Meladema imbricata (Wollaston, 1871)

Bilton, David T. & Ribera, Ignacio, 2017, A revision of Meladema diving beetles (Coleoptera, Dytiscidae), with the description of a new species from the central Mediterranean based on molecules and morphology, ZooKeys 702, pp. 45-112 : 73-76

Meladema imbricata (Wollaston, 1871) Figures 3C, 4C, 5C, 11C, 12C, G, 13C, 14C, 15C, 23, 24A, 25D, F

Scutopterus imbricatus Wollaston, 1871: 220.

Meladema imbricata (Wollaston, 1871): Sharp 1882: 824; Régimbart 1895: 184; Machado 1987: 58; Balke et al. 1990: 364.

Meladema lanio ab. imbricata (Wollaston, 1871): Gcshwendtner 1936: 42.

Meladema imbricatum Branden, 1885: 95.

Meladema lanio f. imbricata Sanfilippo, 1966: 49.

Type locality.

“Madeira” [mislabelled].

Type material

(BMNH). Holotype ♀ (Figure 24A): " Scutopterus // imbricatus , Woll" [HW] Scutopterus // imbricatus Woll// M.E.Bacchus det 1977// HOLOTYPE" [Latin name, describer & last 7 HW] "Holo-// type" [small, circular label, red margin] " Meladema // imbricata (Woll.)// M. Balke det. 1989" [Latin name, describer & 89 HW] (dry pinned, BMNH, Wollaston Collection).

Note that as discussed by Machado (1987), the type specimen must have been mislabelled, as this species is now known to be endemic to the Canary Islands.

Additional material examined

(genotyped specimens). Spain, Canary Islands. 1 ♂ "1998 SPAIN Islas Canarias// La Gomera// El Cedro - stream in laurysilva// D. T. Bilton leg." "M. IMBRICATA" [HW] "G4 Mel// below G3" [HW] "DNA voucher// NHM-IRM3A" (IBE); 1 ♂ "1998 SPAIN Islas Canarias// La Gomera// El Cedro - stream in laurysilva// D. T. Bilton leg." "M. IMBRICATA" [HW] "G1 Mel" [HW] "DNA voucher// NHM-IRM4A" (IBE); 1 ♂ "1998 SPAIN Islas Canarias// La Gomera// El Cedro - stream in laurysilva// D. T. Bilton leg." "DNA voucher// NHM-IRM4b" (IBE); 1 ♀ "15/i/2000 SPAIN Islas Canarias// La Gomera// El Cedro// D. T. Bilton leg." "DNA voucher// NHM-IRM15A" (CBP); 1 ♂ "15/i/2000 SPAIN Islas Canarias// La Gomera// El Cedro// D. T. Bilton leg." "DNA voucher// NHM-IRM15B" (CBP); 1 ♂ "April 1999 SPAIN// Islas Canarias La Palma// Bco. Hoyo Verde Caldera de// Taburiente D. T. Bilton leg." "DNA voucher// NHM-IRM6A" (CBP); 1 ♀ "April 1999 SPAIN// Islas Canarias La Palma// Bco. Hoyo Verde Caldera de// Taburiente D. T. Bilton leg." "DNA voucher// NHM-IRM6B" (CBP); 1 ♀ "April 1999 SPAIN// Islas Canarias La Palma// Bco. Hoyo Verde Caldera de// Taburiente D. T. Bilton leg." "DNA voucher// NHM-IRM6C" (CBP); 1 ♂ "April 1998 SPAIN// Islas Canarias La Palma// Bco. del Rio upper reaches in// Laurisylva D. T. Bilton leg." "DNA voucher// NHM-IRM7A" (CBP); 1 ♂ "1998 SPAIN Islas Canarias// Tenerife// Barranco del Río 1,600m// D. T. Bilton leg." "M. IMBRICATA" [HW] "T7// Mel." [HW] "DNA voucher// NMH-IRM5A" (IBE); 1 ♂ "1998 SPAIN Islas Canarias// Tenerife// Barranco del Río 1,600m// D. T. Bilton leg." "DNA voucher// NMH-IRM5D" (IBE); 1 ♂ "13/i/2000 SPAIN Islas Canarias// Tenerife// Barranco del Rio 1,600m// D. T. Bilton leg." "DNA voucher// NHM-IRM17A" (CBP); 1 ♀ "13/i/2000 SPAIN Islas Canarias// Tenerife// Barranco del Rio 1,600m// D. T. Bilton leg." "DNA voucher// NHM-IRM17B" (CBP). All with " Meladema imbricata // (Wollaston, 1871)// D T Bilton [or I Ribera] det. 2017".

