered the species a member of the genus Microchilus , but Machatschke (1974) considered it a member of the genus Leucothyreus . Neither provided evidence that established justification for placement of the species in either genus.

Although research on the genera of Geniatini has laid the basic foundation for taxonomic characters for the geniatine genera ( Jameson and Hawkins 2005), phylogenetic characters (shared, derived characters based on phylogenetic analyses) would greatly assist in circumscription of the genera. Lacking a phylogenetic hypothesis, I examined exemplars of several genera within the Geniatini . Genera of Geniatini that are easily distinguished from Microchilus based on taxonomic characters (see the key to genera of Geniatini in Jameson and Hawkins 2005) are: Evanos , Geniates , Geniatosoma , Heterogeniates Ohaus , Lobogeniates Ohaus , Mimogeniates Martínez , Rhizogeniates Ohaus , Trizogeniates , and Xenogeniates . This enigmatic species, “beckeri” ( Fig. 6 ), could be keyed to the following genera or confused with the following genera: Microchilus ( Figs. 1-4 ), Leucothyreus , Bolax ( Fig. 5 ), or Eunanus Ohaus ( Fig. 7 ). The following discussion provides generic characters and a comparison of characters observed in “beckeri” as a means of aiding placement of the species in the genus with which it shares the most characters.

The species “beckeri” resembles Microchilus based on its small size (less than 8 mm), distribution in southern Brazil, apex of the labrum with a narrow tooth, and interocular width greater than 6 transverse eye diameters. But “beckeri” does not share the form of the claws (internal and external claws on all legs are incised in Microchilus ; internal claws simple, external claws incised in “beckeri”), the form of the clypeal apex (broadly rounded in Microchilus ; narrowly parabolic in “beckeri”), length of antennal club in the male (club 1.5 times length of segments 2-7 in Microchilus ; club 2 times length of segments 2-7 in “beckeri”), and protibial teeth (two external teeth in Microchilus ; three external teeth in “beckeri”). These characters preclude placement of “beckeri” in the genus Microchilus .

Several characters observed in “beckeri” are shared with members of the genus Eunanus : the narrowly parabolic clypeus, form of the meso- and metatibia, interocular width greater than 6 transverse eye diameters, male antennal club two times the length of segments 2-7, apex of the labrum with a narrow tooth, and small size. But “beckeri” does not share the simple claws observed in members of Eunanus nor the form of the mandible which possesses a rounded, apical tooth. These characters preclude placement of “beckeri” in the genus Eunanus .

A few species of Bolax are small ( B. foveolata Blanchard , B. incogitata Dohrn , B. mutabilis Burmeister , B. vittipennis [Laporte]) and vaguely resemble “beckeri” based on size. However, “beckeri” does not share the form of the labrum (apex with a narrow tooth in Microchilus ; apex with a broad tooth in Bolax ), length of antennal club in the male (club 1.5 times length of segments 2-7 in Microchilus ; club subequal to segments 2-7 in Bolax ), form of the pronotum (lacking longitudinal constrictions in Microchilus ; often with longitudinal constrictions at apex and base in Bolax ). These characters preclude placement of “beckeri” in the genus Bolax .

This leads us to the genus Leucothyreus , the largest genus in the Geniatini with 164 described species. It is a heterogeneous assemblage of species, and several characters are variable within the group (perhaps indicating that the group is not monophyletic). Several characters observed in “beckeri” are shared with members of the genus Leucothyreus : the form of the claws are simple and incised on all legs; protibia with three external teeth; length of protarsomeres 2-5 greater than length of protarsomere 5; base of pronotum not constricted; and surface of pygidium with decumbent, white setae and horizontal striae (striae weak in “beckeri”). No characters observed in “beckeri” necessarily preclude placement in the genus Leucothyreus , but they may be unusual for the genus Leucothyreus : the form of the clypeus (narrowly parabolic in “beckeri”; variable in Leucothyreus including rounded, parabolic, or quadrate) and interocular width (greater than 6 transverse eye diameters in “beckeri”; generally less than 5 transverse eye diameters in Leucothyreus ). Based on the shared characters of “beckeri” and members of Leucothyreus , as well as the few conflicting characters that preclude placement of “beckeri” in the genus, I transfer M. beckeri Martínez to the genus Leucothyreus ( Leucothyreus beckeri [Martínez], new combination).

Based on examination of type specimens, it is clear that L. beckeri and L. bucki are conspecific (see “Taxonomic History”). Although Martínez (1964) noted Ohaus’ label that identified the specimen as “ Leucothyreus bucki ”, the name was not published. Additionally, he thought that this specimen clearly represented a new member of the genus Microchilus rather than Leucothyreus and that the determination by Ohaus was in error. Machatschke (1974), in his description of L. bucki (a name that was in litteris by Ohaus), did not make a connection with M. beckeri because the two species were placed in different genera. Based on examination of type specimens for L. beckeri and L. bucki , these species are conspecific. Leucothyreus bucki Machatschke is herein considered a new junior synonym of Leucothyreus beckeri Martínez.

Figures

Figure 1-7. Habitus images Microchilus species and other Geniatini. 1) Microchilus lineatus, male. 2) M. lineatus, female. 3) M. rodmani, holotype male. 4) M. rodmani, allotype female. 5) Bolax foveolata Blanchard, male. 6) Leucothyreus beckeri, male. 7) Eunanus murinus Ohaus, male. Scale line = 1.0 mm.