Amerus pratensis, (Bretfeld & Griegel, 1999)

Schneider, Clément, 2017, Morphological review of the order Neelipleona (Collembola) through the redescription of the type species of Acanthoneelidus, Neelides and Neelus, Zootaxa 4308 (1), pp. 1-94 : 41-47

Acanthoneelidus pratensis ( Bretfeld & Griegel, 1999) , Bretfeld & Griegel, 2006

Figs 3 B, 4B, 5, 7A, 11–13

Material examined. Paratypes. 1 ♀ on three slides (specimen ‘ad. /5’) and 2 juveniles on three slides (specimens ‘juv. /7’ and ‘juv. /8’), Poland, NW, Stettin , süde. Stettin, Zwischenoderland, 26.vi.1996, A. Griegel leg., ex coll. Griegel, [ SMC].

Diagnosis. White. 300 µm length. Integumentary channels present on head, Th. II/III and abdomen. Crossed connection of channels on head with linea ventralis. Postero-dorsal area of head with 3 + 3 cylindro-conical macrochaetae and generally with stronger chaetae than anterior part of head. Unpaired chaeta a0 present on anterodorsal area of the head. Labium: basomedian fields with 4 + 4 chaetae, basolateral fields with at least 1 + 1 chaetae.

Several cylindro-conical macrochaetae on trunk. Tita I–III each with a long, external chaeta with enlarged and flattened apex. Mucro with posterior lamellae large and wavy.

Description. Size. Specimen ‘ad. /5’ in lateral position approximately matches with the measurement of 300 µm indicated by Bretfeld & Griegel (1999), ‘juv. /7’ extended in dorsal position, reaches 325 µm from labrum to anus.

Pigmentation. Not observed in ethanol, white according to Bretfeld & Griegel (1999). Slides mounted specimens without visible pigments.

Body shape and segmentation. Orthognathous. Ratio height: length of the head could not be observed. Antennae with four distinct antennomeres, apex pointed in a lateral-anterior direction, the greatest angle being between Ant. I and Ant. II, Ant. I directed laterally, the other antennomeres directed more anteriorly (angle of Ant. IV with head axis ~ 45°; Figs 11 A, 13A, B). Total length of antenna ~ 127 µm; max width (Ant. III) ~ 15.8 µm; ratio width: length ~ 0.12; ratio length of Ant. I: Ant. II: Ant. III: Ant. IV ~ 0.27: 0.70: 0.91: 1. Th. I diameter reduced, with steep curves between head/Th. I and Th. I/II. Detail of the connection of head with Th. I could not be analyzed. Th. II, III and abdominal segmentation unclear. The cuticle of juvenile mounted in dorsal view (‘juv. /7’) displays transversal folds on thorax and abdomen, one is roughly delimiting Th. II and III areas, the others are without clear link to a putative segmentation on abdomen. At least Abd. VI is well delimited from Abd. V tergum. Dorso-median line not observed, at least not marked by the granulation, posterior granule T of the line absent. Precoxal areas of Th. II and Th. III without particular anterior enlargement. Ratio length of head: length of thorax (distance from anterior insertion point of subcoxa I to posterior insertion point of subcoxa III): length of abdomen (distance from posterior insertion point of subcoxa III to anus) ~ 1: 0.73: 0.67 (measured on ‘Juv /7’). Each coxa long and bended ( Fig. 13 C, E). On each leg, length of coxa> tibiotarsus ~ femur> trochanter. Abd. VI sternum clearly separated posteriorly from Abd. VI tergum by anal aperture; separated anteriorly from Abd. V sternum by a steep curve; fused laterally with the great abdomen without any distinction in the integumentary granulation. Abd. V sternum with genital plate cryptic, enclosed deeply between Abd. IV and Abd. VI. Abd. IV sternum enlarged, creating a large ventro-posterior side bearing the furca. Abd. I–III sterna reduced with tenaculum being stuck to ventral tube ( Fig. 12 ). Manubrium/dens articular processes: with double convex tips on manubrium; concavity on dens ( Fig. 13 J). Dens with distal part (dd) only slightly wider and longer than proximal part (dp). A delimitation mark between dp and dd can be observed on posterior side but is probably a subsequent fold due to the width difference rather than a suture, there is no articular process ( Fig. 13 I, J). Ratio length of manubrium: dp: dd: mucro ~ 0.75: 0.9: 1: 0.75.

Granulation. Coarse secondary granulation without peculiar pattern. Strong secondary granules occur dorsally on head, thorax and abdomen. Smaller granules are present on antennae, legs, furca and Abd. VI sternum.

