Willowsia jacobsoni (Börner, 1913)

Cipola, Nikolas Gioia, Katz, Aron D., Bu, Yun & Godeiro, Nerivania Nunes, 2025, Systematics of Willowsia jacobsoni (Börner, 1913) (Collembola, Entomobryidae): morphology, postembryonic development, distribution, mitogenome and phylogeny, Zootaxa 5604 (3), pp. 201-233 : 223-224

On W. jacobsoni dorsal chaetotaxy development

In Entomobryoidea, the first nomenclature for the head sutural series was based in Dicranocentrus Schött ( Orchesellidae ), considering eight primary chaetae (S0–7) and based only in adults’ morphology ( Mari Mutt 1979). After, there were different proposals (e.g. Jordana & Baquero 2005; Soto-Adames 2008) and descriptions of many Entomobryoidea, including W. jacobsoni (e.g. Mari Mutt 1981; Katz 2017), labeling differently the more lateral chaetae (S5, S5i or S6) of the sutural series, consequently generating a discrepancy in homology recognition ( Cipola et al. 2014, 2018, 2019, 2022; Zhang et al. 2011, 2017; Pan et al. 2011, 2019; Pan & Zhang 2016; Katz 2017; Cipola & Katz 2021; Viana et al. 2022, 2024; Mandal et al. 2024). Another aggravating factor was that some authors considered the closest chaeta to H eye as belonging to the sutural series (e.g. Mari Mutt 1981; Pan et al. 2011; Pan & Zhang 2016; Katz 2017), while this chaeta (s) may better fit the ‘IO’ series, a more consistent observation considering the shared dorsal chaetotaxy pattern of the Entomobryoidea (e.g. Soto-Adames 2008; Cipola et al. 2014, 2018, 2019, 2022; Pan et al. 2011, 2019; Zhang et al. 2011, 2017; Pan & Zhang 2016; Viana et al. 2022, 2024; Mandal et al. 2024). This makes sense considering that among the genera of Entomobryoidea, at least Homidia jordanai Pan & Zhang, 2011 has all eight primary chaetae (S0–7) already in the 1 st instar ( Pan et al. 2011), while some of these chaetae (e.g. S1, S4, S6) may appear secondarily in other taxa, as in Entomobrya egleri Arlé & Guimarães, 1978 , Homidia breviseta Pan, 2022 in Xiang et al. (2022), Sinhomidia uniseta Pan, Si & Zhang, 2019 (in Pan et al. 2019) ( Xiang et al. 2022; Viana et al. 2024). However, this is not the case of W. jacobsoni , which consistently lacks S1 and S4 chaetae during all its development ( Fig. 9 ). Regardless, from the head development presented here for the first time in Willowsia , it was possible to verify that the 1 st instar of W. jacobsoni is similar to those of other Entomobryinae genera ( Pan et al. 2011, 2019; Xiang et al. 2022; Viana et al. 2024).

The proposal of eight primary chaetae on the sutural series (S0 to S7) corroborates Mari Mutt (1979), and applies to different Entomobryoidea taxa, reinforcing the consistent morphology and its subsequently homology seen at least in the sutural series of dorsal head chaetae. For this reason, we believe this nomenclature needs to remain standardized in future descriptions.

In addition to the analysis of the sutural series, the M0 chaeta is lost at least in adulthood (or earlier) in the above mentioned Entomobryinae taxa (it is absent in E. egleri ), as well as in some Seirinae , like Seira ( Soto-Adames 2008) and Lepidocyrtinus Börner, 1903 (personal observation). However, in W. jacobsoni and in Lepidocyrtinae overall, this chaeta is likely present in all instars ( Barra 1975; Mateos & Winkler 2018; Cipola et al. 2019, 2020; Mateos & Greenslade 2021). The same happens with the Am0 chaeta, which remains stable during the development of W. jacobsoni , but is lost secondarily in Sinhomidia uniseta ( Pan et al. 2019) and an undescribed species of Lepidocyrtinus (personal observation). In this sense, our study provides evidence for the first time of the presence of both chaetae (M0, Am0) in Willowsia , since they are part of the head microchaetotaxy which is usually overlooked to the genus (e.g. Zhang et al. 2011; Pan & Zhang 2016; Katz 2017; Mandal et al. 2024). It is now necessary to verify whether such chaetae are constantly present within Willowsia lineages and may represent a generic characteristic.

The 1 st instar chaetotaxy of Th II to Abd V is now known at least to 14 Entomobryinae species distributed across seven genera: Coecobrya Yosii, 1956 (2), Entomobrya (3), Entomobryoides Maynard, 1951 (1), Homidia Börner, 1906 (3), Sinella (2), Sinhomidia Zhang, 2009 in Zhang et al. (2009) (1) and Willowsia (2) ( Szeptycki 1979; Pan et al. 2011, 2019; Zhang et al. 2019a; Xiang et al. 2022; Viana et al. 2024). The 1 st instar W. jacobsoni has a similar pattern compared to these taxa, but differs especially due to the presence of one extra chaeta (aa) on Th II, which is unclear if it is typical of the genus, as this segment has never been described for this instar before (see Szeptycki 1979; Zhang et al. 2019a). Another difference regards the Abd I, which lacks a typical primary chaeta (m5, a5 was mistakenly named as m5) in W. buski compared to W. jacobsoni , but this needs to be better investigated as it could be an observational mistake (see Szeptycki 1979: 166).

