Tomocerus tropicus, Yu, Yang and Liu, 2018

Yu, Daoyuan, Zhang, Yating, Wang, Ziqiang, Hu, Feng & Liu, Manqiang, 2024, Unravelling the obscure homology: postembryonic development of chaetotaxic traits in a basal hexapod taxon (Collembola: Tomoceridae), Zoological Journal of the Linnean Society 202 (3), pp. 1-20 : 3-9

Tomocerus tropicus

Specimens examined. 15HN5TCJ (1–11), 29.xii.2015, leg. Daoyuan Yu and Chunyan Qin. Cultured in the lab.

Head: First instar ( Fig. 2A ). Numbers of chaetae in each row are as follows. An: 2; A: 4; M: 5; S: 5; ocular: 3; Ps: 3; Pi: 1; Pa: 4; Pm: 3; Pp: 2; Pe: 2. Chaetae Pa2, Pa5, Pp1, Pp3, and Pe2 are macrochaetae, others are mesochaetae.

Second instar ( Fig. 2B ). Primary chaetae A2, A3, A5, M2, S0, S2, S5, S6, Pa6, and Pm1 turn into macrochaetae; most mesochaetae turn into microchaetae except for the row An, Pe, and M4. Secondary mesochaetae An3a, Pe2i, and Pe2e, and microchaeta Pa5a and Pp3e appear constantly, whereas mesochaeta An1a0 and microchaeta A2a0 in the middle line are either present or absent.

Third instar ( Fig. 2C ). Pm1, Pp1, and Pe2 become distinctly smaller (relative size), but still retain the morphology of macrochaetae. Secondary microchaetae An1p and An3a2 are present.

Fourth instar ( Fig. 2D ). The relative sizes of macrochaetae Pm1, Pp1, and Pe2 reduce further. Additional micro- and mesochaetae appear only in row An and along the postoccipital collar.

Adult macrochaetotaxy ( Fig. 10A ). A2, A3, A5, M2, S0, S2, S5, S6, Pa2, Pa5, Pa6, and Pp3 are macrochaetae. Antennal base and postoccipital collar each with a row of chaetae. Other chaetae are all microchaetae, and are often obscured by dense scales.

Th. II: First instar (Fig. 3A). Numbers of chaetae in each row are as follows. Row a: 6; row m: 7 * 1 (outlier); row p: 6 * 1. Chaetae a2–6, m3, m6, p2, p4, and ap5 are macrochaetae, others are mesochaetae. One s-microchaeta and 12–16 normal s-chaetae are present on the lateral side. The s-microchaeta is between a6 and m7. The pseudopore is near m2. An extra macrochaeta is present near p 5 in only one specimen.

Second instar ( Fig. 3B ). Primary chaetae m4 and p3 become macrochaetae; m6 becomes a bothriotrichum; ap5 becomes a mesochaeta; m1, m2, m5, am5, and p1 become microchaetae. Chaetae in the antero-lateral corner (a6, a7, m7, and s-chaetae around them) are moved postero-laterally. Secondary macrochaetae appear anteriorly to row a, forming the collar; secondary mesochaetae appear on the lateral side; about 17 microchaetae appear in a scattered manner. The number of s-chaetae remains unchanged.

Third instar ( Fig. 4A ). The antero-lateral chaetae are moved further postero-laterally. Chaeta p5 becomes a microchaeta. More secondary macrochaetae and mesochaetae appear in the collar. No more microchaeta and s-chaeta appear. Chaeta m4 becomes a mesochaeta (or remains its primary status) in only one specimen.

Fourth instar ( Fig. 4B ). Chaeta a6 becomes a bothriotrichum; ap5 becomes a microchaeta. Number of macrochaetae and mesochaetae in the collar increases further. A few microchaetae are added.

Adult macrochaetotaxy ( Fig. 11A ). The anterior margin of the tergum has a medial ‘collar’ formed by the secondary chaetae m.a2a, m.a3a, and m.a4a. Postero-lateral to the collar and anterior to row a are about eight macrochaetae in an approximate row (hereafter called aa’ series), formed by m.a5a, m.a6a, and some members of the m.a4a. Among the primary chaetae, a2–5, m3, m4, and p2–4 are macrochaetae; a6 and m6 are bothriotricha. Patterns of microchaetae and s-chaetae are not determined.

