Morphological analysis of Mecosarthron Buquet and Xixuthrus Thomson and Reevaluation of Generic Assignment of Xixuthrus domingoensis Fisher (Coleoptera, Cerambycidae, Prioninae)

The characters defining Mecosarthron Buquet, 1840 and Xixuthrus Thomson 1864 are discussed, along with a historical review of the literature that described and classified these taxa. Through morphological examination of these genera and most of the included species, we addressed the systematic placement of Xixuthrus domingoensis Fisher, 1932 that was placed in Mecosarthron by Ivie (1985). We restore its placement in the genus Xixuthrus. The first description of the female of X. domingoensis is provided, along with comparative redescriptions of Mecosarthron gounellei (Lameere, 1903), and M. buphagus Buquet, 1840. We include a key to the species currently in Mecosarthron. Key-Words: Systematics; Hispaniola; South America; Longhorned beetles; Sexual dimorphism; Morphology.


IntroductIon
The taxonomic history and characters of two genera of Prioninae, Mecosarthron Buquet 1840 and Xixuthrus Thomson 1864, are discussed.The relationship of these genera is poorly known since they share some characters but differ in others.The main purpose of this work is to elucidate their taxonomy through examination of the characters in all the relevant species.As a result of this work, we transfer one species from Mecosarthron to Xixuthrus, provide descriptions or redescriptions for three species in these groups, and provide an identification key to Mecosarthron.
According to him, Xixuthrus has the following characters: upper eye lobes just subapproximate; antennae slender, reaching the posterior third of the body, with eleven segments; scape exteriorly toothed, scabrous; antennomere III slightly longer than scape, underside spiny; antennomeres IV-X slender, gradually shorter; antennomere XI longer than X; genae projecting; mandibles elongate, projecting, weakly curved downwards, with four teeth at inner face (two at base and two more apical); palpi elongate; prothorax narrowed anteriorly, laterally toothed or crenulate; scutellum rounded; elytra very elongated, almost flat, subparallel, apex with sutural spine; pro-and mesosternal process almost laminiform; forelegs longer than middle and hind legs, robust; all legs inferiorly with two series of spines; tarsi wide, last segment notably long.Thomson (1864) included Xixuthrus and Mecosarthron among his "Ctenoscelitae", characterized by: upper ocular lobes sub-approximate; scape frequently elongate, slender, rarely short and somewhat thick; antennomere III longer than following (except in Mecosarthron); prothorax wide, crenulate, with sexual dimorphism very distinct; elytra wide, almost flat, laterally enlarged; pro-and mesosternal process laminiform; legs more or less spiny, rarely unarmed."Ctenoscelitae" was a division of "Macrotomitae", and besides Xixuthrus and Mecosarthron included: Ancistrotus, Ialyssus Thomson, 1861, Ctenoscelis, Ergates, Navosoma and Trichocnemis.That group had many exceptions (some of them pointed out by Thomson himself ), and at least one character that does not occur in Mecosarthron and frequently not in Xixuthrus [but weakly so in the type species, X. microcerus (White, 1853)]: pronotum with sexual dimorphism distinct.
Lacordaire (1868) maintained Xixuthrus and Mecosarthron in "Cténoscélides" that also included Ialyssus and Ctenoscelis, and characterized this group as follows: ligulae moderately emarginate; mandibles very distinct, sub-horizontal or tilted down, rarely vertical, almost flat dorsally; antennae filiform; eyes not divided; prothorax usually with sexual dimorphism marked, and in this case, in males finely rugose or abundantly and finely punctate; with shining callosities in males and always rugose and irregular in females, laterally crenulate in both sexes; legs long, tibiae spiny; tarsomere III bilobed.
