Metallactus generosus Suffrian 1866
Creators: name: Sassi, Davide
Publisher: name: Zenodo
Year: 2020
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Coleoptera
Chrysomelidae
Metallactus
Metallactus generosus

description: Metallactus generosus species-group
The brief description that follows is based on males. Medium sized, body length of males 4.4–5.2 mm, of females 4.8–6.0 mm; sexual dimorphism not distinct; colour: head black, with yellow median spot on vertex; pronotum yellow with paired black streaks, to entirely black with yellow posterior angles; elytron with dark median stripe present, from base to beyond the apical clivus, but not reaching the elytral apex, which always remains yellow, extent of lateral yellow margin variable; venter and legs black, or abdominal ventrites laterally and patches on legs yellow; body cylindrical, weakly depressed on elytral disc; vertex wide, flat, with weak to moderate punctation and evident midcranial suture; ocular lines poorly defined; frontoclypeal suture barely detectable. eyes large, reniform, slightly protruding with moderately deep canthus, well separated along their entire median margin, interocular distance only slightly greater in females than males; upper lobes of eyes not convergent to midline; antennae unremarkable (compare with Sassi 2018), slightly longer in males; pronotum vaguely tronco-conical, with posterolateral impressions feeble or absent; pronotal punctation shallow, sparse medially, noticeably denser laterally; pronotal hypomeron generally sublucid, almost glabrous, with sparser and more strongly impressed punctuation than in other parts of ventral surface; prosternal process typical of Metallactus, narrow, grooved, with arched sides and surface covered with long dense setae; elytra feebly convergent to apex in dorsal view, with narrow lateral margins and raised humeri; elytral punctation shallow, only partially arranged in irregular rows; elytral epipleuron smooth, slightly convex, impunctate; pronotal/elytral length ratio about 0.5; elytral lateral margin weakly excised; pterothoracic and abdominal ventrites covered with tiny punctures and short dense whitish setae; pygidium smooth, opaque and covered by minute punctation and short, dense setae; legs without modifications; shaft of aedeagal median lobe cylindrical with surface smooth and devoid of peculiar structures (M. argentinensis sometimes with feeble longitudinal carina on ventral surface); median lobe apex obtusely triangular, blunt, scarcely differentiated from shaft, straight or feebly bent ventrally in lateral view; setose depressions (“hairy dents” in Sassi, 2018) weakly impressed and scarcely delimited, bearing scattered setae; ventral profile moderately swollen in lateral view; number and general shape of endophallic sclerites typical of Metallactus (Sassi, 2018); spermathecal duct long, thin, delicate and coiled, mid section forming a complex tangle near vasculum, but section near bursa copulatrix almost straight or slightly sinuous, well developed, and moderately thickened and pigmented just before junction with bursa copulatrix; vasculum sickle-shaped.
Comments. The overall appearance of the three species is reminiscent of M. kollari and M. hamifer speciesgroups (Sassi 2018; Sassi, 2019), which are most probably related. These species groups all share a simple aedeagal form.
The M. generosus species-group is distinguished externally from these others by the overall colouration of the dorsal parts. The black sutural stripe has not been observed in the other species groups, except rarely in some unusual chromatic forms of Metallactus kollari (Perty, 1832).
Some species described by Suffrian with extensive black elytral pattern may appear similar to the species under study, but their median lobes are noticeably different, with evident protrusions on the ventral surface. I regard the genitalic differences as critical for the definition of the species groups and therefore will treat these species separately. Females are more difficult to deal as they are chromatically more variable than males and lack any diagnostic genitalic characters. Female genitalia are not very useful in species discrimination, as the spermatheca and rectal apparatus appear to vary as much intraspecifically as interspecifically, but few specimens were available for study. Further study may reveal some useful female characters.
To summarize, the chromatic characteristics, together with the external morphology of the median lobe of aedeagus, which in the taxa under examination is extremely simplified, allow separation of the M. generosus speciesgroup from all the others currently known and attributable to Metallactus.
Distribution. (Figs 5–7). The M. generosus species-group is found in central and eastern South America (southern Brazil, Paraguay, Uruguay and northern Argentina). The overall pattern of distribution is similar to those of the M. kollari and M. hamifer species-groups (Sassi, 2018; Sassi, 2019).
description: Published as part of Sassi, Davide, 2020, Revision of the Metallactus generosus species-group with a preliminary evaluation of the effectiveness of the endophallus morphology in distinguishing critical sibling species (Coleoptera: Chrysomelidae: Cryptocephalinae), pp. 148-162 in Zootaxa 4877 (1) on page 150, DOI: 10.11646/zootaxa.4877.1.6, http://zenodo.org/record/4423973