Orphnaeus dekanius Verhoeff, 1938

Orphnaeus dekanius Verhoeff, 1938

Figs 12 View Figure 12 , 13 View Figure 13 , 14 View Figure 14 , 15 View Figure 15 , 16 View Figure 16 , 18 D – H View Figure 18

Examined material.

6 specimens: NHMUK 015991469 , 1 ♂, 40 mm, 73 leg-bearing segments, Picard , Aldabra, 08. 10. 1974 ; NHMUK 015991470 , 1 ♂, 35 mm, 75 leg-bearing segments, Picard , 09. 04. 1974 ; NHMUK 015991471 , 1 ♀, 24 mm, 81 leg-bearing segments, Grande Terre , Aldabra, 03.1974, leg. J. Wilson ; NHMUK 015991472 , 1 juvenile, 13 mm, 81 leg-bearing segments, Grande Terre , Aldabra, 05.1974, leg. J. Wilson ; NHMUK 015991473 , 1 ♂, 37 mm, 75 leg-bearing segments, Pandanus litter, Aldabra , 22. 03. 1974 ; NHMUK 015991474 , 1 ♀, 51 mm, Takamaka (Anse Takamaka), 23–27.02. 1968, leg. B. Cogan & A. Hutson .

Diagnosis. Medium to large size Orphnaeus species, with 73–81 leg-bearing segments and variable but generally present longitudinal bands of dark pigment flanking the central vessel. Mandible with three or four pectinate lamellae. First maxillae with both telopodal and coxosternal lappets present and uniarticulate telopodite. Second maxillary pretarsus spatulate, fringed by acuminate hyaline projections. Posterior trunk metasternites with paired pore fields at the posterior end. Pore fields on posterior metasternites, procoxae and metacoxae bordered by dense groups of setae-like projections. Female gonopods uniarticulate, medially overlapping, with angled, rounded external margin.

Description.

Head and antennae. Cephalic plate with broadly rounded anterior margin and straight posterior margin, overlapping the forcipular tergite. Head approximately as broad as long ( NHMUK 015991473 ) to 1.2 × broader than long ( NHMUK 015991474 ). Antennae approximately 2.5 × longer than head, weakly tapering distally (Fig. 12 A View Figure 12 ). In older specimens, the tapering of antennae and dorsoventral compression of proximal antennal articles are more clearly visible (Fig. 12 C View Figure 12 ). Antennal article XIV 1.9 × longer than the penultimate, with two clusters of sensilla basiconica arranged in lateral pits. Small, spear-like sensilla present at apical end.

Mandibles. Of typical aspect for the genus (Fig. 13 C, D View Figure 13 ). Four conspicuous pectinate lamellae evident, arranged concentrically around distal edge. Proximal to the outermost lamella, isolated projections resembling those on the lamellae are present. Two minute sensilla present laterally.

Labrum and clypeus. Labrum of typical oryid aspect, with short hairlike hyaline projections on middle part (Fig. 13 B View Figure 13 ). Lateral parts incompletely separated from middle part and clypeus by evident sutures. Clypeus with two pairs of postantennal sensilla, a median field of sensilla spanning its mediolateral axis and one pair of prelabral sensilla posterior to these (Fig. 13 A View Figure 13 ). Polygonal reticulation evident.

Maxillae. First maxillae with apically rounded, short coxal projections, bearing 8–11 sensilla (Fig. 14 A View Figure 14 ). Telopodite broadly rounded, uniarticulated, of similar size to the coxal projection, bearing 7–12 sensilla. Both telopodal and coxosternal pairs of lappets present, with distinct spinous reticulation (Fig. 14 C View Figure 14 ). Coxosternal pair of lappets completely obscured by second maxillary telopodite in ventral view. Second maxillary coxosternite with shallow, concave, rounded anterior margin bordered by a row of trichoid sensilla and two groups of trichoid sensilla proximally (Fig. 14 A View Figure 14 ). Metameric pore conspicuous, surrounded by sclerotised rim. Telopodal articles stout. Pretarsus with spine-like projections around entire exterior margin and two pores opening on its dorsal surface (Fig. 14 D, E View Figure 14 ).