That there was confusion about the phylogenetic relationships of Xixuthrus and Mecosarthron is clear by looking at the sequence of treatments by Lameere.Lameere (1901) placed Xixuthrus and Mecosarthron in "Macrotomites" and subtribe "Titanines": "Mecosarthron est le point de départ de Ctenoscelis (y compris Jalyssus?[sic], Aulacocerus, Titanus, Ancistrotus, Apotrophus, probablement de Chalcoprionus, qui m'est inconnu, et vraisemblablement aussi de Macrodontia, genre qu'il m'est impossible de bien juger, n'en ayant pas à ma disposition les espèces inférieures".To Lameere the "Macrotomites" had the eyes unemarginate, with two subtribes: "Macrotomines", in which the ligulae are small and undivided; and "Titanes", in which the ligulae are large and bilobed. To Lameere (1903a) the inclusion of Mecosarthron in "Cténoscélides" was unjustifiable because the ligulae are different, and according to him, the shape of eyes, legs, etc., led to his conclusion that the approximation of Buquet (1840) was quite superficial.Lameere (1903b) placed Xixuthrus and Mecosarthron in "Mécosarthrines" and provided a key to the genera in that group, characterized as follows: sides of prothorax crenulated; ligulae large and bilobed; scape very long."Mécosarthrines" included, besides Xixuthrus and Mecosarthron: Omotagus Pascoe, 1867, Dysiatus Pascoe, 1869, Clinopleurus Lansberge, 1884, and Daemonarthra Lameere, 1903b. Lameere (1912) revised his opinion, stating for "Mécosarthrines" (translated): "This group must take the name of Xixuthrines, genus Mecosarthron to me appearing decidedly, having to be removed to be placed near Basitoxus".He provided no explanation as to why he changed his opinion from 1903.Lameere (1913) maintained Xixuthrus and Mecosarthron in Macrotomini, the former being in the subtribe "Xixuthri" and the latter in "Basitoxi".Lameere (1919) described "Xixuthri" and "Basitoxi", clarifying his opinion of 1912.Comparing the descriptions of the two groups, the differences are almost nonexistent or nearly all characters are variable according Lameere himself, and inadequate for separating the groups of genera.Among the characters recorded for the two groups by Lameere we can mention one in which he did not indicate variation: body convex in "Xixuthri" and weakly convex in "Basitoxi".In the key to the groups of Macrotomini, Lameere separated "Xixuthri" and "Basitoxi" at couplet 1: lateral margins of prothorax on a ridge (crest) and explanate sides, prothoracic episterna wide, antennae with the scape and antennomere III usually somewhat elongate, and body less depressed: "Basitoxi"; lateral margins of prothorax not on a ridge, sides not explanate but declivous, prothoracic episterna more or less narrow, antennae frequently with scape and antennomere III elongated, and body more convex: "Xixuthri".Clearly, the key does not fully distinguish the groups of genera, and nearly all are based on subjective, variable or dubious characters.
Vitali (2008) used a tribal status for "Rhaphipodi" sensu Lameere (1912), including, besides the genera allocated by him, the following ones: Mecosarthron, Mallodonopsis Thomson, 1861, Olethrius Thomson, 1861and Samolethrius Vitali, 2008.Rhaphipodini was characterized by Vitali: "habitus relatively stout; head with Y-shaped suture; labrum small; mandibles with scarce sexual dimorphism; scape flattened, long (reaching the anterior angles of the pronotum or at least abundantly surpassing the posterior margin of eyes in males), not shorter than antennomere III, straight or abruptly bowed at base; pronotum laterally denticulate; elytra with three longitudinal ridges, sometimes obsolete; legs more or less visibly denticulate; anterior legs of males longer and more robust than those of females".Yet according to Vitali (2008: 29), Rhaphipodini (including Mecosarthron) differs from "Xixuthri in the 4 elytral ridges and the concave labrum".
To assess the relationships and phylogenetic placement of Mecosarthron domingoensis (Fisher, 1932), we have analyzed the type species, as well as others, of all the associated tribes or subtribes based on the paper of family group names by Bousquet et al. (2009).To this list, we have added several species of other groups proposed by Vitali (2008) since his classification differs.Clearly, a thorough phylogenetic study involving all genera in these groups needs to be undertaken, but the goal of this work is simply to understand the most appropriate placement of the species currently placed in Mecosarthron.An analysis of the type species in these genera will suffice to achieve that goal.