Compared to other Willowsia taxa, in the 2 nd instar, the Th II of the W. jacobsoni resembles W. japonica (in Zhang et al. 2019a) in all primary series, except m4 as mic and p2 as mac (the opposite in W. japonica ), as well as some secondary chaetae are only present in W. jacobsoni (m5a), while others only occur in W. japonica (a3a, m2i, m4p, p2e). Furthermore, it is noted that W. japonica has a secondary chaeta (a4e2) in the same position as the primary chaeta (aa) of W. jacobsoni , but it is difficult in this case to trace whether they are homologous or not. In Th III, W. jacobsoni differs only by a5 as mes and m5 as mac (mac and mes, respectively, in W. japonica ), in addition to two secondary chaetae (p2e, m6i) present only in the W. japonica (see that m5p was named as “ a6i ” in Zhang et al. 2019a). On Abd I, W. jacobsoni does not hold any secondary chaetae in the 2 nd instar (while m2i is present in W. japonica ), and m6 is a mac (a mic in W. japonica ), but both species have three internal mac as adults, and m6 is lost or was not observed in W. japonica (see Zhang et al. 2011: 15). The Abd II of W. jacobsoni differs from W. japonica by three mac present (m6, p6, el) and one absent (m3e), in addition to some secondary chaetae present (a1e, m3ea) and two unnamed absent ( Zhang et al. 2019a). However, the 2 nd instar of both species has three mes on the post-posterior row. It is noted that in W. jacobsoni , the m3e mac only appears in the 4–5 th instars, while in W. japonica , it appears in the 2 nd instar, plus the a2 mac emerges only during the 3 rd instar or later (and is absent in W. jacobsoni ). Therefore, the development, as well as the final pattern of Abd II mac is quite different between species. In Abd III, W. jacobsoni has four more mac (a7, am6, p6pe, p7) compared to W. japonica , while the latter species has six secondary chaetae (m7a and five unnamed), but both hold mes on the post-posterior row, six in W. jacobsoni and four in W. japonica . Furthermore, the a2 chaeta is modified into a mac only in adults of W. jacobsoni , so this modification probably also occurs in final stages of W. japonica ( Zhang et al. 2011) , but this needs to be investigated. The Abd IV of W. jacobsoni differs from W. japonica and W. cassagnaui Zhang, 2015 (in Zhang et al. 2019a) by two chaetae as mac (A5–6) plus more two (D3, E4) absent in W. cassagnaui . However, in W. japonica and W. cassagnaui these internal chaetae, together with the others, result in a total of seven mac in adults (Si, A5–6, C1, Sm, B5–6) (Zhang 2015; Zhang et al. 2011), while in W. jacobsoni the internal mac is reduced in number, with at least A5 and often A3 and Sm being absent. In different Entomobryoidea, the Si chaeta appears in the 2 nd instar ( Zhang et al. 2019a), but in W. jacobsoni , it atypically appears during the 4 th instar, and remains until adulthood. In this sense, as well as to the other tergal segments, secondary chaetae may appear independently among the three Willowsia species, including on post-posterior row. The development of the pattern of type II sensilla from the 1 st instar to the adult of W. jacobsoni is quite stable, differing only in the emergence of an extra posterior sensillum in the 2 nd instar and another anterior sensillum during the 4–5 th instars ( Fig. 12 ). The distribution pattern of these sensilla is also similar compared to E. egleri instars, only differing in the emergence of one sensillum in the 4 th instar and three sensilla in adults ( Viana et al. 2024). On the other hand, this does not seem to be a rule given that there is a reduction in the number of sensilla between the 1 st and 2 nd instars in H. breviseta ( Xiang et al. 2022) , for example. Finally, comparing W. jacobsoni to W. japonica , there are 10 shared mac on Abd V (a5–6, m2, m5, p1, p3–6), but in W. japonica three secondary chaetae (p4a, p5a, p6e, named as “ pp6 ” in Zhang et al. 2019a) appear earlier.

After this study of the dorsal chaetotaxy development of W. jacobsoni , the species can now be identified and compared in different instars with other congeneric species, a strategy which has already proven successful when applied to diagnose other Entomobryoidea taxa ( Soto-Adames 2008; Xiang et al. 2022). Besides, after comparing the development of W. jacobsoni with other Entomobryinae taxa, it was possible to verify that a large part of the specific pattern of each species is formed during the 2 nd instar or slightly after (except part of the head in W. jacobsoni ), with few changes in primary macrochaetotaxy until adulthood.

Figures

FIGURE 9A–E. Willowsia jacobsoni, development of dorsal head chaetotaxy; A, 1st instar; B, 2nd instar; C, 3rd instar; D, 4–5th instars, arrows indicate instar variation of chaetae as mac or mic; E, adult.

FIGURE 12A–E. Willowsia jacobsoni, development of dorsal Abd IV–V chaetotaxy; A, 1st instar; B, 2nd instar; C, 3rd instar; D, 4–5th instars; E, adult; red arrows on Abd IV indicate secondary sensilla; p6 chaeta on Abd V is named as “ap6” by Szeptycki (1979) and Zhang et al. (2019).