Melanesicus Beaver & Petrov, 2021

Beaver, R. A., Petrov, A. V. & Sittichaya, W., 2021, A new genus of ambrosia beetle from Melanesia (Coleoptera: Curculionidae Scolytinae: Xyleborini), Zootaxa 4949 (1), pp. 163-172 : 164-165

Genus Melanesicus Beaver & Petrov , gen. n.

Type species: Xyleborus partitus Browne 1974: 69 .

Diagnosis. The genus Melanesicus clearly belongs in the tribe Xyleborini ( Wood 1986; Alonso-Zarazaga & Lyal 2009). It is distinguished morphologically from other genera in the tribe by the following combination of characters: 1) antennal funicle 4-segmented (excluding pedicel), club of type 3 ( Hulcr et al. 2007; Smith et al. 2020); 2) pronotum subquadrate in form as viewed from above, as wide as or slightly wider than long, the anterior margin transverse, without asperities, emarginate in the middle when viewed from in front, lacking mycangial tufts at the base; 3) scutellum linguiform, flush with the surface of the elytra or almost so; 4) elytra divided into sharply separated smooth and shining (anterior), and matt (posterior) areas, the latter armed with numerous granules; 5) sclerolepidia absent from dorsal margin of metaventrite; 6) procoxae contiguous; 7) protibiae subtriangular, widest at apical third, the outer margin with 7‒13 small, closely set, socketed teeth in apical two-thirds, the posterior surface without granules.

Description. Female. Antenna with antennal funicle 4-segmented (excluding pedicel), first segment of club occupying one-third to one half of anterior face, its anterior margin concave or sinuate, segment 2 corneous, two sutures visible on posterior face. Eyes coarsely faceted, shallowly to strongly emarginate, upper part smaller than lower part or approximately equal in size. Frons plano-convex, usually with a median, smooth, shining, almost impunctate area extending from anterior margin to upper margin of eyes. Submentum triangular, impressed.

Pronotum subquadrate, as wide as or a little wider than long, anterior angles strongly curved, anterior margin almost transverse, without asperities, emarginate in middle when viewed from in front; anterior slope weakly impressed in middle near anterior margin, armed with very numerous, tiny, pointed asperities, which become more transversely elongate towards summit in middle of pronotum; disc smooth, subnitid or dull, finely, not very densely punctured. Scutellum linguiform to triangular, shining, flush with elytral surface or almost so.

Elytra 1.5‒1.6 times longer than wide, 1.45‒1.8 times as long as pronotum; viewed from above sharply divided into basal smooth, shining, and apical, matt, granulate parts (a deep, saddle-like transverse impression just before declivity in one species); basal part bearing fine strial and interstrial punctures, the serial arrangement rather indistinct; declivity strongly convex, with granules on striae and interstriae, or only on interstriae, or without a distinct arrangement; interstriae 1 broadened and raised at least near apex of declivity, bearing larger granules and projecting beyond apex in lateral view. Metanepisternum narrow with one or two rows of seta-bearing punctures. Metaventrite without a row of sclerolepidia on dorsal margin.

Legs with procoxae contiguous, sternellum swollen, protibiae subtriangular with numerous (7‒13), small, closely set, socketed teeth in apical two-thirds, the posterior surface without granules. Meso- and metatibiae flattened, strongly widened in middle, with numerous, small, socketed teeth.

Male. The male has been described only for M. caledoniae ( Beaver & Liu 2016) . It is generally similar to the female, but smaller and stouter, the eyes are reduced, the pronotum relatively wider, and the elytra relatively shorter. The sculpture of the matt area of the elytra differs in detail from that of the female.

Distribution. The genus is distributed in the Melanesian region from New Guinea to Vanuatu, New Caledonia and Fiji.

Relationships. There is a superficial resemblance to Schedlia Browne, 1950 , in the quadrate shape of the pronotum with a transverse anterior margin, and the division of the elytra into anterior shining, and posterior matt and granulate parts, but Melanesicus has a distinct flat scutellum, and the posterior surface of the protibiae is not granulate. In Schedlia , the scutellum is not visible, the protibiae are granulate posteriorly, and mycangia are present in the elytral bases ( Hulcr & Cognato 2013). The position of the mycangia in Melanesicus is not certain, but they are probably oral. The only species of Melanesicus in which DNA has been investigated ( M. granulosus ) grouped in a weakly supported clade with a species of Ancipitis Hulcr & Cognato, 2013 ( Cognato et al. 2011, 2020). However, the relationship of the two genera seems likely to be distant. As noted by Hulcr & Cognato (2013), Ancipitis has a number of dryocoetine-like characters including a flat submentum, a long ventral side to the head, and narrow tibiae with few large denticles in enlarged sockets, characters which are absent in Melanesicus .

Biology. The biology of Melanesicus has been investigated only in M. partitus and M. deformatus ( Roberts 1977) . The gallery system consists of multiply branched tunnels in one transverse plane. The eggs are laid in clutches, initially of 4 to 6, but later of up to 12 or 14, in the branches, and the larvae of each batch appear to develop and pupate within the branch in which they hatched. The adults of the new generation move around the nest, and undergo a period of maturation feeding within the nest before emergence. Brood sizes in mature galleries can be very large (at least 150 individuals) in M. partitus , but only smaller broods were observed in M. deformatus . The males develop faster than the females. Roberts (1977) found a sex ratio of 4− 7 females: 1 male in young broods, but it is likely that the ratio increases in older galleries.

The occurrence of maturation feeding within the parental gallery by the new generation females implies the possibility that they are also involved in brood care, gallery maintenance and fungal gardening ( Biedermann & Taborsky 2011; Biedermann et al. 2011). Given the large size of some gallery systems of M. partitus investigated by Roberts (1977), it is also possible that some may breed within the parental nest rather than dispersing to found new nests ( Biedermann et al. 2012).

Key to Melanesicus species (females)

1 Elytra strongly, transversely impressed just before declivity. Declivity almost vertical with two rows of small, stout, pointed teeth on upper margin. 2.9‒3.1 mm long. Fiji............................................... deformatus (Browne)

‒ Elytra not strongly impressed before declivity. Declivity less strongly sloping, without rows of pointed teeth on upper margin. ................................................................................................... 2

2 Granules on elytral declivity arranged in uniseriate rows along striae and interstriae, or on interstriae alone.............. 3

‒ Granules on elytral declivity irregularly scattered over surface, at least on apical half of declivity, so that striae and interstriae are obscured......................................................................................... 5

3 Granules on elytral declivity large, almost equal in size on striae and interstriae. 3.5‒3.75 mm long. New Caledonia....................................................................................... caledoniae (Beaver & Liu)

‒ Granules on elytral declivity smaller on striae than on interstriae or absent........................................ 4

4 Strial granules on elytral declivity minute or absent. 3.6‒3.8 mm long. Fiji........................... partitus (Browne)

‒ Strial granules on elytral declivity smaller than those on interstriae, but distinct. 3.95‒4.0 mm long. Vanuatu............................................................................................... tishechkini Petrov sp. n.

5 Larger species, 3.2 mm long. Granules on elytral declivity confused over whole surface, striae and interstriae obscure. New Guinea............................................................................... granulosus (Schedl)

‒ Smaller species, 2.45 mm long. Granules on elytral declivity in strial and interstrial rows near upper margin, becoming confused in apical half of declivity. Fiji................................................... nukuruanus Beaver sp. n.