Unravelling the obscure homology: postembryonic development of chaetotaxic traits in a basal hexapod taxon (Collembola: Tomoceridae)

Abstract

Homology assessment is essential for systematic zoology, but for many taxa and traits, precise recognition of homology can be challenging with limited evidence. Tomoceridae is one of the earliest branches in the elongate-bodied springtails (Collembola: Entomobryomorpha), and is among the commonest and best-known families in the class. However, the homology of some important traits is still obscure in this group, including its characteristic chaetotaxic traits. In this study, we conducted the first rigorous homology assessment of chaetotaxy in Tomoceridae based on ontogenetic evidence. The postembryonic development of chaetotaxy was investigated in three species of Tomocerus. The primary chaetotaxy of first instar larvae show high similarity within Tomocerinae. The postembryonic development involves extensive transformations between chaetal types. The continuous dwarfism of primary chaetae during development leads to apparent oligochaetosis in adults. The thoracic bothriotricha are transformed from primary macrochaetae. The sensory chaetotaxy is stable during postembryonic development, and shows interspecific differences mainly on the mesothorax and the fourth abdominal segment. This study will promote the future use of chaetotaxic traits in the taxonomic and phylogenetic studies of Tomoceridae, and will potentially illuminate the early evolution of Entomobryomorpha.

Introduction

Precise recognition of homology is crucial for systematic, developmental, and evolutionary biology underpinned by comparative analyses of homologous traits. However, the direct empirical test of homology, i.e. observing the process of evolutionary divergence from a common ancestor, is impossible for most macroevolutionary events, thus palaeontological and ontogenetic observations are commonly used as proxies. Ideally, the descent of traits can be documented by continuous fossil records (Smith 1994), but well-preserved fossils for detailed comparison are often found wanting, particularly for soft tissues and tiny organisms. Therefore, the ontogenesis of traits, with some debate (Scholtz 2005), have become an established approach in homology assessment (e.g. Hoffmann and Britz 2006, Cox 2012, Ito et al. 2021). In this approach, homology is identified by shared developmental pathways of traits (Roth 1984).

The wingless hexapods Collembola are among the most numerically abundant and ecologically important animals in soil related ecosystems (Potapov et al. 2023). The classification of Collembola has been primarily based on external morphology, which has also informed phylogenetic inferences (e.g. D’Haese 2003a, Zhang et al. 2019). Among frequently used morphological traits, the patterns of arrangement of various chaetae on cuticle, i.e. the chaetotaxy, is among the most informative (Deharveng 2004). To effectively compare the chaetotaxy among collembolan taxa, the homologization of chaetae is necessary. This is conventionally done by designating each chaeta in a given nomenclature system (e.g. Jordana and Baquero 2005). For some oligochaetotic taxa, this approach can be applied directly in adults because of very limited ontogenetic changes (D’Haese 2003b), but for most plurichaetotic taxa, such as Entomobryoidea Womersley, 1934, it is not feasible because of the extensive addition of secondary elements (neochaetosis) and the shifts of chaetal type and position during postembryonic development. More confusing cases occur in many of the ‘scaled’ taxa, e.g. Lepidocyrtinae Wahlgren, 1906 and Tomoceroidea Szeptycki, 1979. In these groups, the adults are plurichaetotic in a broad sense, but most ‘chaetae’ have been modified into scales. The scales are not only by themselves less informative than chaetotaxy, but also often obscure the observation of other chaetae, notably the smaller ones. The remaining informative chaetae, namely the macrochaetae, are very sparse on the body, leading to an oligochaetotic appearance. Different from the genuine oligochaetosis, the secondary oligochaetosis, or more precisely, ‘oligo-macrochaetosis’, is formed by the deformation of primary patterns, including the shifts of chaetal positions and transformations of large primary chaetae into tiny chaetae and even to scales (Barra 1975, Szeptycki 1979). Therefore, chaetal homology in the plurichaetotic and secondary oligochaetotic groups can only be identified by ontogenetic analyses, i.e. tracing the postembryonic changes of each element in the chaetotaxy.

