Crossodonthina elegans, Kasai & Tanaka & Sawahata, 2023

Kasai, Hiro, Tanaka, Shingo & Sawahata, Takuo, 2023, A new species of the genus Crossodonthina (Collembola: Neanuridae: Neanurinae Lobellini) from Yonaguni-jima in southwest Japan, Zootaxa 5277 (3), pp. 509-520 : 510-516

Crossodonthina elegans sp. nov.

[Japanese name: kubiwa-akafusa-ibotobimushi]

Figs. 1–14 ; Tables 1–3 .

Type material. Holotype: female, Japan, Okinawa prefecture, Ryukyu Islands, Yonaguni-jima, Mt. Yonaguni-dake (alt. 131 m, 24°27′15.7″N, 122°58′43.1″E), 18-XI-2018, Hiro Kasai leg. ( NMNS, NSMT-Ap 614) . Paratypes: 1 female ( NMNS, NSMT-Ap 615) and 1 male ( NMNS, NSMT-Ap 616), the same data and slide as for holotype ; 2 females ( NMNS, NSMT-Ap 617–618) and 1 male ( NMNS, NSMT-Ap 619), the same locality as for holotype, 21- VIII-2020, Keisuke Matsui leg. All deposited into the Apterygota (Ap) collection of the National Museum of Nature and Science ( NMNS; previous name, the National Science Museum, Tokyo: MSMT), Tokyo Province, Japan .

Diagnosis. 3+3 eyes on head. Body ground color red, Ant. I white, Th. I white or yellow. Cephalic chaeta O present. Cephalic tubercle Dl separated from L and So. Mandible with 3 basal teeth, 4 fringed rami, and 1 broad lamella. Maxilla without marginal filaments. Labral formula 0/2, 2. Sensory chaeta on each tubercle L of Abd. I–IV. Tubercles De and Dl of Abd. V separated.

Description. Body length: up to 1.7 mm. Body ground color red, Ant. I white, Th. I white or yellow while alive ( Fig.1 ), completely white in alcohol. Occasionally, sparse white or yellow area between body segments and between tubercles De and Dl from Th. II to Abd. V ( Fig. 1 ). Eyes, 3 + 3, black, 2 on anterior and 1 on posterior of tubercle Oc.

Antenna: Antenna 4-segmented, length subequal to head, Ant. III and IV dorsally fused. Ratio of antennal segments as I: II: (III + IV) = 1: 1.1: 2.4. Ant. I and II with 9 and 11 chaetae, respectively. Ant. III consisting of 5 sensory chaetae, sensillum sgd and sgv weak and blunt, 2 small round rods exposed in separate pits, with sgd well above 2 rods ( Fig. 6 ). S-chaetae of Ant. IV relatively thin. Chaeta d4 very short, Ant. IV with a trilobed apical bulb ( Fig. 6 ). Chaetotaxy of antennae as in Table 1b and Figs 6–7 .

Mouthparts: Buccal cone poorly developed. Labrum truncate and frontal margin weakly curved, chaetal formula 0/2, 2 ( Fig. 10 ). Labium with chaetae (A, C, D) on proximal part of palp (labial papilla x not observed), 4 (E, F, G, f) on submentum and 4 (b, c, d, e) on mentum ( Fig. 11 ). Mandibular head elongated, comprising 1 lamella, 1 fringed ramus inside, and 3 fringed rami outside ( Figs 12–13 ). The inner fringed ramus longest and twice as long as the outer ramus, thinner towards the upper side and thicker towards the lower side, with 2 rows of approximately 30 marginal acute protruding chaetae. Inner and outer margins of the base fused, with 6–7 basal spinal teeth ( Fig. 13A ). The middle lamella short and broad, and 1/3 of the length of the inner ramus, with 2 strong curved basal teeth and 1 small round tooth in between, followed by a row of 12–16 small apical teeth ( Fig. 13B ). The outer ramus consists of 3 fringed rami (long, medium, and short) ( Fig. 13C ). The long ramus plumate-like, with approximately 9 small chaetae (rarely apically bifurcated). Medium and short ramus dendritic, repeatedly branching, with approximately 15 and 12 small chaetae, respectively. The short ramus basely with 1 long ramus (often branched) and 2 short rami; further down with 2 short spine-like chaetae, one of which bifurcated. These outer rami show some variation in the number and location of branching among individuals. Maxillary head consists of 2 subequal styliform rami without ciliates; apex of inner one hook-like with 1 blunt apical tooth and 1 small subapical tooth, and 1 minute tooth near the top between them. The outer rami pointed with needle-like protrusions on the lower side ( Fig. 14 ).

