Metallactus Suffrian, 1866

Sassi, Davide, 2018, Revision of the Metallactus kollari species-group with a new diagnosis of the genus (Coleoptera: Chrysomelidae: Cryptocephalinae), Zootaxa 4413 (1), pp. 57-110 : 62-64

Metallactus Suffrian, 1866

Metallactus Suffrian, 1866: 248 ; Chapuis, 1874: 167; Jacoby, 1889: 124; Chamorro-Lacayo, 2013: 203; Agrain et al. 2017: 58 . Type species: Metallactus kollari ( Perty, 1832) , herein designated.

Head wide, slightly convex. Clypeal area barely delimited (i.e. frontoclypeal suture blurred). Mid-cranial suture apparent, generally extended from clypeal border to vertex. Ocular canthus deep, blunt. Ocular line sometimes narrow but generally apparent, punctate, closely leaning against inner ocular margin, reaching canthus at its ventral end. Interocular distance not particularly different in males and females, compared with other Neotropical Pachybrachina with notoriously reduced interocular distance in males, some species with reduced interocular distance in males but at comparable level with some Palearctic Pachybrachis .

Pronotum slightly subconical to moderately transverse, faintly depressed on disc, weakly and evenly curved at sides, with maximum width slightly posterior to midline. Posterolateral impressions generally weakly marked, often detectable, in correspondence of which posterior margin appears thickened and slightly salient. Pronotal punctation moderately to weakly impressed, generally sparser, shallower on disc.

Elytra subcylindrical, weakly depressed on disc, lateral edge feebly excised, pygidium posteriorly barely exposed. Elytral length more than 2 times pronotal length. Elytral punctation rather shallow, partially arranged in irregular rows, sometimes rows almost completely obliterated.

Sub-humeral lobe of epipleura rather short, smooth, flat or slightly swollen, usually lustrous, not or very sparsely and shallowly punctured, separated from elytral margin by a blunt edge.

Prosternal process rather long and slender, fairly restricted at middle, concave along its median line with distinctly salient edges, hollow gutter-shaped. Apex expanded and leaning against inner and posterior margins of coxal cavities, terminating in subrounded or bluntly triangular posterior margin, sometimes with short, subacute, slightly raised tip. Prosternal process surface coarsely punctured and covered with long, thin setae, often more developed than in any other sector of ventral surface. Shape of the prosternal process corresponds to the short description given by Suffrian (1866).

Metasternum relatively long, fairly convex. Metepisternum long, narrow. Metepimeron barely detectable, basically fused with caudal external angle of metepisternum. Ventral surface of abdomen as a whole short and squat.

Fore legs robust with swollen femora and bulging coxae, swollen and elongated in antero-posterior direction. Male fore legs without relevant clues of sexual dimorphism either in femora or tarsi.

Outline of aedeagus basically tubular, with ventral profile, in lateral view, slightly convex or bulging. Hairy dents differently developed in Metallactus , in some species groups being limited to barely distinguishable, scarcely differentiated surfaces with few setae. In other groups, however, they tend to deepen, assuming particular shapes.

Spermatheca ( Figs 3 h–16h; 19h) rather simple in all observed species. It is basically sickle-shaped, poorly sclerotized, with basal and apical branches barely to moderately swollen and apical tip often slightly bent downwards. Ampulla slightly or fairly shifted on dorsal side of vasculum, always shortened, weakly developed, with insertions of spermathecal duct and accessory gland generally very close to each other.

Rectal apparatus ( Figs 3k /m–16k/m; 19k/m) with two dorsal and one ventral sclerites, does not seem useful in diagnosis, being extremely uniform in shape and habit. In general, apodemes of ventral sclerite oriented outward. Ventral sclerite always ribbon-like, quite high, the anterior margin basically almost straight, posterior one more or less arched. Apodemes of dorsal sclerites membranous, hyaline, bent inwards.

Diagnosis: The genus Metallactus is closely related to Griburius Haldeman, 1849 but it can be distinguished by the following characteristics: body mostly cylindrical, only faintly depressed on pronotal and elytral disc (often distinctly depressed both on pronotum and elytra in Griburius ); pronotum lengthened, PL/PW ratio greater than 0.65 (less than 0.55 in Griburius ); elytral lateral edge only feebly excised, abdomen not exposed posteriorly; colouration never metallic; sexual dimorphism not apparent either in ocular distance or in the structure of fore legs; prosternal process narrow, generally coarsely punctate and covered by long setae, shallowly grooved along its whole length, with lateral edges salient and fairly sinuate in the middle (in Griburius prosternal process is larger, not grooved longitudinally and not restricted in the middle, i.e. lateral margins almost straight); aedeagus never depressed in antero-dorsal direction; endophallus extremely characteristic, as described above. Apodeme of ejaculatory sac is lengthened, whip-shaped, brownish. Its shape in contrast with the one detected genus Griburius , which is short and ball-shaped in the examined species (unpublished data). On the other hand, a true apodeme seems to be missing both in American and in Palearctic species of Pachybrachis . This might represent, therefore, a useful diagnostic character at the genus level.

Female genitalia, in particular the shape of the spermatheca, are quite uniform throughout the species and seem not to be useful for diagnostic purposes.

Among the species of Metallactus defined by the above new diagnosis it is possible to track down a number of relatively homogeneous species-groups based on the external morphology, the chromatic design, and the general shape of the median lobe of the aedeagus. Even though, according to the present state of knowledge, it is not always possible to attribute a true phylogenetic meaning to these groups, it is believed that they can be proposed provisionally, given the practical utility they offer in organizing the systematic review of the whole genus. The following one is a first group of species that includes the type species of the genus.

Figures

FIGURES 3–4. M. kollari (3), M. albipes (4). 3a–b: habitus; 3c–e: median lobe of aedeagus in ventral (c), dorsal (d) and lateral (e) view [1.4 mm]; 3f: sclerites II–III–IV of endophallus [1.2 mm]; 3g: sclerite I and dorsal spicule of endophallus [0.8 mm]; 3h: spermatheca [0.9 mm]; 3k–m: rectal apparatus in dorsal (k) and ventral (m) view [1.3 mm]. 4a–b/a1: habitus; 4c–e: median lobe of aedeagus in ventral (c), dorsal (d) and lateral (e) view [1.1 mm]; 4f: sclerites II–III–IV of endophallus [0.8 mm]; 4f1: same of 4f in ventral view; 4g: sclerite I and dorsal spicule of endophallus [0.3 mm]; 4h: spermatheca [1.2 mm]; 4k–m: rectal apparatus in dorsal (k) and ventral (m) view [1.3 mm]. 3a–b: lectotype; 4a–b: lectotype. 4a1 paralectotype of M. nigrofasciatus n. syn. hd: hairy dent: scI–IV: sclerites I–IV of endophallus; ds: dorsal spicule; lt: lateral thickenings. Square brackets: length of scale segment.