Ontogenetic observations on the chaetotaxy have been applied to Symphypleona (e.g. Nayrolles 1996, 1998, Betsch 1997), Neelipleona (Schneider 2017), Poduromorpha (e.g. Thibaud 1967, 1969, Gruia 1974, Snider 1977, D’Haese 2003b), and Entomobryoidea (e.g. Szeptycki 1979, Soto-Adames 2008, Zhang et al. 2011, 2019), and Isotomidae (e.g. Deharveng 1977, Potapov et al. 2010) in Entomobryomorpha. However, it is only in Entomobryoidea that the postembryonic development of cephalic and tergal chaetotaxy has been extensively recorded and rigorously analysed for homology assessment. These studies have successfully resolved the chaetal homology in the most plurichaetotic groups of Collembola, and made it possible for a comprehensive phylogenetic analysis based on homologized chaetotaxic traits (Zhang et al. 2019). By contrast, the chaetotaxy of another major family of Entomobryomorpha, the Tomoceridae Schäffer, 1896, has never been subject to ontogenetic assessment for homology. Tomocerids are typically oligo-macrochaetotic and scaled Collembola, with apparently simplified and rather conservative adult macrochaetotaxy across species (Yosii 1967). It has been noticed that the chaetotaxy of early instar larvae of tomocerids differs sharply from that of the adults (Goto 1956, Uchida and Chiba 1958), which even led to erroneous establishment of new genera for the juveniles (Denis 1931, Womersley 1942), but rigorous analyses have not been reported. As a result, current interpretations of tomocerids’ chaetotaxy are often merely based on general patterns (e.g. Park et al. 2011, Yu et al. 2014) or attempted homologizations based only on adult status, while the latter may lead to incompatible interpretations (e.g. Barjadze et al. 2016, Chang and Park 2020). The lack of reliable homology assessment has hampered the further exploration of chaetotaxic traits for taxonomic and phylogenetic use in Tomoceridae. Given that Tomoceridae has been recovered as the basal branch of Entomobryomorpha (Yu et al. 2022), the ontogenetic information of its chaetotaxy may also illuminate the early evolution of this order.

To shed light on the obscure chaetal homology in Tomoceridae, we have investigated the postembryonic development of the chaetotaxy of three species of the type genus Tomocerus Nicolet, 1842. In this study, the chaetotaxy of the early instar larvae was studied in detail and compared with the corresponding adult patterns. Homology was determined following established systems for Entomobryomorpha (Szeptycki 1972, 1979, Jordana and Baquero 2005). Particularly, we focused on elucidating the formation of oligo-macrochaetosis, the developmental origin of bothriotricha, and the pattern of specialized sensory chaetae.

Material and methods

Juveniles of three species of Tomocerus have been examined. Living Tomocerus tropicus Yu, Yang and Liu, 2018 was brought from the type locality to the lab and cultured with yeast under 20 °C in an incubator. Eggs were separated into microcosms and monitored once per day. Instars (each usually lasting for 3–5 days) were counted by recording new exuviae after moults. For each instar, two to four individuals were collected and preserved in ethanol. The specimens were then cleared in Nesbitt’s Fluid and mounted in Marc André II Solution on slides for microscopic examination. The observation and collection were continued until the chaetotaxy had transformed to the adult pattern in the fourth instar. Specimens of Tomocerus nabanensis Yu, Yang and Liu, 2018 and Tomocerus nan Yu, Yang and Liu, 2018 were sorted from Berlese samples and determined to each instar by comparing their sizes and morphology to those of the cultured T. tropicus. The first four instars were obtained for T. nabanensis, and the first and third instars were obtained for T. nan. Scanning electron microscopic photos were taken from adult specimens with a Hitachi S-4800 High Resolution Scanning Electron Microscope (SEM). All specimens are preserved at Nanjing Agricultural University.