Xixuthrus and Mecosarthron
As the convoluted literature on tribal affinities suggests, Xixuthrus and Mecosarthron have more morphological features in common than different, especially when we compare some species such as M. tritomegas Lameere, 1920 andX. microcerus (White, 1853).Among the features in common, the more notable are: prothorax margins with numerous small spines; prosternal process large (although very variable in shape, mainly in the species currently placed in Xixuthrus); epipleuron at humerus smooth and margined dorsally with a distinct carina; 3-4 elytral costae visible; profemora in male rugose-scabrous; tibiae (particularly first pair) lined with numerous denticles and small spines; two moveable spurs adjacent to one another at apex of metatibia; tarsal pads 1-3 with narrow, glabrous middle region.
The scape and antennomere III in Xixuthrus vary in form and length.The same occurs in Mecosarthron, if M. tritomegas really is a true Mecosarthron (see comments below).
The first protarsomere in Xixuthrus is also variable and does not distinguish this genus from Mecosarthron, although in the species included originally in Xixuthrus by Lameere (1903b) (X.microcerus, X. costatus (Montrouzier, 1855) and X. heros (Heer, 1868)) it is strongly narrowed at base and very elongate, while in Mecosarthron it is moderately narrowed at the base and less elongate.
The protibia in Xixuthrus (mainly in males) is proportionally narrower and longer (Fig. 1) than in Mecosarthron buphagus (Fig. 2), or at least it is strongly enlarged apically (as in X. lameerei Marazzi et al., 2006).However, the holotype male of M. tritomegas is aberrant in that feature (Fig. 7B), because the protibia is the same as in many Xixuthrus (strongly variant from middle and hind tibiae).
Xixuthrus (and also Dysiatus Pascoe, 1869 [Fig. 8]) differs from Mecosarthron by having the prothorax of males microsculptured (sexual punctation of Lameere (1903b)) and the elytral costae visible, frequently pronounced.In Mecosarthron, the prothorax of males is not microsculptured, and the elytral costae are not pronounced (or not well delimited).In M. gounellei (Lameere, 1903a) the elytral costae are distinct, but they are not well delimited as in Xixuthrus.In some males of Xixuthrus, the sexual punctation of the prothorax is not evident, but the sculpturing of the pronotum differs from Mecosarthron: in Xixuthrus it is finely punctate and frequently with callosities distinctly contrasting with the areas around them; in Mecosarthron the pronotum is distinctly more coarsely punctate, and there are no contrasting callosities.Marazzi et al. (2006) synonymized Clinopleurus Lansberge, 1884 with Xixuthrus, based on the similarity of shape of the mesosternal apophysis, allometric length of the mandible, length of the palpi, and shape of pronotum.Currently, Xixuthrus is divided into two subgenera based on the development of the protibia: Xixuthrus sensu stricto, type species Macrotoma microcera White, 1853, and X. (Daemonarthra), type species Daemonarthra helleri Lameere, 1903b.The species of Xixuthrus (Xixuthrus) have the protibia slender, frequently very long, not uniformly enlarged from the base towards apex and not appreciably thickened at apex, and tarsomere I notably long.Xixuthrus (Daemonarthra) do not have the apex of the protibia notably thickened.However, there are species with intermediate features, such as X. solomonensis Marazzi & Marazzi, 2006, and other species that have the protibia not fitting the subgeneric definitions, such as Xixuthrus (Xixuthrus) thomsoni, X. (X.) lameerei, and X. (X.) arfakianus, suggesting that the validity of the subgeneric classification needs to be re-examined.