Th. III: First instar ( Fig. 3A ). Numbers of chaetae in each row are as follows. Row a: 7; row m: 5; row p: 6. Chaetae a4, m6, p1, p3, and p5 are macrochaetae, others are mesochaetae. One s-microchaeta and six normal s-chaetae are present on the lateral side. The s-microchaeta is between a7 and m7. One normal s-chaeta is close to the s-microchaeta, the other five are close to a4, a6, m7, p5, and p6, respectively. The pseudopore is approximately between a1 and m2.

Second instar ( Fig. 3B ). Primary chaetae m6 becomes a bothriotrichum; most mesochaetae become microchaetae except for a7, m7, p2, and p6. Secondary mesochaetae appear on the lateral side; five microchaetae appear near m3, m6, p1, p2, and p5, respectively. The number and position of s-chaetae remain unchanged.

Third instar ( Fig. 4A ). Chaetae p2 and p6 become microchaetae. No other significant changes occur except for the addition of lateral mesochaetae and occasionally a microchaeta beside p1.

Fourth instar ( Fig. 4B ). Chaeta a7 becomes smaller. Other characters remain the same as for the third instar.

Adult macrochaetotaxy ( Fig. 11A ). Chaetae a4, p1, p3, and p5 are macrochaetae; m6 is a bothriotrichum. Patterns of microchaetae and s-chaetae are not determined.

Abd. I: First instar (Fig. 3A). Numbers of chaetae in each row are as follows. Row a: 4; row m: 3; row p: 5. Chaetae m2–4 are macrochaetae, others are mesochaetae. Five normal s-chaetae are present in a posterior row. Four s-chaetae are posterior to m2, m3, m4, and p5, respectively; one s-chaeta is between p5 and p6. The pseudopore is approximately between a1 and m2.

Second instar ( Fig. 3B ). Mesochaetae become microchaetae except for p5 and p6. A secondary mesochaeta appears anteriorly to p6; two microchaetae appear beside p5. The number and position of s-chaetae remain unchanged.

Third instar ( Fig. 4A ). A secondary mesochaeta appears externally to p6. Other chaetae remain unchanged.

Fourth instar ( Fig. 4B ). A secondary mesochaeta appears beside p6. Other chaetae remain unchanged.

Adult macrochaetotaxy ( Fig. 11A ). Chaetae m2–4 are macrochaetae. Patterns of other chaetae are not determined.

Abd. II: First instar ( Fig. 3A ). The chaetotaxy is almost identical to that of Abd. I, except that row a has one more mesochaeta (a6).

Second instar ( Fig. 3B ). Mesochaetae become microchaetae except for a6, p5, and p6. A secondary mesochaeta p5e appears between p5 and p6. Two secondary microchaetae appear between a6 and p5, and between m4 and p5, respectively. The number and position of s-chaetae remain almost unchanged, except that the most lateral one is placed more anteriorly.

Third instar ( Fig. 4A ). Chaeta a6 is moved forward. Secondary chaeta p5e becomes a microchaeta. Additional mesochaetae appear on the lateral side. An additional microchaeta appears externally to p5e. Other chaetae remain unchanged.

Fourth instar ( Fig. 4B ). No significant changes occur except that a secondary microchaeta appears anteriorly to m3.

Adult macrochaetotaxy ( Fig. 11A ). Chaetae m2–4 are macrochaetae. Patterns of other chaetae are not determined.

Abd. III: First instar ( Fig. 3A ). Numbers of chaetae in each row are as follows. Row a: 5; row m: 6 * 1; row p: 6. Chaeta a5 is a bothriotrichum, m3 is a macrochaeta, others are mesochaetae. One s-microchaeta and five normal s-chaetae are present. The s-microchaeta is between m6 and p6. One normal s-chaeta is close to the s-microchaeta, the other four are close to m3, m4, m5, and p3, respectively. The pseudopore is close to m1.

Second instar ( Fig. 3B ). Primary chaetae m6, p1, p3, and p6 become macrochaetae; most mesochaetae become microchaetae except for a7, m7, and p7. About six secondary microchaetae appear in a scattered manner. The number and position of s-chaetae remain unchanged.

Third instar ( Fig. 4A ). Chaetae p6 and p7 are either macro- or mesochaetae. Additional mesochaetae appear on the lateral side. A few more microchaetae are also added.

