The Rare Richardsitas Betsch (Collembola, Symphypleona, Sminthuridae): A New Species from Australia with Comments on the Genus and on the Sminthurinae

Richardsitas Betsch is a small genus of Sminthurinae with only two species described so far, both from Madagascar. It resembles other Sminthurinae with long antennae, especially Temeritas Richards. Here we provide the first record of Richardsitas from Australia, Richardsitas subferoleum sp. nov., which is similar to R. najtae Betsch and R. griveaudi Betsch in males’ large abdomen chaetotaxy and presence of tenent-hairs on tibiotarsi II–III, but lacks mucronal chaeta and has 28 segments on the fourth antennal segment plus a unique pair of sensilla on the second. We also provide an updated genus diagnosis to Richardsitas, a key to its species, a discussion of the affinities of Temeritas and Richardsitas to other Sminthurinae, and an updated key to this subfamily.

Herein we describe in detail the first species of Richardsitas from Australia and update the generic diagnosis. We also provide a key to Richardsitas species and discuss its affinities with other Sminthurinae genera. Finally, based on our survey of the Sminthurinae, we provide an updated key to the extant genera.

Materials and Methods
The specimens were preserved in ethanol, cleared in Nesbitt's solution, and mounted in glass slides using Berlese's medium. Morphological studies and raw drawings were made with a Leica DM750 microscope with an attached drawing tube. Photographs were taken with the same microscope, with a Leica MC170 HD camera using LAS V. 4.12 software. Final figures were improved and organized in plates using CorelDraw X8 software. Type series was deposited at the South Australian Museum, Adelaide, Australia, under the acronym of SAMA.
The terminology used in descriptions follows Betsch and Waller [25] for head and large abdomen chaetotaxy and Betsch [26] for small abdomen chaetotaxy, using as a model the species in Medeiros and Bellini [24]; Fjellberg [27] for labial palp papillae and Cipola et al. [28] for labral chaetotaxy.
The abbreviations and symbols used are: Abd-abdominal segment; Ant-antennal segment; Th-thoracic segment. Chaetae present or absent are marked with white arrows, unpaired chaetae on frontal head and trunk are marked with a '*' on drawings, extra chaetae on head without clear homologies are circled. Head, trunk (thorax + abdomen), and furcal chaetotaxy are given by half body in the text description. The chaetal labels are marked in bold in the text.
Description. Body (head + trunk) length of type series ranging between 0.8-1.37 mm, males average 0.87 mm, females average 1.27 mm, type series average 1.11 mm, holotype (male) with 0.95 mm. Habitus typical of the genus. Body colour in ethanol mottled pink, antennae mostly dark mauve, distal third of Ant III, and distal half of Ant IV whitish. Body chaetae slightly serrated and acuminate.
Etymology. From Latin, subfero = to endure, to tolerate; and oleum = oil. The new species has survived widespread oil extraction on Barrow Island for many years.
Distribution, Habitat and Conservation. Barrow Island is Class A Sanctuary protected by Western Australian legislation. It is a continental island being only 56 km offshore the Western Australian coast and with about 236 km 2 of total area. Most of the island is covered with hummock grassland (Triodia sp.) with scattered shrubs, herbs and rare Ficus trees [31]. It is in the wet/dry tropics with rain falling almost entirely in the summer months and then generally in short, sharp heavy downpours. The climate of the area is "BWh" following the Köppen-Geiger climate classification, which means an arid main climate with desert-like precipitation and overall "hot arid" temperatures [32].
Although Barrow Island is a sanctuary, it is inserted in a prolific oil field area. Its biota has been exposed to commercial oil extraction for the past decades, and more recently to natural gas processing as well [31].
Richardsitas subferoleum sp. nov. type specimens were all collected in pitfall traps after a rare heavy rainfall of nearly 50 mm within 24 hours in March 2006 and on none of the other seven sampling occasions from 2005 to 2012. The only other known local springtail species that responded in a similar way to a significant rainfall event was Pygicornides sp. The collection sites in 2006 were all clustered around the location of the planned gas plant before construction. How much of the original vegetation of hummock grassland with scattered shrubs and herbs remains undisturbed now after construction, is not known. A sole collection was made in May 2007 and a few more specimens of Richardsitas subferoleum sp. nov. were found at that time, near to a new airfield. Again, the vegetation was largely hummock grassland with some native grasses and shrubs. It is likely that Richardsitas subferoleum sp. nov. is widespread on the island but is only active after a significant rainfall event mainly in summer.
Remarks. Richardsitas subferoleum sp. nov. resembles R. najtae and R. griveaudi by its Temeritas-like habitus, presence of dimorphic short candle-shaped chaetae on dorsal large abdomen of males, presence of capitate tenant-hairs on tibiotarsi II-III, absence of oval organs on all tibiotarsi and ventral chaetotaxy of dens following the formula 1:1:2:2:2:2:3. However the new species is unique in its combination of 28 antennal subsegments on Ant IV (30 in the other two species), 4 zones of short candle-shaped chaetae on large abdomen of males (3 in R. griveaudi) and mucronal chaeta absent (present in the other two species). The new species also differs in the reduced number of dorsal short candle-shaped chaetae of males (16-17 on each side), while R. griveaudi has about 26 and R. najtae about 58. The overall morphology of such chaetae is constant in the new species while in both R. griveaudi and R. najtae there are smaller and larger chaetae combined at least in the posterior zone of the large abdomen. Lastly, the new species has a peculiar organ on dorsal Ant II with two modified sensilla, absent in the other two species of Richardsitas. The main differences between the three species are summarized in Table 1.
Richardsitas najtae and R. griveaudi were recorded from semi-arid forests in south and southwest Madagascar, at the same latitude as Barrow Island [5,15,16]. This may be significant especially as both islands experience to some extent the same climate, especially in the summer rainfall.