Additional material examined (non-genotyped specimens). Spain, Canary Islands. 1 ♂ "April 1998 SPAIN// Islas Canarias la Gomera// El Cedro stream in Garajonay// laurisylva D. T. Bilton leg." (CBP); 1 ♀ "April 1998 SPAIN// Islas Canarias La Palma// Bco. del Río upper reaches in// Laurisylva D. T. Bilton leg." (CBP); 1 ♂ "Islas Canarias: Tene-// rife, 9.-10.vi. 1989// Bco. del Río 1100m.// Balke & Hendrich leg." " Meladema // imbricata // M. Balke det 2011" "M. Balke// BMNH(E) 2013-119" (BMNH); 1 ♂ "I. Canarias/ Tenerife// Bco. del Rio, 1400m// 9.-10.6.1989, Bach// Balke/ Hendrich, leg." " Meladema // imbricata Woll.// HENDRICH det. 1995" [Latin name & describer HW] (CBP); 2 ♂♂, 2 ♀♀ "9.-10.6.89 Islas Canarias// Tenerife, Barranco// del Rio, ca. 1000m// Balke, Hendrich, Fery leg." [HW] " Meladema // imbricata // Woll.// Fery det. 89" [HW] (CBF); 2 ♂♂, 1 ♀ "ESP. Tenerife// Bco. del Río 1600m// 2.xi.1991 AN Nilsson" " Meladema imbricata // (Wollaston, 1871)// Det. AN Nilsson 1991" (CBP); 1 ♂ "April 1998 SPAIN// Islas Canarias Tenerife// Bco. del Río Upper// Reaches D. T. Bilton leg" (CBP). All with " Meladema // imbricata (Wollaston, 1871)// D T Bilton [or I Ribera] det. 2017".

Description

(based on all material examined).Size: Holotype TL = 22.13 mm; EL = 15.79 mm; MW = 10.22 mm. Other material examined TL = 18.05-21.38 mm; EL = 13.57-15.62 mm; MW = 8.83-9.98 mm.

Colour. Dorsum (Figure 3C), dark reddish brown to yellow. Labrum yellowish; clypeus yellowish except central 1/4 red to blackish, connected to dark pigmentation on frons. Frons with transverse pale strip anterolaterally, adjacent to pale parts of clypeus, otherwise dark reddish brown. Medial, paired interocular patches on frons yellow; strongly transverse apicolaterally, almost reaching channel around interior margin of compound eye. Pronotum dark reddish brown on disc; narrowly reddish along anterior margin; lateral margins broadly yellowish to pale red. Elytra yellowish brown, with black irrorations; pattern much more clearly visible when lifted (Figure 4C). Legs yellowish brown to black; posterior tibiae and tarsi darkest. Antennae and maxillary and labial palpi yellowish to reddish. Venter reddish brown; prementum and posterior genae yellow; mentum and submentum reddish. Pronotal hypomeron and shoulder, outer portion of elytral epipleurs and apex of metacoxal process yellowish.

Head. Labrum shining, with medium to fine, sparse punctures. Reticulation absent anteriorly, clearly evident in posterior half, here fine and composed of small, isodiametric to slightly transverse meshes. Clypeus weakly shining, with medium to fine, sparse punctures and traces of very fine, shallow, close punctures. Frons weakly shining, entire surface with coarse, open reticulation, becoming stronger and more evident posteriorly. Meshes transverse to isodiametric apically and medially, strongly elongate posteriorly and onto vertex. Paired epicranial foveae on anterior frons, one immediately behind the other. Anterior foveae transverse, posterior foveae elongate oval. Foveae all strongly reticulate; anterior and posterior foveae linked by reticulated channel. Internal and posterior borders of compound eyes distinct, raised relative to level of adjacent cuticle. Lateral margins bordered by distinct narrow channel; deeper anteriorly than posteriorly and continuing behind posterior margin of eye onto vertex. Channel with dense punctures, bearing long, stiff, yellow recumbent to decumbent setae.