Based on the morphological characters of the type species of Mecosarthron and Xixuthrus, we return Mecosarthron domingoensis to Xixuthrus (Xixuthrus), re-establishing the original combination.The arguments used by Ivie (1985) to place this species in Mecosarthron are not supported.However, we need to say that some features found in X. (X.) domingoensis agree better with those found in Dysiatus than in Xixuthrus, as for example, the very distinct sexual punctation in males (Fig. 5E) [for males of Dysiatus melas see Fig. 8A].The differences pointed out by Pascoe (1869) ("mandibulae crassae, subverticales, productae, rectae, apice abrupte arcuatae, intus bidentatae … Antennae graciles, dimidio corporis longiores; scapo subelongato, depresso, scabro, intus denticulato; articulo tertio multo breviore … Prothorax transversus, utrinque denticulatus, angulis anticis productis, rotundatis, apice bisinuato, basi truncate …") to separate Dysiatus from Xixuthrus do not really distinguish these genera.Pascoe (1869) separated Xixuthrus and Dysiatus in a key: anterior angles of the prothorax rounded (Xixuthrus); anterior angles of the prothorax produced (Dysiatus).This feature is variable in the species actually placed in Xixuthrus and does not exclude X. domingoensis from Dysiatus.Pascoe (1869) also wrote: "tarsi antici articulo basali haud elongato".This is not true: the photographs of the holotype of Dysiatus melas Pascoe (Figs. 8A,B) show that tarsomere I is elongate, as in some species of Xixuthrus, although not notably elongate as in X. microcerus and X. domingoensis.Gressitt (1959) also separated Xixuthrus from Dysiatus in a key, using the form of anterior angles of prothorax: rounded in the species of Xixuthrus from New Guinea and projected forward in Dysiatus.However, this character does not distinguish the genera, and reveals a mistake: the anterior angles of Xixuthrus (Daemonasthra) helleri, a species that occurs in New Guinea and was mentioned by Gressitt (1959), are as in Dysiatus.Furthermore, other species recently described, such as X. lameerei Marazzi et al. (2006), also from Papua New Guinea, have the anterior angles of the prothorax as in Dysiatus.
Similarly, the characters used by Lameere (1903b) are inadequate to distinguish Xixuthrus from Dysiatus: "Antennes à 3 e article notablement plus court que le 1 er ; mandibles renflées à la base; tarses antérieurs à 1 er article non allongé", for Dysiatus; and "Antennes à 3 e article au moins presque aussi long que le 1 er ; mandibules non renflées à la base tarses antérieurs à 1 er article allongé", for Xixuthrus.The shape and punctation of the prothorax in males, type of mandible, and length of antennomere III is very similar in D. melas and X. domingoensis, but the length and shape of the protibia and tarsomere I (longer and finer) and the presence of elytral pubescence [absent in males of D. melas], allow the exclusion of Fisher's species from Dysiatus.According to Lameere (1912) the females of D. melas have the elytra slightly pubescent.
The geographical isolation of X. (X.) domingoensis suggests that the species may have been introduced.However, no known species of Xixuthrus in the Pacific Islands (its geographic center of diversity) match this species.Thus, it is most likely a native species, although it is challenging to explain the presence of a species of Xixuthrus on an Atlantic island, while it doesn't occur in Africa or North, Central, or South America.
The original description of the male (Figs.4A-C, 9) is detailed and needs few additional comments.Nevertheless, Fisher (1932) recorded some features that do not agree with the holotype or with the specimens more recently collected: "Elytra five times as long as pronotum" (actually just longer than four times); and "eyes … separated from each other on the top by about the width of the upper lobe" (actually, the distance between upper lobes is equal to about 1.5 X the width of the lobe, and a little greater than its length).Since the original description was based only on males, we present the first description of the female below, followed by comments on sexual dimorphism in this species.
Description (female, Fig. 4D-F): Integument reddishbrown, mostly matte due to pubescence; darker on head, pronotum, venter, and appendages than on elytra; mostly covered with very short, fine, translucent to slightly golden pubescence.