Remarks on the Distribution and Morphology of the Richardsitas Species
The unusual morphology of the three Richardsitas species, with Temeritas-like habitus combined with a single capitate tenent-hair on tibiotarsi II and III and males with short candle-shaped or spine-like chaetae on dorsal large abdomen, is unmatched among the subfamily Sminthurinae (Table 2).
It strongly suggests the genus is monophyletic, although a rigorous phylogenetic analysis must confirm this hypothesis. In this sense the disjunct distribution of Richardsitas in Madagascar and Western Australia may point at least to two different scenarios: a Gondwanan origin to the genus, about at least 100 million years ago with the break of East Gondwana; or a more recent colonisation through the Indian Ocean, similarly to the model proposed by Christiansen and Bellinger [33] to Hawaii colonisation. If the first hypothesis is true, it is highly possible that relict populations of Richardsitas were isolated in Madagascar, Australia (as in Barrow Island), and other localities after the breakup of the former Gondwana supercontinent. In either case, considering its distribution, it is possible Richardsitas has a wider distribution around the Indian Ocean ( Figure 6).

Remarks on the Distribution and Morphology of the Richardsitas Species
The unusual morphology of the three Richardsitas species, with Temeritas-like habitus combined with a single capitate tenent-hair on tibiotarsi II and III and males with short candle-shaped or spinelike chaetae on dorsal large abdomen, is unmatched among the subfamily Sminthurinae (Table 2). It strongly suggests the genus is monophyletic, although a rigorous phylogenetic analysis must confirm this hypothesis. In this sense the disjunct distribution of Richardsitas in Madagascar and Western Australia may point at least to two different scenarios: a Gondwanan origin to the genus, about at least 100 million years ago with the break of East Gondwana; or a more recent colonisation through the Indian Ocean, similarly to the model proposed by Christiansen and Bellinger [33] to Hawaii colonisation. If the first hypothesis is true, it is highly possible that relict populations of Richardsitas were isolated in Madagascar, Australia (as in Barrow Island), and other localities after the breakup of the former Gondwana supercontinent. In either case, considering its distribution, it is possible Richardsitas has a wider distribution around the Indian Ocean ( Figure 6). Except for the previously discussed features, the overall morphology of Richardsitas is remarkably similar to Temeritas, as originally stated by Betsch [15], and antennal morphology supports that the first genus is similar to Temeritas stricto sensu ingroup [24]. Other features shared by Richardsitas and Temeritas are the presence of 8 + 8 eyes, absence of postantennal chaeta, presence of trochanteral spine, smooth D bothriotrichium and dens with 13 ventral (anterior) chaetae (Table 2). These characteristics, including the long subsegmented antennae, are also found in Galeriella and Keratosminthurus, with exception of eyes being absent in Galeriella (dental ventral chaetotaxy unknown in this genus) and head dimorphic features in Keratosminthurus [9,18]. Because of these similarities, we consider such Sminthurinae genera are possibly closely related within Temeritasgroup. Specialisations like loss of eyes and body pigments in Galeriella are related to a troglobiont Except for the previously discussed features, the overall morphology of Richardsitas is remarkably similar to Temeritas, as originally stated by Betsch [15], and antennal morphology supports that the first genus is similar to Temeritas stricto sensu ingroup [24]. Other features shared by Richardsitas and Temeritas are the presence of 8 + 8 eyes, absence of postantennal chaeta, presence of trochanteral spine, smooth D bothriotrichium and dens with 13 ventral (anterior) chaetae (Table 2). These characteristics, including the long subsegmented antennae, are also found in Galeriella and Keratosminthurus, with exception of eyes being absent in Galeriella (dental ventral chaetotaxy unknown in this genus) and head dimorphic features in Keratosminthurus [9,18]. Because of these similarities, we consider such Sminthurinae genera are possibly closely related within Temeritas-group. Specialisations like loss of eyes and body pigments in Galeriella are related to a troglobiont way of life. We are not including in this group other genera like Janusius Bretfeld, 2010 [34], Sminthurus and Spatulosminthurus, as they do not share the metatrochanteral spine, a feature which may be significant to separate them from other Sminthurinae [2] ( Table 2). Because of variable morphology we did not include Pararrhopalites in Temeritas-group; most of its species have fewer than 15 subsegments on Ant IV [23,24]. Nevertheless, at least a few species of Pararrhopalites resemble Temeritas ormondae-group [24].
Our diagnosis of Richardsitas mainly fits the one proposed by Betsch [5,15], with some additions, especially the variation of Ant IV subsegments and presence or absence of mucronal chaeta. Such differences are considered as interespecific variations within other Sminthurinae genera, such as in Temeritas (Table 2). On the other hand, the lack of further data concerning labrum, labium, ventral head, legs, among other features of R. najtae and R. griveaudi, prevents us providing additional notes on differences/diagnostic attributes of Richardsitas. For instance, none of Betsch's descriptions show modified sensilla on the Ant II as seen in Richardsitas subferoleum sp. nov. Betsch [15,16] described Ant II and made notes on antennae of R. najtae and R. griveaudi, and so we consider both lack such an organ.