The cephalic and tergal chaetotaxy of the slide-mounted specimens were illustrated and studied with a Nikon Eclipse Ni Microscope. The prothorax is devoid of chaetae and thus not treated. Only the left sides are shown because the chaetotaxy is generally bilaterally symmetrical. The nomenclature and homology of cephalic chaetotaxy followed the system proposed for Entomobryoidea by Mari-Mutt (1979), Jordana and Baquero (2005), and Soto-Adames (2008), in which the cephalic chaetae are arranged in rows named after their positions: antennal (An), anterior (A), medio-ocellar (M), ocular (p, q, r, s, t, v, Mari-Mutt 1986), sutural (S), post-sutural (Ps), postero-anterior (Pa), postero-medial (Pm), postero-posterior (Pp), and postero-external (Pe). The system of tergal chaetae followed the a-m-p (anterior-medial-posterior) row system proposed for Poduromorpha by Yosii (1956, 1960) and adapted to Entomobryomorpha by Szeptycki (1972, 1979) and Deharveng (1977). The homology of primary tergal chaetae (chaetae present in the first instar) with respect to Entomobryoidea and Isotomidae was determined after Yu et al. (2016b) and Zhang et al. (2019) with a few modifications, mainly following the homologization criteria of Zhang et al. (2019).

Under the aforementioned systems, chaetae are named after the following rules. Primary chaetae matching in the rows are numbered from the internal to the external side (0 for the unpaired chaetae in the middle line of terga), according to their determined homology with respect to a supposed ‘complete form’ (e.g. Szeptycki 1979, Jordana and Baquero 2005), then each chaeta is named with the abbreviated row-name plus its number in the row. For example, the inner-most primary chaetae in the anterior (a) row of a tergum is named a1. Where a primary chaeta is not in any of the existing rows, it is named after a primary chaeta in the row preceded by a letter indicating its relative position to this chaeta (a: anterior, p: posterior, i: internal, e: external). For example, am5 is a primary chaeta anterior to m5. Secondary chaetae are named after the nearest chaeta occurring in earlier instars followed by a letter indicating their relative position to this chaeta. Where two or more closely located secondary chaetae appear in the same direction, numbers are added to distinguish them. For example, if two secondary chaetae are anterior to a2, they are named a2a and a2a2, respectively; if a third chaeta external to a2a appears in the next instar, it is named a2ae. To briefly describe a group of closely located secondary chaetae, series of chaetae are named after the main chaeta preceded by the letter m. For example, chaetae a2a, a2a2, a2a3, and a2ae can be called m.a2a. In the illustrations, primary chaetae of the same row and secondary chaetae related to them are connected by dotted lines, and the initial letter indicating the row is only given in the name of the first primary chaeta in the row. Because patterns of most lateral chaetae and secondary microchaetae show high variability among individuals and are not important for systematics, they are not treated in detail in this study.

Terms and abbreviations: primary/secondary chaetae: chaetae present in the first instar/appearing in later instars; micro-/meso-/macrochaetae, s-chaetae, bothriotricha: see results ‘Chaetal types’; Th.: thoracic segment; Abd.: abdominal segment.

Results

Chaetal types

Microchaetae and mesochaetae

These two types are acuminate ordinary chaetae. Under an optical microscope, the microchaetae are smooth, whereas the mesochaetae are smooth to faintly ciliated (Fig. 1A). Under the SEM the microchaetae are striated or slightly ciliated (Fig. 1B). These two types are the most abundant chaetae on the head and body in the adult stage. In living and ethanol preserved specimens, both the two types of chaetae attach to the cuticle in a small acute angle, i.e. they are almost parallel to the long axis of body. The only major difference between them is that mesochaetae are distinctly thicker and longer. In the first instar the size difference is so obscure that all chaetae of this type are treated as mesochaetae.