Chaetal morphology:Ordinary dorsal chaetae of four types ( Fig. 5 ): Ml, Mc, me, and mi. Macrochaetae (Ml) about 100–160 µm, thick, straight, sheathed, apically rounded and slightly knobbed longitudinally with slightly serrated margins. Some lateral chaetae weakly acuminate with slightly sheathed. Short macrochaetae (Mc) morphologically similar to Ml and at most approximately 1/2 the length of Ml. Mesochaeta (me) similar to ventral chaetae: thin, smooth, and pointed. Microchaetae (mi) quite pointed, simple, without a socket, and at most approximately 1/10 the length of Ml, sometimes move away from tubercles.Among mi, chaeta D, chaeta De2, and microchaetae of tubercle Dl on the head: chaeta Di3 (posterior medial chaetae) and chaeta De4 (anterior medial chaetae) of Th. II–III, chaeta De3 (medial chaetae) of Abd. I–III, chaeta Di3 (posterior chaetae), and chaeta Dl4 (anterior medial chaetae) of Abd. V, which are particularly small and easily overlooked. Dorsal sensory chaetae of two types ( Fig. 5 ): s and ms. Sensory chaetae (s) thin, smooth, and equal or longer than mesochaetae.

Cephalic tubercles and chaetotaxy: Dorsal central area consists of 6 separate tubercles: tubercle Cl with 2 Ml (F) and 2 me (G); 2 tubercles An, each with 1 Ml (B), 1 Mc (E), and 2 Mc or me (C, D); tubercle Fr with 2 Ml (A) and 1 Mc or me (O); and 2 tubercles Oc, each with 1 Ml (Ocm) and 2 Mc (Oca, Ocp). Dorsal posterior area with 4 separate tubercles: 2 tubercles Di, each with 1 Ml (Di1) and 2 tubercles De, each with 1 Ml (De1) and 1 mi (De2), chaetae De2 sometimes absent. Dorsal lateral area with 2 separate tubercles and 2 fused tubercles (L + So): 2 tubercles Dl, each with 1 Ml and 2 mi and 2 tubercles (L + So), each with 2 Ml, 2 Mc, and 5 me ( Fig. 3 ). Group Vi with 6+6 chaetae. Groups Vea, Vem and Vep with 4, 3 and 4 chaetae respectively. Dorsal chaetotaxy of head as in Table 1a and Figs 2–3.

a) Cephalic chaetotaxy—dorsal side.

Body tubercles and chaetotaxy: Th. I with 3 + 3 tubercles; the interval of tubercles Di wider than those on the other body segments, with tubercles De and Dl shifted outwards. Th. II–III with 4 + 4 tubercles (Di, De, Dl, L); tubercle Dl shifted forward and tubercle L shifted backward or to the ventral side ( Fig. 2 ). Abd. I–V with 4 + 4 tubercles (Di, De, Dl, L); tubercle De on Abd. V small, with only 1 sensory chaeta, and clearly separated from tubercle Dl; tubercle L on Abd. V shifted towards the ventral side. Abd. VI with 1 tubercle on each side and no cryptopygy ( Fig. 4 ). Dorsal chaetotaxy as in Table 2 and Figs 2, 4 . Genital plates with 15–21 and 20 chaetae in females and males, respectively. Furcal remnant with 3 mesochaetae ( Fig. 9 ). Ventral tube comprises 4 + 4 chaetae (1 + 1 proximal and 3 + 3 distal). Ventral chaetotaxy as in Table 2 and Fig. 9 .

Legs: Tibiotarsus III with 18 chaetae (T1–T4, A1–A7, B1–B6, M); B4 and B5 shorter than the inner edge of the unguis and pointed. Unguis ventral with 1 inner tooth and basal granules. Two minute chaetae (each with anterior and posterior surfaces) on the apical row of Fe on legs I–III. Unguiculus and tenant hair absent ( Fig. 8 ). Chaetotaxy of legs as in Table 2 and Fig. 8 .

Ecology. Found under the fallen leaves and bark of decayed branches in evergreen broad-leaved forests dominated by Castanopsis sieboldii var. lutchuensis and Adinandra yaeyamensis .

