Abacaecus walterrossii, Allegro & Giachino, 2020

Allegro, Gianni & Giachino, Pier Mauro, 2020, Abacaecus n. gen. walterrossii n. sp., a notable hypogean carabid species from Sierra Leone, Western Africa (Coleoptera, Carabidae, Pterostichinae), Zootaxa 4766 (3), pp. 435-446 : 437-442

Abacaecus walterrossii n. sp.

( Figs. 1 , 2 , 3 , 4 )

Type locality. Sierra Leone, Eastern Province, near Lalehun, Gola Rainforest National Park (7° 39.595’N, 10° 56.308’W).

Type Series. Holotype ♂: Sierra Leone, Eastern Province, near Lalehun, Gola Rainforest National Park , 2.XI.2013, Walter Rossi legit (CGi).

Differential diagnosis. A medium-sized (ABL: 9.56 mm) wingless abacetine with eyes strongly reduced, scarcely depigmented body, a single pair of supraorbital setae, mentum tooth with simple rounded apex, second antennomere a little eccentrically inserted on first, a scutellar setigerous pore at base of stria 2, deeply impressed elytral striae, discal setae on elytra absent, apex of each elytron forming a distinct tooth, prosternal intercoxal process lozenge shaped, dorsal face of tarsomeres smooth and with a distinct medial carina, depressed at sides, mentum tooth not exceeding level of epilobes, paramedial pits of mentum small, ostium long and wide, almost reaching basal bulb. It is easily distinguished from the unique other blind African species belonging to Pterostichinae (excluding Drimostomatina) recorded to date, Pterostillichus caecus , which is characterized by smaller size (ABL: 7.15 mm), bifid mentum tooth, second antennomere almost centrally inserted on first, absence of parascutellar setigerous pore at base of stria 2, elytral striae nearly obsolete, prosternal intercoxal process widely rounded, dorsal face of tarsomeres rugulose and without medial carina, paramedial pits of mentum large, ostium long, but narrow and not reaching basal bulb.

Description. Habitus: ABL: 9.56 mm (HT ♂). Body parallel-sided, moderately shiny, concolorous reddish brown with head, antennae and tibiae a little darker (fig. 1). Female unknown.

Head longer than wide, narrow in comparison with pronotum (fig. 3a); eyes strongly reduced, only a small vestigial depigmented area is visible, without any evidence of ommatidia (fig. 2a); a single pair of supraorbital setae; dorsum densely and delicately punctate; microsculpture not evident except at high magnification; frontal impressions deep, elongate divergent, ending at mid distance between clypeofrontal suture and supraorbital seta; tempora superficially wrinkled (fig. 2a); a marked supraorbital carina dilated above antennal insertion; labrum rectangular, with six setigerous punctures on anterior margin; clypeus trapezoid, rectilinear anteriorly, with two setigerous punctures closer to lateral margins than to anterior one.Antennae moderately long, densely pubescent from second fourth of segment 4, with terminal three articles surpassing base of pronotum; second antennomere a little eccentrically inserted on first; mandibles elongate, with well defined and glabrous scrobe and apex pointed and slightly hooked; mentum (fig. 2b) clearly broader than long, anterior margin shallowly emarginate; epilobes rather broad, laterally prominent; mentum tooth simple, widely rounded and not exceeding level of epilobes; mentum glabrous, except for two paramedial setae; paramedial pits distinct but small; suture between mentum and submentum present; submentum with two pairs of setae, one medial and one lateral; labial and maxillary palps fusiform; labial palpomere 2 with 2 large medial setae.

Thorax: PL: 2.34 mm; PW: 2.83 mm. Pronotum densely and minutely punctate throughout, without any distinct microsculpture even at>50x magnification; form subquadrate, broader than long at middle, widest at anterior third; side margins hardly rounded at anterior half and delicately sinuate backwards; hind angles right angled and lacking of seta; posterior margin slightly concave between basal impressions, which are linear and markedly impressed (fig. 2c); anterior margin with front angles acute and markedly protruding (fig. 3a); a pair of lateral setae at anterior third; lateral marginal bead continuous and dilated before hind angles; anterior submarginal sulcus very superficial; medial longitudinal sulcus fine, ending well before apex and base.

Elytra: EL: 5.37 mm; EW: 3.10 mm. Parallel-sided, moderately convex and flattened on disk, widest at base, with right-angled shoulders (fig. 2c); distinctly sinuate before apex. Surface moderately shiny; microsculpture not evident except at high magnification. Epipleura with distinct external plicae (‘crossed epipleura’). Intervals moderately convex, minutely and sparsely punctate; striae deeply impressed for whole length, delicately punctate. Parascutellar stria connected to stria 1; basal section of stria 1 absent; scutellar setigerous pore present at base of stria 2 (fig. 2c). Basal margin incomplete, reaching level of stria 1. Each elytron ended apically with a distinct tooth (fig. 3b). Discal setigerous punctures absent; stria 7 with two setigerous punctures near apex; umbilicate series of 7 (humeral) + 4 (preapical) + 3 (apical) setigerous punctures in stria 8, widely interrupted between the humeral and preapical group. Hind wings absent.