Remarks on Some Sminthurinae Genera
According to Bernard and Wynne [35] the diagnoses of subfamilies of Sminthuridae are partially supported by overlapping taxonomic characters, such as the presence, absence or shape of the neosminthuroid chaetae in the parafurcal area, ventral dens chaetotaxy and number of Ant IV subsegments. The unreliability of subfamily diagnoses makes the placing of some genera like Keratosminthurus uncertain [9]. The same applies to other Sminthurinae genera, which in some cases cannot clearly be distinguished from each other ( Table 2). A large study concerning the evolution and validity of internal taxa of Sminthurinae is needed to better delimit which morphological features are of phylogenetic significance in this group. Legends: [] = species references; ∆ = Temeritas-group; † = extinct; + = present; -= absent; ? = unknown/unclear; * = some species of Temeritas have sexual dimorphism on parafurcal area, the males can have plumose chaetae nearside the genital opening (see Medeiros and Bellini 2019); ** = here we did not consider Parrarrhopalites indianus Baijal and Argarwal, 1972 [39], since its antennae description is unclear; *** = genus inquirenda, see footnote of Sminthurinae genera key.
Regarding the extinct taxa listed as Sminthurinae in Bellinger et al. [4], at least one genus may not belong to the subfamily, Brevimucronus Christiansen and Pike, 2002 [6]. The description of its antennae morphology and measurement are ambiguous, and the genus apparently is related to Dicyrtomidae. The authors described the fourth antennal segment possibly bearing a large apical bulb, but it is most likely a reduced Ant IV since it has a few chaetae. If this is true, then the antennae are elbowed between Ant II and III, as seen in extant Dicyrtomidae (see Christiansen and Pike [6] (p. 180, Figure 36)) [6]. Other extinct genera are similar to Sminthurinae, but there are several uncertainties about their morphology (see Table 2). At least the identity of extinct Sminthurinae genera without the metatrochanteral spine should be taken with caution as an incomplete understanding of morphology could hide taxa more related to other subfamilies of Sminthuridae or even other families of Symphypleona.
Based on our survey of the Sminthurinae we provide the following key to its extant genera.

Conclusions
With the description of R. subferoleum sp. nov., there are now three described species of Richardsitas. Although the new species fits the genus diagnosis, it shows a remarkably reduced number of short candle-shaped chaetae on male's dorsum, and a peculiar organ on apical Ant. II. The new species expands the genus distribution to Australia. Richardsitas morphology supports it is closely related to Temeritas, a Holotropical genus with species recorded from Madagascar and Australia as well. Sminthuridae subfamilies and genera's diagnoses are partially based on overlapping features, especially among the Sminthurinae, and should be investigated under a wide phylogenetic analysis to better circumscribe them and to delimit which morphological features have phylogenetic significance.