Australobius chagosensis Popovici & Edgecombe, 2024

Popovici, George, Edgecombe, Gregory D. & Hall, Daniel W., 2024, New Chilopoda from the Chagos Archipelago, Journal of Natural History 58 (41 - 44), pp. 1885-1915 : 1888-1891

Australobius chagosensis Popovici & Edgecombe sp. n.

( Figures 1 , 2 (A–l))

urn:lsid:zoobank.org:act:4FC7008A-F1DD-4D39-8831-E642AB062676

Etymology

The specific epithet reflects the origin of the two known specimens, both from Diego Garcia in the Chagos Archipelago.

Type material

Holotype, NHMUK015619673 , ♀, Diego Garcia, 7.359°S, 72.432°E, leaf litter, 02 July 2022, leg. W. Rabitsch. Paratype NHMUK015626351 , juvenile ♀, Point Marianne Culture site, 7.319°S, 72.427°E, leaf litter, 22 June 2022, leg. W. Rabitsch.

Diagnosis

Small-sized Australobius , body length 7–10 mm. Antenna with 19–20 articles. 4 + 4 ocelli of similar size. Anterior margin of forcipular coxosternite with 3 + 3 teeth; small spiniform porodont situated between the medial and outermost teeth. Tergites 9, 11 and 13 with small posterior triangular projections. Ultimate leg pair with greatly reduced tarsus 2, less than 1/3 length of tarsus 1; ultimate leg apical claw simple. Female gonopods with 3 + 3 slender spurs and a trifurcate claw.

Description

Habitus. Body length 10 mm (holotype), 7 mm (paratype). Colour in ethanol dark brown with violet pigmentation in the anterior part of the body and the proximal articles of the legs. Head and antennae. Cephalic plate of subequal length and width; frontal margin with medial notch ( Figure 1 (A)). 4 + 4 ocelli, arranged in two rows comprising two ocelli each ( Figure 1 (B,C)). Principal ocellus only slightly larger than seriate ocelli, all of which are of similar size. Tömösváry’s organ small, positioned below the principal ocellus ( Figure 1 (B)). Antenna with 19 (holotype, one complete antenna)–20 articles; 2.4 times longer than the cephalic capsule and 25% of body length ( Figure 1 (A)).

Forcipular segment. Anterior margin of forcipular coxosternite only slightly protruding, with 3 + 3 teeth of roughly equal size ( Figure 1 (D)). Porodont slender, spiniform, situated between the medial and outermost teeth ( Figure 1 (E,F)). Dental margin continuous with the remainder of the anterior margin. Median diastema broadly U-shaped (holotype: Figure 1 (D)) or rounded V-shaped (paratype: Figure 1 (E)).

Trunk. Large tergites wrinkled, with evident lateral but indistinct posterior margination. Posterior angles of tergites 9, 11 and 13 of holotype with small, feeble triangular projections ( Figure 1 (G)) but these are completely absent in the paratype ( Figure 2 (A)). Coxal pores on legs 12–15, round, arranged as 2, 3, 3, 3/2, 3, 3, 3 (paratype) or 3, 3, 4, 4/3, 4, 5, 4 (holotype) ( Figure 2 (I)).

Ultimate leg-bearing and postpedal segments. Ultimate leg telopodite slender and elongate ( Figure 2 (C)).Tarsus 2 greatly reduced in size,only 24.5% (paratype) – 31% (holotype) of the length of tarsus 1 ( Figure 2 (C–F)). Telopodite of leg pair 14 similarly elongate but tarsus 2 not greatly reduced in size ( Figure 2 (B,G)). Pretarsus without accessory claws, main claw bearing a minute anteroventral spine ( Figure 2 (H)). Female gonopods with 3 + 3 elongate, slender spurs ( Figure 2 (J,K)); apical claw with well-developed lateral and medial denticles ( Figure 2 (I)). Gonopods of immature female paratype with 2 + 2 minute spurs ( Figure 2 (L)). Male unknown.