Macrochaetae

In the taxonomic descriptions of Tomoceridae, macrochaetae are often defined as ordinary chaetae with larger sockets than others, because most of them are easily lost during slide preparation and thus the morphology is often unknown. Observation of intact Tomocerus specimens, either living or ethanol preserved, revealed four morphological types of chaetae on these large sockets (Fig. 1A). Type I is straight, blunt, and club-like, distributed on the head and most body segments (Fig. 1B); type II is curved, tapering and with rounded apex, usually on Abd. III, IV, and V; type III is slender, curved, and acuminate, also distributed on posterior segments. These three types of chaetae are usually from slightly to intensely serrate, and their bases always attach almost vertically to the cuticle. In comparison, the fourth type, which is distributed mainly around antennal bases and on the lateral side of the head and body, is morphologically identical to the mesochaetae. Therefore, only types I, II, and III are essentially different from the mesochaetae. Moreover, in the early instars, many type I chaetae are smaller than mesochaetae, making it impossible to distinguish them by size. Therefore, to avoid confusion, here we use the morphological criteria, but not the size criteria to differentiate macrochaetae and mesochaetae, i.e. we define macrochaetae as the chaetae of types I, II, and III regardless of their sizes.

Bothriotricha

This type of chaetae is extremely slender and intensely ciliated (Fig. 1A, D). In adult Tomocerinae, bothriotricha are consistently distributed on several terga on a relatively fixed pattern, i.e. 1 or 2 on Th. II, 1 on Th. III, 1 on Abd. III, and 2 on Abd. IV.

S-chaetae

Chaetae of this type are different from ordinary chaetae in morphology and texture, and can be further classified into normal s-chaetae and s-microchaetae. The normal s-chaetae are slender, subcylindrical, and apically rounded (Fig. 1A). They are usually thin-walled and thus more hyaline than ordinary chaetae under optical microscopes. Under the SEM, they are genuinely smooth, and usually with several basal rings (Fig. 1E). Their lengths can vary from as long as a microchaeta, to as long as a large mesochaeta, depending on their positions on the body. In this work, we term the distinctively elongated normal s-chaetae on Abd. IV the ‘long s-chaetae’ to distinguish them from other s-chaetae of normal length. Compared to the normal s-chaetae, s-microchaetae are thicker, darker, and more tapering (Fig. 1A, F). They are always subequal to or smaller than the size of a microchaeta, and often lie in shallow furrows in the cuticle.

Scales

The morphology of scales in Tomoceridae has been well known (Tullberg 1872) and is consistent across species. Most scales are broad, apically rounded, and with strong longitudinal ribs. Some scales on the ventral side of dens (the middle segments of furca) can be rather narrow. In Tomocerus, scales are absent at the first instar and emerge at the second instar. The details of the pattern of scales and its development are not treated here.

Chaetotaxy

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 p5 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. Externo-lateral 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.

Tomocerus nabanensis

Specimens examined. 17BN1TJ (1–8), vii.2017, leg. Shengjie Liu.

Head

First instar (Fig. 5A). The pattern is identical to that of T. tropicus.

Second instar (Fig. 5B). The chaetal transformation is identical to that of T. tropicus. The neochaetosis is also similar, with the exception that the secondary chaeta An1a appears instead of An1a0 and A2a0.

Third instar (Fig. 5C). Macrochaetae Pm1, Pp1, and Pe2 become mesochaetae. Pp3 becomes distinctly smaller. Secondary chaetae An1p, An3a2, An3i, A2a0, and Pe3i appear.

Fourth instar (Fig. 5D). Pm1, Pp1, Pp3, and Pe2 become microchaetae. Additional micro- and mesochaetae appear only in row An and along the postoccipital collar.

Adult macrochaetotaxy (Fig. 10B). The pattern is very similar to that of T. tropicus, but Pp3 is a microchaeta.

Th. II

First instar (Fig. 6A). The patterns of ordinary chaetae, s-microchaetae, and pseudopores are identical to that of T. tropicus. About 10–13 normal s-chaetae are present.

Second instar (Fig. 6B). Primary chaetae m1, m2, and p3 become macrochaetae; m6 becomes a bothriotrichum; ap5 becomes a mesochaeta; m4, am5, and p1 become microchaetae. Chaetal movement and neochaetosis of macro- and mesochaetae are similar to those of T. tropicus, but a4 is devoid of associated secondary chaetae at this stage. About 10 microchaetae appear in a scattered manner. The number of s-chaetae remains unchanged.