Remarks. This is a unique species among members of the genus due to the white or yellow color of its Ant. I and Th. I. Crossodonthina elegans sp. nov. is most similar to C. laterisensillata Ohira, Kataoka, Tanooka & Nakamori, 2022 ; however, the two differ in a number of characteristics as follows: cephalic tubercles on lateral part (Dl separated to L and So in C. elegans sp. nov., Dl fused to L and So in C. laterisensillata ), basic body macrochaetae (apically rounded in C. elegans sp. nov., pointed in C. laterisensillata ), number of chaetae on tubercle L of Abd. I and III (5 in C. elegans sp. nov., 4 in C. laterisensillata ), and number of chaetae on tubercle L of Abd. IV (5 or 6 in C. elegans sp. nov., 7 in C. laterisensillata ). More differences were apparent in C. elegans sp. nov.: mouthparts, mandible lamella (12–16 upper teeth in C. elegans sp. nov., 22 upper teeth in C. laterisensillata ), mandible outer ramus (basal rami weakly developed in C. elegans sp. nov., developed in C. laterisensillata ), and maxillary ciliate on apex (absent in C. elegans sp. nov., present in C. laterisensillata ). The new species is also similar to C. tridentiens Yue & Yin, 1999 , C. choui Jiang & Zhang, 2012 , and C. clavata Jiang & Wang, 2021 , in that it has 3 + 3 eyes, cephalic chaeta O present, cephalic tubercle Oc with 3 chaetae, cephalic tubercle L fused to So, and thick macrochaetae that are apically rounded. However, the following features can distinguish these species: C. choui has a maxilla with 3 rami (2 in C. elegans sp. nov.), 2 ordinary chaetae on the tubercle Di of Th. II–III (3 in C. elegans sp. nov.), and sensory chaetae absent on tubercle L of Abd. I–III (present in C. elegans sp. nov.); C. clavata has a mandible with 4 basal teeth (3 in C. elegans sp. nov.), cephalic tubercle Dl fused to L and So (separated in C. elegans sp. nov.), and 3 ordinary chaetae on tubercle De of Th. II (4 in C. elegans sp. nov.). C. tridentiens is particularly similar in mandible morphology to that of C. elegans sp. nov. in having 3 prominent basal teeth and dendritic rami. However, in C. elegans sp. nov., the upper part of the basal tooth is a board-like lamella (fringed lamella in C. tridentiens ), with 3 medium-sized rami (2 in C. tridentiens ). The new species can be distinguished from the above species by the characters listed in Table 3 .

Figures

FIGURE 1. Crossodonthina elegans sp. nov.: habitus of living specimen.

FIGURES 2–5. Crossodonthina elegans sp. nov.: 2, habitus and dorsal chaetotaxy (holotype); 3, dorsal chaetotaxy of head; 4, dorsal chaetotaxy of Abd. IV–VI; 5, type of body chaetae.

FIGURES 6–11. Crossodonthina elegans sp. nov.: 6, dorsal chaetotaxy of Ant. III–IV; 7, ventral chaetotaxy of Ant. III–IV; 8, chaetotaxy of leg III; 9, ventral chaetotaxy of abdomen; 10, labrum; 11, labium.

FIGURES 12. Crossodonthina elegans sp. nov.: photomicrograph of the mandible.

FIGURES 13–14. Crossodonthina elegans sp. nov.: 13, mandible (A, inner ramus; B, middle lamella; C, outer ramus); 14, maxilla.

Tables

TABLE 1. Cephalic chaetotaxy of Crossodonthina elegans sp. nov.

Tubecle	Number of setae	Type of setae	Name of setae
Cl	2	Ml	F
	2	me	G
An	1	Ml	B
	1	Mc	E
	2	Mc or me	C, D
Fr	2	Ml	A
	1	Mc or me	O
Oc	1	Ml	Ocm
	2	Mc	Oca, Ocp
Di	1	Ml	Di1
De	1	Ml	De1
	1	mi	De2 (rarely absent)
Dl	1	Ml	uncertain
	2	mi
L+So	2	Ml	uncertain
	2	Mc
	5	me
b) Chaetotaxy of antennae.
Segment, Group	Number of chaetae	Segment, Group	Number of chaetae adult
I	9	IV	or, 8 S, i, 12 mou, 7 brs, 2 iv
II	11
III	5 sensilla AOIII
ve	5	ap	10 bs, 5 miA
vc	4	ca	3 bs, 3 miA
vi	4	cm	3 bs, 1 miA
d	5	cp	8 miA, 1 brs

TABLE 2. Postcephalic chaetotaxy of Crossodonthina elegans sp. nov.

Terga					Legs
	Di	De	Dl	L	Scx2	Cx	Tr	Fe	T
Th. I	Ml	Ml+Mc	Ml	-	0	3	6	13	19
Th. II	Ml+Mc+mi	2Ml+Mc+mi+s	2Ml+Mc+s+ms	Ml+2Mc	2	7	6	12	19
Th. III	Ml+Mc+mi	2Ml+Mc+mi+s	2Ml+Mc+s	Ml+2Mc(Mc+me)	2	8	6	11	18
Terga					sterna
Abd. I	Ml+Mc	Ml+Mc+mi+s	Ml+Mc	Ml+Mc+2me+s	VT: 4
Abd. II	Ml+Mc	Ml+Mc+mi+s	Ml+Mc	Ml+Mc+2(1)me+s	Ve: 4
Abd. III	Ml+Mc	Ml+Mc+mi+s	Ml+Mc	Ml+Mc+2me+s	Ve: 3–4, Fu: 3
Abd. IV	Ml+mi	Ml+mi+s	2Ml+Mc	2Ml+2Mc+1(0)me+s	Vei: 1, Vec: 2, Vel: 4, Vl: 4–5
Abd. V	Ml+Mc(mi)+mi	s	3Ml+mi	3Mc+2(1)me	Ag: 3, Vl: 1
Abd. VI	6Ml+me				Ve: 14me+3(2)mi, An: 2–3 mi