Forcipular segment. Forcipular tergite 3.2 × broader than long. Exposed surface of forcipular coxosternite 2.2 × broader than long (Fig. 12 D View Figure 12 ). Chitin lines absent. Anterior margin rounded, deeply concave. Pleural sutures strongly converging posteriorly. Trochanteroprefemur ~ 1.5 × broader than long, with evidently rounded external face. Tarsungulum stout, large, entirely covered by anterior edge of cephalic plate, with smooth inner concavity. Opening of venom gland channel immediately proximal to tip of tarsungulum. All forcipular articles without denticles.

Trunk. Last five or six trunk metasternites with two posteriorly located pore fields (Fig. 15 D View Figure 15 ), anteriorly bordered by dense clusters of hairs (Fig. 15 A View Figure 15 ). All other trunk metasternites with four pore fields, two anterior and two posterior, of equal size on metasternites 2–46, the anterior gradually decreasing in size until disappearing on metasternites 74–76. One single row of paratergites present beginning from the second leg-bearing segment, becoming very conspicuous on the eighth leg-bearing segment. General setation of sclerites sparse.

Ultimate leg - bearing and postpedal segments. Ultimate leg-bearing segment metasternite variably trapeziform, 2.3 × broader than long (Fig. 16 A, B View Figure 16 ). Posterior edge with dense field of hairs and occasionally small clusters of pores. Coxopleuron stout, without coxal organs. Telopodite of ultimate leg-pair only moderately inflated in both males (Fig. 16 B View Figure 16 ) and females (Fig. 16 A View Figure 16 ), with dense fields of hairs present on the ventral side of all articles. Pretarsus absent (Fig. 16 C View Figure 16 ). Metatarsus with small hair-like projections at its apical edge.

Female gonopods usually uni-articulated (Figs 15 D View Figure 15 , 16 D View Figure 16 ), occasionally with an anterior notch or asymmetrical articulation. Lateral edge smooth, rounded, forming strongly acute angle with posterior edge of first genital sternite. Male gonopods biarticulated, bearing 16 setae (Fig. 16 E View Figure 16 ).

Remarks.

Orphnaeus dekanius was originally described from Trivandrum (Thiruvanathapuram), India ( Verhoeff 1938), and maintained as a valid species under Orphnaeus until its reassignment to Nycternyssa , justified by the description of the female gonopods as “ uni-articulate ”. A detailed re-evaluation of the status of Nycternyssa is provided below.

Subsequent to its original description, there is no evidence that other specimens had been assigned to either O. dekanius or N. dekania prior to recent records from the Chagos Archipelago ( Popovici et al. 2024). Based on morphology alone, the present specimens are considered conspecific with those collected in the Chagos Archipelago and match the original description of O. dekanius . The most salient diagnostic trait allowing for reliable differentiation of O. dekanius from O. brevilabiatus is the presence of dense clusters of minute setae bordering the posterior pore fields of the former (Fig. 15 A View Figure 15 ). Although this character was illustrated by Verhoeff in the original description of O. dekanius ( Verhoeff 1938: tafel 8, fig. 61), no mention was made of it in the text of the description. It is unambiguously shared by all specimens studied here from near the type locality of O. dekanius (specimens from Sri Lanka listed above under “ Specimen data ”), and from other localities in the Western Indian Ocean, and is consistent in both sexes in specimens. Setae are sparse in the smallest studied Aldabra specimen (13 mm) but are clustered by a body length of 24 mm. As these clusters of setae are completely absent in specimens identified as O. brevilabiatus from near its type locality in Myanmar (Fig. 17 View Figure 17 ) and other localities in mainland and maritime Southeast Asia, we maintain the validity of O. dekanius and O. brevilabiatus even in light of the variability of female gonopodal articulation in the latter (Fig. 18 A – C View Figure 18 ). The incomplete description of Orphnaeus meruinus Attems, 1909 does not allow for reliable separation from O. dekanius , given the inconsistencies in how diagnostic characters for these two taxa are coded in past literature. One salient difference from all O. dekanius specimens in our sample is the greatly inflated ultimate leg telopodite in males assigned to O. meruinus collected in Oman ( Lewis and Gallagher 1993), a character that separates these species even when accounting for body size.