Third instar (Fig. 7A). The changes are very similar to those in T. tropicus, except that about five additional microchaetae appear.

Fourth instar (Fig. 7B). The changes are very similar to those in T. tropicus, including the transformation of a6 into a bothriotrichum.

Adult macrochaetotaxy (Fig. 11B). Collar and aa’ series of macrochaetae are well developed. Chaetae a2–5, m1–3, and p2–4 are macrochaetae; a6 and m6 are bothriotricha.

Th. III, Abd. I, and II

The first instar (Fig. 6A), postembryonic development (Figs 6B, 7), and adult macrochaetotaxy are almost identical to those of T. tropicus, except for minor differences in the transformation and addition of microchaetae.

Abd. III

First instar (Fig. 6A). The pattern is almost identical to that of T. tropicus, with minor difference in some chaetal positions.

Postembryonic development (Figs 6B, 7). The changes are very similar to those in T. tropicus, except that the transformation of p6 and addition of microchaetae occur later.

Adult macrochaetotaxy. The pattern is identical to that of T. tropicus.

Abd. IV

First instar (Fig. 6A). The patterns of ordinary chaetae, bothriotricha, and pseudopores are identical to those of T. tropicus. A total of 21–25 normal and 12–13 long s-chaetae are present. The five most posterior long s-chaetae are the longest (subequal to the length of Abd. V), and are each associated with a mesochaeta in row p (p1–5). The other long s-chaetae are subequal in length.

Postembryonic development (Figs 6B, 7). The changes are very similar to those in T. tropicus, except for minor and apparently random differences in microchaetae and lateral mesochaetae.

Adult macrochaetotaxy. The pattern is identical to that of T. tropicus.

Abd. V

First instar (Fig. 6A). The pattern of ordinary chaetae is almost identical to that of T. tropicus, except that in one case p4 is absent. Six normal s-chaetae are present, with two in front of row a, two between row a and m, one posterior to p6, and one beside el. Occasionally, the s-chaeta posterior to a3 is absent and an extra s-chaeta is present posterior to p6.

Postembryonic development (Figs 6B, 7). The changes are almost identical to those in T. tropicus, except that some mesochaetae become microchaetae in different instars.

Adult macrochaetotaxy. The pattern is identical to that of T. tropicus.

Abd. VI

The chaetotaxy at the first two instars (Fig. 6) is identical to that of T. tropicus.

Status in the later instars and adult is not observed.

S-chaetotaxy from Th. II to Abd. V

Normal s-chaetae: 10–13/6/5/5/5/21–25 + 12–13 (long)/6

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

Tomocerus nan

Specimens examined. 15HN2TJ (1–5), 25.xii.2015, leg. Daoyuan Yu and Chunyan Qin.

Head

First instar (Fig. 8A). The pattern is identical to that of T. tropicus.

Third instar (Fig. 8B). Chaetal transformation similar to that of T. nabanensis at the third instar, but S0 and Pp3 become microchaetae. Secondary chaetae An1a, An1p, An3a, An3a2, An3a3, Pa5a, Pp1i, and Pp3e appear.

Adult macrochaetotaxy (Fig. 10C). The pattern is very similar to that of T. nabanensis, but S0 is a microchaeta.

Th. II

First instar (Fig. 9A). The patterns of ordinary chaetae, s-microchaetae, and pseudopores are very similar to those of T. tropicus, except that a2 is a mesochaeta. Six normal s-chaetae are present on the lateral side.

Third instar (Fig. 9B). Primary mesochaeta p3 becomes a macrochaeta; a6 and m6 become bothriotricha; macrochaetae a5 and ap5, and mesochaetae except for a7, m7, and p6 become microchaetae. Chaetal movement and neochaetosis of macro- and mesochaetae are very similar to those of T. tropicus. About 15 microchaetae appear in a scattered manner. The pattern of s-chaetae remains unchanged.