Pronotum. Posterior margin weakly sinuate laterally (Figure 3C). Surface somewhat shining, strongly rugose. Reticulation meshes large, open, flat and with varying sizes and orientations. Transverse irregular row of medium punctures bearing long, yellow recumbent to decumbent setae 1/5 behind anterior pronotal margin; interrupted briefly in centre, obscured by reticulation inside lateral margins but continuing inside lateral third of posterior margin. Reticulation weak and obsolete anterior to transverse row, surface here clearly doubly punctate, with very fine, dense and medium, sparse to very sparse punctures. Scattered medium punctures visible elsewhere, amongst meshes of reticulation. Centre of disc with traces elongate, narrow, slit-like fovea, typically obscured by reticulation, but traceable as an elongate reticulation chan nel. Lateral margins slightly raised, shining, without rugose sculpture and with fine, scattered punctures.

Elytra. Shining, with short, transverse, usually straight or weakly curved crescentic striolae of varying sizes and density (Figures 5C, 25D, F). Striolae shallow and moderate on shoulder and anterior disc, widely separated; becoming closer, larger and more curved posteriorly. Posterior third of elytra with an almost scaly appearance (Figure 3C); striolae here almost touching each other laterally. Surface between crescentic striolae doubly punctate and reticulate (Figure 5C); with very fine, close and medium, sparse to very sparse punctures (the latter bearing short, peg-like setae). Reticulation fine, somewhat obsolete, meshes isodiametric; more evident in posterior half, sometimes obscuring very fine punctation. Puncture rows well-marked, continuous almost to elytral apices; punctures shallower posteriorly than anteriorly.

Venter. Prementum shining, tumid in centre, with fine and medium, sparse punctures. Mentum shining; central projection with shallow median emargination. Lateral lobes with very fine, close punctures, scattered, whitish recumbent to decumbent setae and longitudinal wrinkles. Submentum shining, with transverse wrinkles. Central 1/4 with medium, sparse punctures bearing long, white-yellowish, erect setae. Gula shining, with sparse, shallow, transverse wrinkles; patch of medium-coarse punctures posterolaterally. Genae shining, strongly reticulate; meshes transverse anteriorly and posteriorly, almost isodiametric in centre. Prosternum shining, with weak, low irregular transverse ridges laterally. Arched in centre and with fine, moderate to close punctures laterally, bearing long, white-yellowish, recumbent to erect setae; punctures and setae extending in an irregular row onto process, just below arch. Process lanceolate, arched; apex acuminately rounded. Centre of prosternum and process with double punctation of very fine, close to very close and medium, sparse to moderate punctures. Pronotal hypomeron shining, impunctate. Elytral epipleurs shining, with fine wrinkles; irregular puncture row close to internal margin, from centre to close to apex, punctures bearing fine, whitish, erect setae. Metaventrite shining, central portion with reticulation reduced to sparse, transverse scratches and very fine, close and fine to medium, sparse punctures. Metaventral process strongly reticulate, with transverse to elongate, rugose meshes and traces of fine, sparse punctures; small central area with reticulation of very small meshes. Metaventral process relatively broad; apex acuminate and upturned slightly anterolaterally. Metacoxal lines not reaching anterior border of metacoxae, disappearing approx 1/10 from margin. Internal laminae of metacoxae shining, sculpture as on centre of metaventrite. Metacoxal lobes sculptured as internal laminae, strongly rounded, with irregular, elongate field of medium to coarse punctures close to lateral margins, bearing fine, white, recumbent to erect setae. External laminae of metacoxae shining, smooth close to process, but with strong reticulation elsewhere; reticulation meshes very elongate posteromedially, to transverse anteriorly. Abdominal ventrites shining. Ventrites 3-5 with cluster of golden, erect setae anteromedially. Ventrite 1 with elongate reticulation throughout. Ventrite 2 with similar reticulation; absent close to centre. Ventrite 3 with elongate reticulation laterally, becoming transverse close to smooth central 1/5. Reticulation of ventrites 4 and 5 restricted to lateral third and with superimposed elongate furrows. Ventrites 2-5 doubly punctate; very fine, moderate and fine, sparse to very sparse punctures; punctures most evident in areas without reticulation. Ventrites 3-5 with transverse irregular row of long, yellowish, recumbent to decumbent setae laterally. Ventrite 6 (Figure 11C) with very fine, moderate punctures and medium to coarse, sparse to moderate punctures; punctures coarser close to apex. Elongate, semicircular wrinkles and channels apicolaterally and centrally; apicolateral sculpture extending basally around central portion of ventrite. Some punctures in channels bearing elongate, whitish, erect setae.