Head coarsely and confluently punctate between the eyes; punctures of differing sizes; area between the posterior edge of eyes and occiput densely punctate-granulate; punctures becoming less dense on antennal tubercle; pilosity short and moderately dense between eyes, with equal abundance, density, and length between eyes and occiput.Labrum centrally strongly concave, arcuate at base; pilosity long, moderately dense, projecting forward.Eyes large, with only slight indention near antennal tubercle; from lateral view, eye extends the entire height of head from vertex to gula, somewhat broader ventral to tubercle than dorsally; from ventral view, eye extends nearly length of head from posterior margin to just before mandible base.Minimum distance between upper eye lobes about 2/3 length of antennomere III; distance between lower lobes greater, but less than length of antennomere III.Area around eye margin not sulcate; strongly depressed between eyes and antennal tubercles with central sulcus extending from between antennal tubercles to occiput.Gula between ventral lobes of eyes strongly vermiculate, lacking pubescence.Gena strongly produced anteriorly into broad, but blunt tooth below mandibular insertion.Mandibles ventrally projecting, from 0.5 to 0.6 times the length of the head; with strong, acute, broad apical tooth, smaller tooth at middle, and weak tooth at base; coarsely, confluently punctate on outer surface, except for teeth (anterodorsally); mostly smooth on underside (posteroventrally); pilosity short, sparse, limited to margins and base.Antennae reaching about middle of elytra or slightly surpassing middle.Scape about as long as antennomeres II-III-IV together (about length of head); dorsally flat, coarsely, shallowly, sparsely punctate, otherwise smooth and shiny, without denticles; neither vermiculate or asperate.
Pronotum much broader posteriorly than anteriorly; strongly multispinose on lateral margins (10-15 well defined spines), with well-defined spines at anterolateral and posterolateral corners; densely punctate; punctures mostly confluent but of different sizes; weakly depressed at middle with small, shiny, impunctate region just anteromedially, extending in a partial medial line posteriorly, but not attaining posterior margin; pronotal sculpturing forming several incomplete, elevated ridges laterally on disk.Most pronotal punctures each bear a single, short, inconspicuous, translucent seta, but otherwise the pronotum appears shiny and glabrous.Prosternum moderately but shallowly punctate-vermiculate, becoming less dense at sides and base of prosternal process.Prosternal process wide, slightly narrowed between procoxae; widened again and rounded on apex.Pubescence of prosternum short, sparse, but more conspicuous than on pronotum.
Elytra with mostly matte finish, but with shiny areas around base, suture, and costal ridges; mostly covered with very short, fine, translucent to slightly golden pubescence; very finely punctate; each micropuncture bearing a single, short, translucent seta.Elytra each with three incomplete costae equally spaced between suture and lateral margin; integument with microrugulae throughout, visible under high magnification.
Femora and tibiae mostly smooth but with scattered asperites throughout and microspinules irregularly scattered along inner margin; protibiae each with two confluent spurs apicomesally and a spine apicolaterally.Protibiae slightly curved laterad from base to apex.All femora about as long as tibiae; overall length of legs similar, but middle legs slightly shorter.
Remarks on sexual dimorphism: Sexual dimorphism in X. domingoensis is most apparent in the structure of the forelegs, pronotum, antennae, and mandibles.The legs (especially protibiae and profemora) are strongly spinulose and asperate in males and very weakly so in females.The protibiae of males are slightly but distinctly curved mesally toward apex (inward curvature) while in females they are outwardly curved toward apex.The protibiae in males are distinctly longer than the profemora (ratio of 1.2-1.3),while in females, they are of approximately the same length.The pronotum in males is very evenly, densely, confluently punctate and lacking ridges.There are 4 small, shiny, semi-impunctate regions on the disk in males: 1 on either side and just anterior to middle, and one posterolateral to that, on each side.In females, the pronotum is as described above, with punctures mostly confluent but of different sizes and with a small, shiny, impunctate region anteromedially, extending into a partial median line posteriorly.The antennae of females reach to approximately the middle of the elytra, while in males, the antennae reach to approximately the apical fourth of the elytra.In females, the mandibles are approximately half the length of the head, while in males, the mandibles are greatly enlarged and nearly as long as the head.
Geographical distribution: Xixuthrus domingoensis is known only from the Dominican Republic (Provinces of Santiago, San Pedro de Macorís, and Altagracia) on the island of Hispaniola.