Adult macrochaetotaxy (Fig. 11C). Besides the collar and aa’ series, chaetae a3, a4, m3, and p2–4 are macrochaetae; a6 and m6 are bothriotricha.

Th. III, Abd. I, Abd. II, and Abd. III

The primary patterns (Fig. 9A), postembryonic changes (Fig. 9B), and final patterns are almost identical to those of T. tropicus, except for minor differences in chaetal positions and fewer microchaetae.

Abd. IV

First instar (Fig. 9A). The patterns of ordinary chaetae, bothriotricha, and pseudopores are similar to those of T. tropicus, but pm7 and ap2 are absent, and p7 is a mesochaeta. About 14–17 normal and 11–14 long s-chaetae are present. The five most posterior long s-chaetae are the longest (shorter than Abd. V), and are each associated with a mesochaeta in row p (p1–5). The other long s-chaetae are subequal in length.

Third instar (Fig. 9B). Primary chaeta p7 becomes a macrochaeta. Other changes are almost identical to those in T. tropicus.

Adult macrochaetotaxy. The pattern is identical to that of T. tropicus.

Abd. V

The primary pattern (Fig. 9A), postembryonic changes (Fig. 9B), and final pattern are almost identical to those of T. nabanensis, except that some mesochaetae become microchaetae in different instars.

Abd. VI

The primary chaetotaxy (Fig. 9A) is identical to that of T. tropicus.

Status in the later instars and adult is not observed.

S-chaetotaxy from Th. II to Abd. V

Normal s-chaetae: 6/6/5/5/5/14–17 + 11–14 (long)/6

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

Discussion

Postembryonic development and homology of macrochaetotaxy

The primary chaetotaxy (s-chaetae excluded) of the three examined Tomocerus spp. are almost identical, with only minor interspecific variations on Th. II and Abd. IV. The pattern is also very similar to those described in Tomocerina varia (Folsom, 1899) (Yu et al. 2016b) and Tomocerus minor (Lubbock, 1862) (Zhang et al. 2019), and less so to that of Pogonognathellus flavescens (Tullberg, 1871) (Szeptycki 1972), suggesting that the primary chaetotaxy is potentially informative for supraspecific classification and phylogeny.

During postembryonic development, drastic changes occur to the primary chaetotaxy. According to our observation, the oligo-macrochaetosis is caused by the extensive ontogenetic transformations of primary chaetae into microchaetae, but not into scales. For example, in the first instar, Pp1, Pm1, Pe2, and Pe3 near the posterior margin of the head are prominent macrochaetae and mesochaetae, while after several instars, they become merely microchaetae in the postoccipital collar. Some microchaetae transformed from the other types, e.g. ap5 on Th. II, are longer and distinguishable from the secondary microchaetae, whereas others are unrecognisable by morphology from the secondary ones. Similar dwarfism of chaetae during postembryonic development has also been reported in other Collembola taxa with dense scales, e.g. Lepidocyrtinae (Barra 1975, Szeptycki 1979), probably as a result of evolutionary convergence.

Despite the extensive dwarfism of primary chaetae, macrochaetae are still increased in some areas, either through transformation or neochaetosis. The transformation occurs on the head, Th. II, Abd. III, and Abd. IV. Notably in the anterior areas of head, all macrochaetae are transformed from primary mesochaetae. Interestingly, Pm1 on the posterior margin of the head transforms into a macrochaetae in the second instar, but finally merges into the postoccipital collar in the form of a microchaeta. The neochaetosis of macrochaetae only occurs in the foremost area of Th. II, where macrochaetae associated with row a are continuously added, forming a dense mesothoracic ‘collar’. The chaetal homology in this area is relatively difficult to determine due to regional plurichaetosis. If we follow the ‘large-ones-first’ criterion in Entomobryoidea, i.e. primary chaetae are often larger than adjacent secondary ones (Zhang et al. 2019), then the secondary chaetae around a3 and a4 should appear both anteriorly and posteriorly to the primary chaetae in T. tropicus, and a3 and a4 should have moved anteriorly and merged into the collar since the second instar. However, in T. nabanensis whose a4 has no associated secondary chaetae at the second instar, it is clear that a4 remains at its original position during the development, while secondary chaetae are only added anteriorly to it. The relative position between a2, a3, and a4 suggests that the position of a3 is also static. Therefore, considering the high interspecific similarity of final chaetotaxy in this area, we conclude that a3 and a4 do not merge into the mesothoracic collar in all the three species, and thus abandon the ‘large-ones-first’ criterion in interpreting this area. In this respect, the formation of the mesothoracic collar in Tomocerus is different from that in Entomobryoidea (Szeptycki 1979, Soto-Adames 2008, Zhang et al. 2011), in which most of the primary chaetae in row a (except for a5) have been surrounded by additional macrochaetae, and thus merged into the ‘collar’ since the third instar.

