Premnobius, Eichhoff, 1878

Atkinson, Thomas H., Petrov, Alexander V. & Flechtmann, Carlos A. H., 2018, New records and combinations in Neotropical Premnobius Eichhoff (Coleoptera: Curculionidae: Scolytinae: Ipini) with an illustrated key to New World species, Insecta Mundi 658, pp. 1-11 : 3-4

Key to females of Premnobius in the New World

The Neotropical native species of Premnobius are clearly unlike any other species of ambrosia beetles known from the Neotropics and are recognizable by the long slender body with a concave declivity with all prominences on the margins of the declivity. There is a superficial resemblance to Dinoxyleborus Smith (2017) (Xyleborini) because of the elongate, slender form, but are distinguishable because of the acute marginal spines, antennal characters and impressed submentum ( Smith 2017). There is also a resemblance to some species of Sampsonius Eggers, 1933 (Xyleborini) , but species in this genus typically have two prominent teeth on the anterior margin of the pronotum, and the elytral declivity is flattened, generally with teeth on the declivital face and the submentum is also impressed.

Neotropical Premnobius species differ from those of Acanthotomicus Blandford, 1894 (Ipini) by the elongate cylindrical pronotum which is at least 2/3 the length of the elytra with an elevated antero-lateral margin ( Fig. 1–4 ). All species of Premnobius express strong sexual dimorphism, with flightless males, while in Acanthotomicus they are normal. The four species treated here have dense pubescence on the lateral margins of the declivity with the most prominent tubercles or projections on the lower part of the declivital crest. The pronotum of species Acanthotomicus is shorter, without the raised antero-lateral margin. The declivity of Acanthotomicus has numerous small tubercles on the lateral margins of excavated area; those of the declivity of the male are larger than those of the female. The largest tubercles are on the upper part of the declivity and abundant pubescence is lacking.

The following key will distinguish all females of all species of Premnobius known from the New World, both native and exotic.

1. Declivity with a pair of quadrate spines on lower lateral margins, their height subequal to width at base; spines on lateral margins often blunt or digitate ( Fig. 1 , 2 , 3 , 4 )............... 2

— Declivity without pair of quadrate spines on lower margins; all granules or spines on lateral margins acutely pointed ( Fig. 5 , 6 ).............................................. 4

2(1). Quadrate elevation near middle of declivity in lateral view, slightly displaced medially from lateral crest ( Fig. 3 )............................................ P. flechtmanni (Wood)

— Quadrate elevations on posterior 1/3 of declivity in lateral view, on lateral crest ( Fig. 1 , 2 , 4 ) 3

3(2). Lateral margin of declivity with long slender, digitate projection on base of declivity; height of projections subequal to that of quadrate elevation on lower margin ( Fig. 2 )....................................................... P. assiduus (Schedl)

— Lateral margins of declivity without projections on base of declivity; a pair of small pointed tubercles may be present on the declivital face, mesad of lateral margin ( Fig. 1 , 4 )....... 4

4(3). Prothorax elongate, length 1/3 of total body length; declivity more abrupt, occupying ¼ of elytral length in dorsal aspect; in lateral view quadrate elevation separated from lower margin by less than 3× its width ( Fig. 1 ) P. perezdelacrucei Petrov and Atkinson

— Pronotum less elongate, length ¼ total body length; declivity more gradual, occupying 1/3 of elytral length in dorsal aspect; in lateral view quadrate elevation separated from lower margin by more than 5× its width ( Fig. 4 ).......................... P. neoadjunctus (Schedl)

5(1). Interstria 1 on declivity with a row of small pointed tubercles; raised lateral margin of declivity with small pointed granules; transition from elytral disc to declivity pronounced, not gradual ( Fig. 6 )................................................. P. cavipennis Eichhoff

— Interstria 1 on declivity without any granules; several large, pointed tubercles near base and midpoint of declivity on lateral margin; base of declivity extends gradually anteriorad at base along insterstria 1 ( Fig. 5 ).............................. P. ambitiosus (Schaufuss)

Figures

Figure 1. Premnobius perezdelacrucei Petrov and Atkinson. female paratype. A) Dorsal view. B) Lateral view. C) Frontal view. D) Declivity. Photos by T.H. Atkinson.

Figure 2. Premnobius assiduus (Schedl), female paratype. A) Dorsal view. B) Lateral view. C) Frontal view. D) Declivity. Photos by T.H. Atkinson.

Figure 3. Premnobius flechtmanni (Wood), female. A) Dorsal view. B) Lateral view. C) Frontal view. D) Declivity. Photos by T.H. Atkinson.

Figure 4. Premnobius neoadjunctus (Schedl), female holotype. A) Dorsal view. B) Lateral view. C) Frontal view. D) Declivity. Photos by T.H. Atkinson.

Figure 5. Premnobius ambitiosus (Schaufuss), female. A) Dorsal view. B) Lateral view. C) Frontal view. D) Declivity. Photos by T.H. Atkinson.

Figure 6. Premnobius cavipennis Eichhoff, female. A) Dorsal view. B) Lateral view. C) Frontal view. D) Declivity. Photos by T.H. Atkinson.