Integumentary channels. Integumentary channels present on head and trunk. Exact pattern could not be determined on head, at least extending to the back of the head and probably laterally on forehead. On thorax and abdomen, the channels represent a paired tree with at least seven terminal branches, passing laterally between legs II and III and rooted to the linea ventralis ( Figs 7 A, 12). The channel edges are thin and smooth.

Chaetae. Seven morphological variations of chaetae on body and trunk: (i) ordinary chaetae; (ii) s-chaetae, short and swollen chaetae (s1–s3; s5) found on trunk; (iii) τ-chaetae, long and flexible chaetae with basal ring implanted in a small depression of the integument, weakly contrasted in light microscopy, found on trunk; (iv) wax rod secretory crypts associated to sensory field (sf1–6); (v) free wax rod secretory crypts (wrc1–8); (vi) inner swollen chaetae of sensory fields and (vii) neosminthuroid chaetae.

Wax secretory rods and sensory fields. Six pairs of sensory fields (sf1–6) on head and trunk, each field associated with a wax rod secretory crypt. On head: sf1 preantennal, without inner swollen chaeta; sf2 postantennal with a short, candlelight-shaped inner chaeta ( Figs 3 B, 11A). On trunk: sf3 dorsally on Th. II with three globular inner swollen chaetae ( Figs 7 A, 12); sf4 on precoxal area of Th. II with two globular inner swollen chaetae ( Figs 7 A, 12); sf5 on precoxal area of Th. III and sf6 on dorso-posterior part of the abdomen, each with an inner swollen chaeta ( Figs 7 A, 12). Eight pairs of free wax rod secretory crypts including seven in Th. III tergum area (wrc1–7), one in abdominal terga area (wrc8; Figs 7 A, 12).

Mouth parts. Mandibula as described by Bretfeld & Griegel (1999), maxilla could not be precisely analyzed.

Labium. Badly preserved on available material. Basomedian field with at least four mesochaetae and basolateral field with a least one mesochaeta on tubercle ( Fig. 11 C). Palp could not be precisely observed, apparently with six papillae including the hypostomal one.

Maxillary outer lobe and oral fold. Maxillary palp with a mesochaeta in the sub-basal papilla, a strong, straight mesochaeta in the apical papilla ( Fig. 11 C). Rest of the chaetotaxy ambiguous: either sublobal plate with two micro-hairs and palp with a third sub-apical micro-hair, or sublobal plate with three micro-hairs and palp without a third hair ( Fig. 11 C). Oral fold with two microchaetae ( Fig. 11 C).

Labrum. Labrum rather plane, ordinary ( Figs 2 E, 11B). a-row with two pairs of mesochaetae (a1, a2), subapical and in dorsal position; m- and p-rows each with two pairs of mesochaetae (respectively m1, 2 and p1, 2) and an unpaired mesochaeta (respectively m0 and p0; Fig. 11 B). All chaetae apparently with smooth edges, pointed upward, and clearly smaller than the pre-labral mesochaetae ( Fig. 11 B). Labral ridge sclerotized, apparently bearing a row of apical hairs as a comb of simple, thin hairs ( Fig. 11 B).

Head chaetotaxy. Postero-dorsal area with ten pairs of chaetae (9 + 9 mesochaetae and 1 + 1 macrochaetae), the four posterior pairs the largest ( Figs 3 B, 11A). Postero-lateral area devoid of chaetae. Antero-dorsal area dorsally with 12 + 12 micro and mesochaetae, three unpaired chaetae ( Figs 3 B, 11A). Antero-lateral area with two pairs of chaetae ( Figs 3 B, 11A). The postero-dorsal chaetae stronger than the antero-dorsal chaetae. Ventral area with 2 + 2 post-labial chaetae, the axial ones longer than the external ones ( Figs 4 B, 11C).

Antenna chaetotaxy. Ant. I with three chaetae on dorsal side ( Fig. 13 A, B). Ant. II with four chaetae in an apical whorl, dorsal side ( Fig. 13 A, B). Ant. III with 10 chaetae in four rough whorls (distribution from base to apex: 1, 2, 2 and 5 chaetae); with four apical S-chaetae (S1–S4), S1 and S4 long and tubular and S2 and S3 short, in shape of light-bulb ( Fig. 13 A, B). Ant. IV with 12 chaetae including two in apical position and two strong macrochaetae sub-apical in posterior position, proximal chaetae longer, distal chaetae shorter ( Fig. 13 A, B). With seven S-chaetae including from the most basal to the most apical: three in a whorl on dorsal side (one with rounded apex, the two other could not be fully observed); one in dorsal side with pointed apex; one short with pointed apex (Sx) on postero-ventral side; one long and thick with rounded apex (Sy) on postero-dorsal side; one long with apical curve and rounded apex in posterior position ( Fig. 13 A, B). With two sub-apical sensory rods with truncated apex (sa), equal in size; sub-apical organite short and thick, in basal position in regard to the rods sa, and apical position in regard to the s-chaeta Sx ( Fig. 13 A, B). Sub-apical organite (Or) on a small papilla ( Fig. 13 A). Due to the lack of clearness, the distinction between sa and the two apical ordinary chaetae is not so clear, however what could be observed seems to be in agreement with the original description ( Bretfeld & Griegel 1999). Ornamentation could not be studied.