Plectrotaxy. As in the following tables (C, coxa; P, prefemur; T, tibia; t, trochanter; a. anterior; m, median; p. posterior).

Plectrotaxy of female holotype ( NHMUK015619673 ).

			Ventral					Dorsal
Leg pair	C	t	P	F	T	C	t	P	F	T
1	−	−	−	a	m	−	−	amp	p	−
13	−	m	amp	amp	am	?	−	amp	ap	p
14	−	m	amp	amp	a	a	−	amp	ap	−
15	−	m	amp	amp	−	a	−	amp	p	−

Plectrotaxy of female paratype (NHMUK 015626351).

			Ventral					Dorsal
Leg pair	C	t	P	F	T	C	t	P	F	T
1	−	−	−	a	m	−	−	amp	a	−
13	−	m	amp	mp	m	−	−	mp	ap	p
14	−	m	amp	amp	−	−	−	mp	p	−
15	−	m	amp	am	−	−	−	amp	p	−

Discussion. The specimens from Diego Garcia compare most closely to A. sculpturatus ( Pocock, 1901) (revised by Eason 1973), from southern India, Sri Lanka, the Laccadives and the Maldives, and A. palnis ( Eason, 1973) from Sri Lanka ( Eason 1993). Similarities include the low number of ocelli, no more than diminutive posterior projections on tergites 9, 11 and 13, slender spurs on the female gonopods, and plectotraxy. However, examination of the types of both A. sculpturatus and A. palnis , as well as specimens from the Maldives assigned to the former, shows a substantially longer tarsus 2 than in both specimens of A. chagosensis ( Figure 2 (M,N) vs Figure 2 (C)). Additional characters in which A. chagosensis differs include ocelli more homogeneous in size, and 3 + 3 (vs 4 + 4 or 3 + 4) teeth on the anterior margin of the forcipular coxosternite.

The position of the porodont, between the second and third teeth, is shared with Australobius malayicus ( Verhoeff, 1937) ; however, A. chagosensis can be distinguished from that Malaysian species by the fewer ocelli (four vs six) and 3 + 3 vs 4 + 4 coxosternal teeth.

Figures

Figure 1. Australobius chagosensis sp. n. Holotype NHMUK015619673 except (E) paratype NHMUK015626351. (A) Cephalic plate and antennae, dorsal view. (B) Ocellar field and Tömösváry’s organ, lateral view. (C) Ocellar field, lateral view. (D) Forcipular segment, ventral view. (E) Anterior margin of forcipular coxosternite, ventral view. (F) Anterior margin of forcipular coxosternite, ventral view. (G) Tergites 8–11, dorsal view. Abbreviations: med, median diastema; pd, porodont; To, Tömösváry’s organ.

Figure 2. (A–J) Australobius chagosensis sp. n. (A–D, H–K) Holotype NHMUK015619673; (E–G,l) Paratype NHMUK 015626351. (A) Posterior third of trunk (TT 8–14), dorsal view. (B) Leg 14, lateral view. (C) Leg 15, lateral view. (D) Tarsus 2 of left leg 15, lateral view. (E) Tarsus 2 of left leg 15, dorsal view. (F) Distal end of left leg 15, lateral and dorsal views.(G) Tarsus 2 of left leg 14,lateral view. (H) Pretarsus of left leg 15, lateral view. (I) Segment 15 and postpedal segments, ventral view. (J,K) Gonopod spurs. (L) Gonopods of immature female, ventral view. (M) Australobius sculpturatus (Pocock, 1901), NHMUK 1952.12.11.73–74. Distal end of right leg 15, lateral view. (N) Australobius palnis (Eason, 1973). Holotype, BMNH(E) #200319 Chilo 1894.10.24.66. Distal end of left leg 15, lateral view. Abbreviations: ta1, tarsus 1; ta2, tarsus 2.