Ventral surface (thorax and abdomen): prosternum and proepisterna glabrous, minutely punctate. Mesosternum densely and rugosely punctate. Metepisterna short, delicately punctate; metepimera large, broadly rounded. Prosternal intercoxal process lozenge shaped, unbordered, blunt at apex (fig. 2e). Sterna shiny, glabrous except one pair subapical setae at apex of VII (HT ♂).

Legs moderately slender. Dorsal face of femora with 3-4 short setae; posterior face with 1 seta in profemora, 2 in mesofemora and 0 in metafemora. Metatrochanters glabrous and slightly shorter of half length of metafemora. Protibial antennal clearing organ well developed, with 2 clip setae. Protibiae robust, slightly bent inwards at apical fifth, with a delicate longitudinal keel at inner face, a strong apical spur, 3 outer apical spines and a small apical tooth; mesotibiae with a dense group of finer setae just before middle at inner face; metatibiae longitudinally furrowed. Dorsal face of tarsomeres smooth, with a delicate medial carina, depressed at sides (fig. 2d). Protarsomeres 2-4 triangular, distinctly dilated and delicately bidentate at apex, slightly asymmetrical (HT ♂); mesotarsomeres (at a lesser extent bidentate at apex too) and metatarsomeres 2-4 less dilated and symmetrical; tarsomere 5 ventrally glabrous, dorsally with two setae; claws smooth.

Male genitalia: median lobe of aedeagus with moderately enlarged basal bulb and medioapical part distally almost rectilinear in lateral view, slightly bent downwards at apex (fig. 3c); median lobe with spatulate apical lamella, almost truncate at apex in dorsal view (fig. 3d). Ostium long, reaching basal bulb. Right paramere elongate, narrowing towards apex and blunt at tip; left paramere oval, blunt at tip.

Remarks. The strongly transverse mentum with anterior margin shallowly emarginate, as well as the second antennomere eccentrical (even if only moderately) with respect to first, both likely synapomorphic features for Abacetini ( Will & Park, 2008) , together with the presence of the parascutellar setigerous pore at base of stria 2, provide reasonable evidence to support the taxonomic arrangement of A. walterrossii n. sp. inside the tribe. In any case, the set of its morphological traits as well as its hypogean life habit, almost unique inside the tribe (see also remarks concerning Pterostillichus caecus ), suggest it belongs to a separated phyletic lineage. Other features shown by A. walterrossii n. sp. occasionally appear in other species of Abacetini : for instance the apically toothed elytra (see Metabacetus vandoesburgi Straneo, 1948 from Java, in Will & Park, 2008), the carinate dorsal surface of tarsi (African species of the genus Abacetus ) and the triangular and bidentate protarsomeres 2-4 (African genus Distrigidius , see Jeannel, 1948). At present knowledge, without any molecular support, it is really hard to speculate about the closest relatives of this species.

The most evident adaptive trait to the hypogean life of Abacaecus is microphthalmy, but other morphological features are also consistent with this way of life, such as the elongate form of body, very different from what is seen in most Abacetini , or the markedly protruding front angles of pronotum, which are present in many other species adapted to the subterranean environment and are best interpreted as the result of convergent evolution. It is quite impressive, in this sense, the habitus resemblance of A. walterrossii n. sp. with the Iberian Pterostichini Molopina Zariquieya troglodytes Jeannel, 1924 and Z. boumortensis Faille, Fresneda & Bourdeau, 2011 , Oscadytes rovirai Lagar, 1975 and Henrotius jordai (Reitter, 1914) , all of them adapted to hypogean life and phylogenetically not closely related to Abacetini .

Distribution and ecology. Geographical distribution: this species is recorded only from Gola Rainforest National Park, near Lalehun, Eastern Province, Sierra Leone, Africa.

Life habits: the unique specimen was collected in the banks of a water stream (fig. 4) covered by lush vegetation during wet season (November); the strongly reduced eyes indicate an adaptation of this species to the hypogean environment.

Etymology. This species is dedicated to its collector, Walter Rossi, a world renowned specialist in entomoparasitic fungi, as a token of our esteem and as a sign of gratitude for the gift to the authors of the specimen of the new species.

Figures

FIGURE 1. Abacaecus walterrossii n. sp., image of holotype habitus (scale bar = 1 mm).

FIGURE 2. SEM images of body parts of Abacaecus walterrossii n. sp.: a) head in right lateral view, b) mentum, c) base of pronotum and elytra, d) metatarsum, e) prosternal intercoxal process.

FIGURE 3. Optical microscope images of Abacaecus walterrossii n. sp.: a) head in dorsal view, b) apex of elytra, c) aedeagus in left lateral view, d) aedeagus in dorsal view.

FIGURE 4. Temporary stream in Gola Rainforest National Park, Sierra Leone (type locality of Abacaecus walterrossii n. sp.) during the dry season.