Th. II–Abd. V terga chaetotaxy. Th. II–Abd. V terga with a total of 50 + 50 chaetae, 9 + 9 τ-chaetae all in the thoracic region, 4 + 4 s-chaetae (s1, s2, s3, s5) and 8 + 8 free wax rod secretion crypts ( Figs 7 A, 12). Dorso-lateral area of Th. II with 10 + 10 chaetae including 1 + 1 macrochaetae, 4 + 4 mesochaetae and 5 + 5 microchaetae; 2 + 2 τ-chaetae posterior to sf3 ( Figs 7 A, 12). Precoxal areas of Th. II with 5 + 5 chaetae including 1 + 1 macrochaetae, 3 + 3 mesochaetae, 1 + 1 microchaetae; 1 + 1 τ-chaetae; 1 + 1 s-chaetae (s1; Figs 7 A, 12). Dorso-lateral area of Th. III–anterior abdomen with 14 + 14 chaetae including 1 + 1 macrochaetae, 7 + 7 mesochaetae and 6 + 6 microchaetae; 4 + 4 τ-chaetae; 1 + 1 s-chaetae (s3) and 5 + 5 free wax rod secretory crypts (wrc1–4, wrc7; Figs 7 A, 12). Precoxal areas of Th. III with 6 + 6 chaetae including 2 + 2 macrochaetae, 1 + 1 mesochaetae and 3 + 3 microchaetae; 2 + 2 τ-chaetae; 1 + 1 s-chaetae (s5) and 2 + 2 free wax rod secretory crypts (wrc5, wrc6; Figs 7 A, 12). Posterior area of abdomen with 15 + 15 chaetae including 1 + 1 macrochaetae, 10 + 10 mesochaetae and 4 + 4 microchaetae; 1 + 1 free wax rod secretory crypts (wrc8); 1 + 1 s-chaetae (s2; Figs 7 A, 12). s1, s5 tubular, s2, s3 bean-shaped ( Fig. 12 ).

Legs chaetotaxy. On leg I: coxa with a microchaeta; trochanter with a mesochaeta (median); femur with six chaetae (the short, distal one may be a special swollen chaeta but the poor preservation prevents a definitive conclusion); tibiotarsus with 17 chaetae in roughly four whorl distributed from the most proximal to the most distal as 5, 3, 2, 7 ( Fig. 13 C). On leg II: subcoxa 1, subcoxa 2 each with a mesochaeta; coxa unclear, at least devoid of strong chaetae; trochanter with three microchaetae (2 proximal, 1 median); femur with seven chaetae (two short median microchaetae, 5 distal mesochaetae); tibiotarsus with 16 chaetae in roughly four whorls distributed from the most proximal to the most distal as 4, 3, 2, 7 ( Fig. 13 D). On leg III: subcoxa 1 with a macrochaeta; subcoxa 2 with a mesochaeta; coxa with two chaetae (a proximal mesochaeta, a median microchaeta); trochanter with four chaetae (2 proximal, 2 distal); femur with 8 mesochaetae (1 proximal, 4 median, 3 distal); tibiotarsus with 15 chaetae in roughly four whorls distributed from the most proximal to the most distal as 4, 3, 2, 6 ( Fig. 13 E). There is least a long, flattened external chaeta and three pairs of internal thickened chaetae on each tibiotarsus ( Fig. 13 C–E). See Tables 9, 10, 11 for a summary.

Claws. Each pretarsus bearing two chaetae as short microspines with rounded apex, one on anterior side and one on posterior side ( Fig. 13 F–H). Unguis: la and lp not seen, however could be masked by the external edge of lamella L ( Fig. 13 F–H). Lamella Bp absent ( Fig. 13 F–H). Unguiculus: basal tubercle with a bulk anterior process and a thinner posterior process, both well-developed ( Fig. 13 F–H). Presence or absence of crests of unguiculal lamella (Ca, Cp) could not be verified on available material.

Abd. VI. Tergum: a-row with nine mesochaetae (two pairs and an unpaired), m-row with 2 + 2 mesochaetae. Each anal valve with a microchaeta (av, Fig. 12 ). Sternum with 7 + 7 chaetae ( Fig. 12 ).

Genital plate. Female with maybe 1 + 1 microchaetae (unclear on material). Male unknown.