Male. Foretarsi (Figure 12C) with articulo-setal counts as follows (base to apex): row 1, 8; row 2, 10; row 3, 10; row 4, 8. Curved, golden setae bordering articulo-setal field dense, particularly basally. Foretarsal claws (Figures 12C, 13C) elongate, curved. Mesotarsi (Figure 12G) with articulo-setal counts as follows (base to apex): row 1, 8; row 2, 9-10; row 3, 8-9; row 4, 7 (2 clusters, 4 on inner side, 3 on outer side, situated laterally). Curved, golden setae bordering articulo-setal field relatively dense, especially basally. Mesotarsal claws (Figure 12G) elongate, curved. Abdominal ventrite 6 (Figure 11C) with apex rounded, with well-marked median emargination. Median lobe asymmetrical (Figure 14C), sinuation strong, approximately 1/4-1/3 from apex; ventral margin of apical portion weakly concave in lateral view. Parameres (Figure 14C) with inner margin almost right-angled at base; outer and inner margins undulated slightly.

Female. As male, except for simple fore and mesotarsi and differently shaped abdominal ventrite 6 (with bluntly pointed apex - Figure 15C).

Variation. The size and density of the crescentic striolae on the elytra differs somewhat between individuals and localities (e.g. Figure 25D, F). At least some of this variation may be due to hybridization with M. coriacea (see below), making the extent to which this is truly intraspecific unclear. On La Palma, however, an island with no known populations of M. coriacea , and no genetic evidence of hybridization, the crescentic striolae are relatively very large and dense in some females (Figure 25F), approaching the situation seen in some females of M. lanio (see below).

Differential diagnosis.

Morphologically somewhat intermediate between M. coriacea / M. lepidoptera sp. n. and M. lanio . From M. coriacea and M. lepidoptera sp. n. M. imbricata can be distinguished on its different dorsal colouration, particularly the strongly mottled elytra, with much smaller, sparser crescentic striolae, as well as the less strongly sinuate posterior pronotal margin, details of the male genitalia (median lobe with sinuation further away from apex, with concave ventral margin in lateral view) and the last abdominal ventrites of both sexes. The habitus of M. imbricata is also typically more elongate than either of the above species (Figure 3). There are also additional minor differences in dorsal and ventral sculpture, as described above. With the exception of some females (see below), M. imbricata can be separated from M. lanio on the presence of crescentic striolae on the elytra. The male genitalia of the two species are also different, the sinuation of the median lobe of M. lanio occurring further from the apex than M. imbricata (see Figure 14C, D). M. imbricata also differs from all individuals of M. lanio in its less elongate habitus (Figure 3) and the much stronger sculpture of the metacoxae and abdominal ventrites.

Distribution.

Endemic to the western Canary Islands (Figure 23), being erroneously reported from Madeira in the original description, as discussed by Machado (1987). We have only seen material from a single locality on Tenerife (upper reaches of Barranco del Río), one on La Gomera (El Cedro, in the laurel forest of Garajonay National Park), and two streams on La Palma (Barranco del Río and Barranco Hoyo Verde, situated on opposite sides of the Caldera de Taburiente). Lüderitz et al. (2010) report the species from one additional locality each on La Gomera and Tenerife, although the Tenerife locality (Barranco del Infierno, 500 m) has been visited by DTB in the early 2000s, when it contained only M. coriacea , as reported by Malmqvist et al. (1995). Additionally, neither of these records are mentioned by Lüderitz et al. (2016), casting some doubt on both of them. The number of permanent stream systems on the Canary Islands has declined seriously in recent decades, as a result of unsustain able water use. Streams have been variously diverted, piped, dammed, and negatively affected by abstraction directly from aquifers (Malmqvist et al. 1993, 1995, Kelly et al. 2002, Lüderitz et al. 2010, 2016). As a consequence, M. imbricata , which appears to be restricted to permanent reaches at relatively high altitude, particularly in forested regions, is very rare, being listed as Critically Endangered (1Ac) in the IUCN Red List ( Foster 1996a). The species is also potentially threatened by hybridization with M. coriacea (see Ribera et al. 2003 and below), a process which may be further facilitated by ongoing climate change favouring the expansion of this more eurytopic species. Lüderitz et al. (2016) suggest that M. imbricata may have disappeared from the El Cedro stream on La Gomera recently, apparently being replaced by M. coriacea between 2006 and 2013. It is not clear, however, whether the same stream reaches were sampled on these two occasions. Work establishing the current status of this species in the Canary Islands is clearly a conservation priority.