Type data: Holotype male (Fig. 9A-C), from Dominican Republic (Santiago), collected by Giuseppe Russo Gounelle, in 1926, deposited  Redescription: Male (Fig. 2A-C).Integument piceous to dark reddish-brown except for apical 3/4 of elytra which are lighter reddish-brown; elytra moderately shiny with inconspicuous, very short, fine, translucent pubescence.Head integument with shagreened appearance, more coarsely developed between eyes and on antennal tubercles, more finely developed on vertex and occiput; very few defined punctures present; pilosity short, sparse, becoming denser in indistinct patches behind eyes; broad at base (10-12 mm).Labrum centrally moderately concave, arcuate at base; pilosity of moderate length and density, projecting forward.Eyes large, singularly lobed, with only very vague indication of indentation at side near antennal tubercle; from lateral view, eye extends nearly entire height of head from vertex to gula, somewhat broader ventral to tubercle than dorsally; from ventral view, eye occupies about 50% of length of head from anterior genal apex to posterior pronotal margin.Minimum distance between upper eye lobes barely less than length of antennomere III; distance between lower lobes just greater than length of antennomere III.Area around eye margin not sulcate; strongly depressed between eyes on vertex.Gula between ventral lobes of eyes strongly vermiculate, lacking pubescence.Gena strongly produced anteriorly into broad tooth below mandibular insertion.Mandibles ventrally projecting, from 0.7 to 0.8 times the length of the head; with strong, acute, broad apical tooth, smaller tooth at middle, and weak tooth at base; coarsely, confluently punctate on outer surface, except for teeth (anterodorsally); mostly smooth on underside (posteroventrally); pilosity short, sparse, limited to margins and base.Antennae reaching to at least apical fourth of elytra.Scape elongate (11-13 mm), slightly longer than antennomeres II-III-IV together (extending to anterior third of pronotum); with denticles or spinules on mesal-ventral margin, weakly asperate on dorsolateral margin.
Elytra weakly shiny, mostly covered with very short, fine, translucent pubescence; lacking punctures; microrugulate and shagreened throughout.Elytra each with two vaguely indicated, minimally raised costae equally spaced from suture laterally, and a third vague depression between lateral-most costa and lateral margin.
Remarks on sexual dimorphism: Sexual dimorphism is not as apparent as in Xixuthrus species.Males have the pronotum less broadened posteriorly than anteriorly as compared to females, but sculpturing is very similar.Males have more strongly multispinose pronota laterally (12-15 spinules laterally) as compared to females (less than 10 poorly defined spinules laterally).Head of males slightly broader at pronotal insertion than in females.Mandibles only slightly smaller in females compared to males, but in both cases very similar to overall head length.The front and middle legs are strongly spinulose and asperate in males and very weakly so in females.The protibiae of males are straight to slightly curved mesally toward apex (inward curvature) while in females they are outwardly curved toward apex.The antennae of females reach to about the middle of the elytra, while in males, the antennae reach to at least the apical fourth of the elytra.
Redescription: Male (Fig. 3A-C).Integument dark brown, shining; mandibles blackish; tarsi and distal antennomeres slightly lighter.Head coarsely and confluently punctate between the eyes, almost vermiculate; area between the posterior edge of eyes and occiput moderately, finely granulate; pilosity short and sparse between the eyes, distinctly more abundant between eyes and occiput.Labrum centrally, strongly concave, coplanar with clypeus at base; pilosity long, abundant, projected forward.Area behind eyes granulated between apex of upper ocular lobes and approximately the basal third of lower eye lobes (granules coarser and sparser than dorsal area of head behind upper lobes and slightly finer and closer behind lower lobes); area behind apical two-thirds of lower eye lobes transversely sulcate.Gular area strongly vermiculate; pilosity of short setae intermixed with long setae (more distinct in lateral view).Eyes wide; distance between upper lobes equal to or slightly wider than the length of antennomere III; distance between lower lobes slightly greater than between upper lobes.Mandibles from 0.5 to 0.6 X the length of the head; coarsely, confluently punctate, deeper on outer surface; pilosity short, moderately sparse.Antennae surpassing middle of elytra.Scape about as long as antennomeres III-IV together; dorsally flat, coarsely, confluently punctate, mainly on basal half; ventrally vermiculate, mainly on basal two-thirds, without denticles; distinctly surpassing posterior edge of eyes.