Abd. IV sternum. With 3 + 3 chaetae and 1 + 1 strong neosminthuroid chaetae ( Fig. 12 ). Neosminthuroid chaetae covered with small spicules on whole length ( Fig. 12 ).

Abdominal appendages. Manubrium with 4 + 4 posterior chaetae, arranged in two rows of two chaetae parallel with the axis of symmetry, the axial row with shorter chaetae than the external row ( Fig. 13 I). Proximal part of dens with one postero-median chaeta. Distal part of dens with five posterior, strong chaetae and three antero-distal spine-like chaetae, each with a basal ring at the possible exception of A1 ( Fig. 13 I, J). Mucro elliptical with large, waved, lamellae and a round apex ( Fig. 13 I, J). Tenaculum with two medians lobes without chaeta, with 4 + 4 apical teeth ( Fig. 12 ). Ventral tube without posterior lobe and with 2 + 2 apical mesochaetae ( Fig. 12 ).

Discussion. The genus is monotypic. It has various affinities with other genera of Neelipleona : labrum morphology, claw morphology and presence of s-chaetae s5 are similar to Neelides ; general chaetotaxic pattern of the body and secondary granulation are similar to Megalothorax ; a unique pair of neosminthuroid chaetae is similar to Neelus . The integumentary channels on Th. II/Th. III are comparable to those of M. perspicillum . It also shares some characteristics with Megalothorax of the incertus -group sensu Schneider & D’Haese (2013): 4 + 4 teeth on the tenaculum, globular inner chaetae of sensory fields, presence of s-chaetae s3 and 3 + 3 chaetae on Abd. IV sternum. The presence of only one inner chaeta in sf5 is similar to the juvenile of Megalothorax . A. pratensis was described with four large and blunt teeth on the external, posterior lamella of mucro ( Bretfeld and Griegel 1999). I was not able to distinguish those teeth from the waves of the lamella.

Figures

FIGURE 2. External morphology of the mouth region. From labrum to labium (A) Neelides folsomi (simplified), (B) Megalothorax perspicillum and (C) Sphaeridia pumilis. Labium (D) Parisotoma sp. ventral side. Labrum (E) without anterior process, (F) with anterior process. Legend: blf = basolateral field of labium; blf tub. = tubercle of the basolateral field; bmf = basomedian field of labium; mol = maxilla outer lobe; of = oral fold; * indicates in Sphaeridia pumilis the chaeta putatively homologous with the chaeta on tubercle of basolateral field in Neelipleona.

FIGURE 3. Diagram of the dorsal chaetotaxy of the head. (A) Neelides folsomi; (B) Acanthoneelidus pratensis; (C) Neelus murinus; (D) Megalothorax nigropunctatus; (E) Megalothorax perspicillum; (F) Megalothorax granulosus, integumentary channels not represented. In gray, chaetae absent in juvenile.

FIGURE 4. Diagram of the ventral chaetotaxy of the head. (A) Neelides folsomi; (B) Acanthoneelidus pratensis; (C) Neelus murinus; (D) Megalothorax nigropunctatus; (E) Megalothorax minimus; (F) Megalothorax granulosus.

FIGURE 7. Diagram of the chaetotaxy of the trunk. (A) Acanthoneelidus pratensis, (B) Megalothorax perspicillum, the plain gray patches on the abdomen indicate integumentary marks.

FIGURE 11. Acanthoneelidus pratensis. (A) Chaetotaxy of the head, dorsal side, missing chaetae – probably broken – indicated in dotted line; (B) labrum with pre-labral row of chaetae and maxilla outer lobe, dorsal side; (C) basal fields of labium, oral fold, maxilla outer lobe and post-labial part of the head, lateral side. Legend: blf = basolateral field of labium; bmf = basomedian field of labium; of = oral fold; arrows indicate hairs that could either be third sublobal hairs or sub-apical hairs of the palp.

FIGURE 12. Acanthoneelidus pratensis. Chaetotaxy of the trunk terga chaetotaxy, Abd. IV and VI sterna, tenaculum and ventral tube, lateral side. Legend: av = chaetae on anal valves; s- = swollen chaetae, ns = neosminthuroid chaeta; τ = τ-chaeta basal ring; wrc 1 – 8 = free wax rod secretory crypts. Arrow indicates unpaired chaeta. Some unclear chaetae are drawn with dotted line (reported in Bretfeld & Griegel 1999).

FIGURE 13. Acanthoneelidus pratensis. Chaetotaxy of the antenna, (A) dorsal side, (B) ventral side; chaetotaxy of the legs, (C) leg I from coxa to foot, (D) leg II from trochanter to foot, (E) leg III from coxa to foot, arrows indicate the long, flattened chaeta on each leg; claws (F) I, (G) II, (H) III; furca, (I) posterior side, (J) anterior side.