Transformation of thoracic bothriotricha

Among all the issues about chaetal homology in Tomocerinae, the identities of thoracic bothriotricha have been the most mysterious. The bothriotricha are important sensors of airborne vibration (Drašlar 1973), and are thus among the key functional traits for an epigeic lifestyle. Szeptycki (1972) has shown that the first instar of P. flavescens is devoid of thoracic bothriotricha, and this has been also confirmed by Yu et al. (2016b) in T. varia and Zhang et al. (2019) in T. minor. Yu et al. (2016a) suggested these chaetae are either secondary elements or transformed primary chaetae, but no conclusion was given.

This puzzle has been resolved in the present study. As shown in our results, the transformation of m6 on Th. III from a macrochaeta to a bothriotrichum is obvious, because the general pattern of chaetotaxy does not change during postembryonic development. By contrast, the chaetotaxy of the antero-lateral area of Th. II where the bothriotricha are located shows drastic change during early instars. Secondary chaetae are added, and the relative positions of primary chaetae are also changed, probably due to the allometric growth of different tergal areas. Using the unique s-microchaeta as a landmark, we can notice a gradual shift of a6, a7, and m7 postero-externally from their original positions since the second instar. Alongside this change, m6 transforms to the inner bothriotrichum at the second instar, and a6 becomes gradually thinner and more ciliate until it turns completely to the outer bothriotrichum at the third or fourth instar (Fig. 12). During the transformation, the intermediate morphology and position of a6 are almost identical to the bothriotrichum-like macrochaetae in some species (Yu et al. 2016a), suggesting homology among them. Therefore, given also that the presence of bothriotrichum-like macrochaetae in adults is derived from the 2-bothriotricha pattern (Yu et al. 2021), the former status is probably a neotenic feature.

S-chaetotaxy

The s-chaetotaxy is among the most informative characters for the taxonomy and phylogeny of Collembola (Deharveng 1979, Potapov 1989, Zhang et al. 2015). Before our study, the complete s-chaetotaxy had been reported for the first instar of P. flavescens (Szeptycki 1972), T. varia (Yu et al. 2016b), and T. minor (Deharveng 1979, Zhang et al. 2019). Since only a few specimens of three species were examined, the taxonomic value and postembryonic development of s-chaetotaxy in Tomocerinae were unclear. In the present study, we have confirmed that, similar to most other Collembola (Deharveng 2004), the s-chaetotaxy of the observed species remains constant during postembryonic development. Although a few intraspecific variations have been seen among individuals, the number and pattern of s-chaetae are relatively stable intraspecifically, and more different interspecifically. The strongest interspecific variation in s-chaetotaxy occurs in Th. II and Abd. IV, where the three Tomocerus spp. show different levels of ‘sensillary polychaetosis’ (Potapov et al. 2006). Other terga have consistent s-chaetotaxy across the three species, with the single exception that T. tropicus has one more s-chaeta on Abd. V. Compared with the three species studied previously, the s-chaetotaxy described here is more similar to that of T. minor, suggesting this character may be useful for generic classification, but more species need to be examined. On a higher taxonomic level, the presence of mixed short and long s-chaetae on Abd. IV resembles that of some basal Entomobryoidea (Zhang et al. 2019), probably suggesting a phylogenetic signal.