Gibbosities of pronotum less punctate on inner face than outer face and areas around them; pilosity short, moderately sparse, more abundant on small area close to posterior angles.Prosternal process wide and rounded at apex.Elytra coarsely, abundantly punctate; elytral costae present, but not well defined; pilosity short, sparse.Profemur scabrous on lateral surface (visible on dorsal view), vermiculate or punctate laterad; ventral surface with small spines on apical half (only near apex in small specimens); rugose-punctate on dorsal surface; meso-and especially metafemora less strongly sculptured than profemora.Tibiae with similar length, slightly, but distinctly enlarged from base to apex; protibiae with smaller spines on ventral surface (almost absent in smaller specimens).
Female (Fig. 3D-E): Examined only from a photograph.General appearance similar to that of male.Head distinctly smaller; antennae reaching apex of basal third of elytra; scape reaching approximately to the apex of posterior edge of eyes.
Geographical distribution: Mecosarthron gounellei is known only from northeastern Brazil (Bahia).
Type data: Holotype male (Fig. 6), from Brazil: Santo Antonio da Barra, Bahia, collected by Gounelle, on 11.XII.1888,deposited at MNHN.Remarks: Mecosarthron gounellei differs notably from the other species of the genus by having the prosternal process distinctly enlarged and rounded at the apex (Fig. 3B) and the elytra distinctly punctate and sparsely pubescent (Figs.3A, C, D).In M. buphagus (Figs.2B, E) and M. tritomegas (Fig. 7B) the prosternal process is narrowed towards the apex and the elytra are distinctly pubescent and not notably punctate (see Figs. 2A,D and 7A,respectively).
According to Lameere (1903a) the specimen was "capture à San Antonio da Barra, province de Bahia, au Brésil, le 11 décembre 1888".Santo Antonio da Barra is now Condeúba, and the vegetation in the area is transitional between forest, cerrado (characterized by extensive savanna formations crossed by gallery forests and stream valleys, that includes various types of vegetation), and caatinga (composed of stunted trees and thorny bushes, found in areas relatively xeric areas in Brazil).However the locality is situated near the area with vegetation characteristic of a "seasonal forest", with general appearance of a forest with trees reaching 25 to 30 meters in height, covered with lianas and epiphytes.This Brazilian region displays notable endemism of Cerambycidae (e.g., Antodice lenticula Martins & Galileo; Adetus minimus Breuning; Dorcasta quadrispinosa Breuning; Corimbion balteum Martins; and Minibidion confine Martins).Generally specimens from this region are not common in the large collections of insects in Brazil (with the exception of MNRJ, where there is a good number of specimens collected by C.A. Campos Seabra in Pedra Azul (Minas Gerais), a place somewhat near Condeúba, but placed in the area of "Seasonal Forest").Lameere (1903b) recognized that the species is very distinct when compared with M. buphagus, and wrote: "C'est, parmi les Mécosarthrines, une forme très primitive qui ne peut se rattacher qu'à un type tel que Omotagus Lacordairei".Given that Lameere had a very broad generic approach (for example, with his concept of Callipogon Audinet-Serville, 1832, that included as subgenera Navosoma Blanchard, 1846 and Spiloprionus Aurivillius, 1897), some could conclude that this unusual species merits a separate genus.Lameere (1903b) wrote about the antennae: "Ce Mecosarthron [M.buphagus], supérieur au précédent, nous offre la répétition de ce phénomène d'allongement du 1 er article des antennes qui s'est produit également dans l'évoultion des Xixuthrus, mais, chez ces dernieers, le phénomème [sic] s'accompagne d'un allongement du 3 e article des antennes".However, although in Xixuthrus the scape can vary in length, the variation in form is not pronounced, while the form of scape of M. buphagus is very different from that of M. gounellei.
Although we do not know the exact locality where the specimen was collected in Brazil, it is likely that it is from Central Brazil, because Franceschini (2002) recorded a female of Appula sericatula Gounelle, 1909 from IRSN collected by Bonneuil in Goiás.