Fourth instar ( Fig. 4B ). Chaetae a7 and m7 are moved more externally. No other significant changes occur.

Adult macrochaetotaxy ( Fig. 11A ). Chaetae m3, m6, p1, p3, p6, and p7 are macrochaetae; a5 is a bothriotrichum. Patterns of other chaetae are not determined.

Abd. IV: First instar ( Fig. 3A ). Numbers of chaetae in each row are as follows. Row a: 7; row m: 7 * 3; row p: 7 * 4. Chaeta a2 and a5 are bothriotricha, m6 and p7 are macrochaetae, others are mesochaetae. About 26–32 normal and 19–23 long s-chaetae are present. Lengths of the long s-chaetae gradually increase from the anterior ones to the posterior ones. The five longest (longer than Abd. V) and most posterior s-chaetae are each associated with a mesochaeta in row p (p1–5). The pseudopore is close to ap1.

Second instar ( Fig. 3B ). Primary chaeta p6 becomes a macrochaeta; most mesochaetae become microchaetae except for a7 and m7. Three secondary microchaetae appear near m4, p1, and p6, respectively. The pattern of s-chaetae remains unchanged.

Third instar ( Fig. 4A ). No other significant changes occur than a few added microchaetae.

Fourth instar ( Fig. 4B ). No significant changes occur. In only one specimen an additional long s-chaeta appears occasionally beside p4 on one side.

Adult macrochaetotaxy ( Fig. 11A ). Chaetae m6, p6, and p7 are macrochaetae; a2 and a5 are bothriotricha. Patterns of other chaetae are not determined.

Abd. V: First instar ( Fig. 3A ). Numbers of chaetae in each row are as follows. Row a: 4; row m: 4; row p: 7. Externolateral chaetae (el) are present. All chaetae in row m are macrochaetae, others are mesochaetae. Seven normal s-chaetae are present, with two in front of row a, three between row a and m, one posterior to p6, and one beside el. The pseudopore is absent.

Second instar ( Fig. 3B ). Most mesochaetae become microchaetae except for p5 and p7. Secondary chaetae appear only at lateral side, including a very lateral macrochaeta. The pattern of s-chaetae remains unchanged.

Third instar ( Fig. 4A ). No significant changes occur.

Fourth instar ( Fig. 4B ). No significant changes occur.

Adult macrochaetotaxy ( Fig. 11A ). Chaetae m2, m3, m5, and m6 are macrochaetae. The secondary lateral macrochaeta is moved further lateral and not considered as a dorsal chaeta. Patterns of other chaetae are not determined.

Abd. VI: First instar ( Fig. 3A ). Numbers of chaetae in each row are as follows. Row a: 2; row m: 4; row p: 5. All chaetae in row m are macrochaetae, others are all mesochaetae. No s-chaetae or pseudopore are present.

Second instar ( Fig. 3B ). Chaetae a0, a3, p1, and p4 become microchaetae.

Third instar ( Fig. 4A ). No significant changes occur.

Fourth instar ( Fig. 4B ). A microchaeta is added between p3 and p4.

The adult macrochaetotaxy is not observed.

S-chaetotaxy from Th. II to Abd. V

Normal s-chaetae: 12–16/6/5/5/5/26–32 * 19–23 (long)/7

S-microchaetae: 1/1/0/0/1/0/0.

Figures

Figure 2. Cephalic chaetotaxy of Tomocerus tropicus. A, First instar; (B) second instar; (C) third instar; (D) fourth instar.

Figure 3. ºergal chaetotaxy of Tomocerus tropicus. A, First instar; (B) second instar.Open circle with a slash: pseudopore, the same in following figures.

Figure 4. ºergal chaetotaxy of Tomocerus tropicus. A, Ŋird instar; (B) fourth instar.Half black half white circle (e.g., m4 on Ŋ. II in Fig.4A): chaetae that are either macro- or mesochaetae.

Figure 10. Cephalic chaetotaxy of the adults. A, Tomocerus tropicus; (B) Tomocerus nabanensis; (C) Tomocerus nan.

Figure 11. ºergal macrochaetotaxy of the adults. A, Tomocerus tropicus; (B) Tomocerus nabanensis; (C) Tomocerus nan. Only Ŋ. II is shown for T.nabanensis and T.nan since other segments are identical to those of T. tropicus.