The 1/1/0/0/1/0/0 pattern of s-microchaetae is consistent among the species studied here, but is, however, different from most patterns reported for other Tomocerinae before, i.e. 1/0/0/0/0/0/0 for T. varia (Yu et al. 2016b), 1/0/0/0/1/0/0 for T. minor (Zhang et al. 2019) and Monodontocerus Yosii, 1955 (Yu et al. 2014), and 1, 1/1, 1, 1, 1, 0 for PlutomurusYosii, 1956 (Chang and Park 2020). Barjadze et al. (2016) reported numerous ‘microsensillae’ in Plutomurus revazi Barjadze, Baquero, Soto-Adames, Giordano & Jordana, 2016. Only Yu and Deharveng (2015) reported the same 1/1/0/0/1/0/0 pattern for Yoshiicerus caecus (Yu & Deharveng, 2015). If all these reports are correct, then the s-microchaetotaxy should be highly variable within Tomocerinae. However, this conclusion may be doubtful. On the one hand, the s-microchaetae can be extremely small (e.g. the one on Th. III), so they are easily obscured by other chaetae or simply omitted in some observations. On the other hand, the differentiation between s-microchaetae and normal s-chaetae, and even ordinary microchaetae, can be sometimes subjective. For example, Szeptycki (1972) did not even distinguish s-microchaetae from other s-chaetae in P. flavescens, but the absence of s-microchaetae in this species is unlikely. Therefore, more careful examination is required to clarify the true status of s-microchaetae within Tomocerinae.

Another remaining issue is the identity of ‘rod-like s-chaetae’ described in the adult specimens of some species of Monodontocerus and Yoshiicerus Yu, 2023 (Yu et al. 2014, Yu and Deharveng 2015). These special s-chaetae appear to be an intermediate form between the normal s-chaetae and s-microchaetae, and may be easily mistaken for the latter. Our observation did not reveal any rod-like s-chaetae in the early instars of the three Tomocerus spp. More specific study is needed to determine whether they are transformed from normal s-chaetae in some taxa.

Conclusion

We found that the primary chaetotaxy of the studied Tomocerus spp. is highly congruent among species, whereas differences in adult chaetotaxy are mainly attributed to ontogenetic divergences. The transformation of ordinary chaetae into bothriotricha suggests a broader homology between these two chaetal types in Entomobryomorpha. The s-chaetotaxy on different terga turned out to be informative for systematics on different taxonomic levels, but the difficulty in observation needs to be addressed before future application. In general, the characteristics in primary chaetotaxy and postembryonic changes support a peculiar position of Tomoceridae within the order. The patterns revealed by us are potentially applicable to other genera, but further investigation is still preferred, particularly for Plutomurus and Pogonognathellus.

Given the functional divergence between various types of chaetae, the diversity of chaetotaxic traits and their ontogenetic patterns may provide further information for the adaptive radiation among Collembola. For example, the transformation from macrochaetae to bothriotricha may suggest a functional trade-off between mechanical defence and sensation of airflow. Further observations in other groups of Collembola will help to disentangle the evolutionary trajectories within this ancient lineage of terrestrial arthropods.

Acknowledgements

We are grateful to Dr Chunyan Qin and Dr Shengjie Liu for their kind assistance during the field work.

CRediT statement

Daoyuan Yu (Conceptualization, Formal analysis, Writing—original draft, Funding acquisition), Yating Zhang (Visualization), Ziqiang Wang (Investigation), Feng Hu (Writing—review & editing) and Manqiang Liu (Writing—review & editing, Funding acquisition, Supervision)

Conflict of interest

The authors declare no conflict of interest.

Funding

This work was supported by the National Natural Science Foundation of China (grant numbers 42271059, 41971063) and the National Science and Technology Fundamental Resources Investigation Program of China (grant number 2018FY100300).

Data Availability

The data underlying this article are available in the article.

References