According to Lameere (1920), M. tritomegas closely resembles M. buphagus, but has very different antennae, more similar to those of Ialyssus Thomson, 1864 (the scape is proportionally shorter, weakly surpassing the posterior edge of eyes and antennomere III is longer than the scape, twice as long as antennomere IV, and about as long as IV-V together).In our examination, the form of the scape is more similar to males of Ialyssus tuberculatus (Olivier) than to males of M. buphagus.However, many species of Macrotomini have the scape somewhat similar to M. tritomegas, while M. buphagus has an unusual scape.That character alone suggests that M. tritomegas is not a true Mecosarthron, however, another character, not mentioned by Lameere, is the form of the tibiae: In M. tritomegas, the form of the foretibiae is not similar to that of the meso-and metatibiae, and it is identical to that in Xixuthrus.This is strange because that strong difference (shape and length) among the tibiae does not occur in M. buphagus and M. gounellei in which the protibiae are little differentiated.Lameere (1920) suggested that M. tritomegas is intermediate between M. gounellei and M. buphagus.However, M. tritomegas has no characters linking M. gounellei and M. buphagus.Comparing the scape in males of M. gounellei with males of M. buphagus, it is possible to see that they have some similarities in shape (e.g., form, basal curvature), although it is smaller and not ventrally rough.In M. tritomegas the scape is less uniformly enlarged from base to apex and also less curved at the base.Mecosarthron tritomegas is not intermediate between the other species of the genus because the scape is smaller than in the others.Antennomere III in males of M. gounellei and M. buphagus are very similar in size and equal to or just longer than IV, while in M. tritomegas antennomere III is distinctly longer, about twice the length of IV.Thus, using this feature, M. tritomegas is not intermediate between M. buphagus and M. gounellei.Last, the foretibiae in males of M. gounellei and M. buphagus are very similar (size, shape, and similarity with the  Lameere (1920) states further, on Ialyssus: "son existence [M.tritomegas] rend précaire le maintien du genre Jalyssus [sic] que ne diffère de Mecosarthron que para l'absence de pubescence et la présence d'une ponctuation sexuelle sur le prothorax du mâle".However, the pronotum in males of Ialyssus tuberculatus, which is distinctly different from that in Mecosarthron due to the distinct microsculpture, is not the only difference between Mecosarthron and Ialyssus: In Ialyssus the mandibles and tarsi are shorter (mainly tarsomere V), and the pubescence of the elytra is inconspicuous.Melzer (1919), who agreed with nearly all of Lameere's proposals in his "sous-groupes", and considered them as tribes, wrote: "O habito do Julyssus [sic] effectivamente é bastante semelhante ao Mecosarthron, mas embora seja sua lingueta bilobada, a grande difference sexual entre ♂ e ♀ do Jalyssus [sic] demonstram, que o mesmo não deve ser relacionado com aquelle gênero", commenting that Ialyssus is very similar to Mecosarthron, but the strongly sexually dimorphic conditions suggests that it cannot be closely related to that genus.
Upon examination of the photos of the holotype of M. tritomegas sent by Noël Mal (IRSN), we note that apparently the prothorax + head were glued to the mesothorax, and that the elytra were pubescent.Mal (pers.comm.)confirmed, "there is a tiny +/-dense pilosity quite apparent under enlargement but reduced on front of elytra (erosion?).The pronotum pubescence is longer but not so dense".Based on the photos and information sent by Noël Mal we think the possibility exists that the holotype is a composite specimen comprised of the body of a male of M. buphagus and head and prothorax of another species.The strong difference between the protibiae and meso-and metatibia, also corroborate the hypothesis of a composite specimen.However, we recognize the possibility that it is an unusual species, known only from the holotype, or an incorrectly labeled specimen originating from elsewhere in the world, because, for example, in Xixuthrus there is this kind of differentiation among the tibiae.We searched for specimens that match M. tritomegas in the largest entomological collections in Brazil (DZUP, INPA, MCNZ, MNRJ, MZSP) and in the USNM, but without success.