The Puzzling Falcomurus Mandal (Collembola, Orchesellidae, Heteromurinae): A Review

Simple Summary Springtails are tiny microarthropods found mainly in soil habitats around the globe. Falcomurus is a genus of Heteromurinae (Orchesellidae), currently with a single species from India. Here, we revise the genus, transferring Dicranocentrus litoreus Mari-Mutt and D. halophilus Mari-Mutt to Falcomurus and describing two new taxa from marine littoral habitats in Australian archipelagos. We discovered the morphology of Falcomurus is quite similar among its species, but some characters of its chaetotaxy (the distribution and morphology of body chaetae) are useful to clearly separate them from each other. Abstract Falcomurus Mandal is currently a monotypic genus of Heteromurinae described from India in 2018. Its key characters are the first antennal segment subdivided, the second undivided and the third annulated; the first abdominal segment lacking macrochaetae; and the presence of a sinuous modified macrochaeta on the proximal dens. Some details of its morphology were recently put in doubt, and so its genus status and affinities remain uncertain. Here, we revise the genus based on the type material of Dicranocentrus litoreus Mari-Mutt, as well as provide the description of two new species from Australian archipelagos and a reinterpretation of the chaetotaxy of Falcomurus chilikaensis Mandal and D. halophilus Mari-Mutt. After our revision, Falcomurus shows a well-conserved chaetotaxy and overall morphology, which allowed us to provide an updated generic diagnosis. While the antennae morphology of Falcomurus resembles that of Dicranocentrus Schött, its dorsal sensillar and macrochaetotaxy suggest it is closely related to Heteromurus Wankel, as originally stated by Mandal. The main features useful to separate Falcomurus species are the head, mesothorax and fourth abdominal segment chaetotaxy. We also provide a key to its five species, a comparative table and notes on the affinities and distribution of Falcomurus.


Introduction
Recent studies reviewing the systematics within the Entomobryoidea have improved the understanding of the relationships between the families, subfamilies, tribes and genera, plus identified new more reliable diagnostic characters [1][2][3]. In particular, the affinities and internal organisation of the Orchesellinae sensu Soto-Adames et al. [4] were revised, and this taxon was reinstated as a full family, as earlier proposed [3,5]. Other studies have provided more evidence to support the validity and possible relationships within the Orchesellidae and suggested the sensillar pattern of the trunk segments may better distinguish its lineages [3,[6][7][8].

Taxonomic Summary and Genus
Type species: Falcomurus chilikaensis Mandal, 2018. Remarks on the genus: Falcomurus was first described by [15] as a monotypic genus from India. However, two other species described earlier, Dicranocentrus litoreus, from the Philippines, and D. halophilus from Papua New Guinea, mostly comply with Mandal's description, with the exception of the absence of Ps2 mac on the dorsal head and presence of an unguiculus outer tooth, and are herein transferred to Falcomurus.
As originally noted by [17], D. litoreus and D. halophilus show a different morphology compared to all other Dicranocentrus species, with enlarged a1-2 chaetae on the distal labrum, a sinuous ciliate mac on the proximal dens, mucronal spine absent and a reduced dorsal head and trunk macrochaetotaxy. Mari-Mutt compared the dorsal chaetotaxy of his two species to those of Heteromurus, in lacking the dorsal head S0 chaeta, Th. III with five main mac (four in Heteromurus) and Abd. I devoid of mac [17,37,38]. This dorsal chaetotaxy, combined with antennae with five segments (only Ant. I subdivided) and mucro without spines, is more similar to the morphology of Heteromurus s. str. than to Dicranocentrus, although the annulated Ant. III is more similar to Dicranocentrus [8,33]. After our morphological study of the three previously described and the two new species of Falcomurus presented here, several shared features were noted between them and used to revise and complement the original diagnosis of the genus, especially regarding the main dorsal macrochaetotaxy. This character is mostly stable among the species, with few variations on the anterior and posterior head, Th. II m4 complex and p series, plus central Abd. IV. Further characteristics may be shared among Falcomurus species, such as the presence of only two enlarged distal mac on the anterior face of the ventral tube [15] (p. 81, Figure 11, and the remarks we present on F. chilikaensis) and elements of the lateral chaetotaxy of the posterior head and trunk segments, which were omitted in [15,17]. On the other hand, the Ant. II undivided described for F. chilikaensis, After our morphological study of the three previously described and the two new species of Falcomurus presented here, several shared features were noted between them and used to revise and complement the original diagnosis of the genus, especially regarding the main dorsal macrochaetotaxy. This character is mostly stable among the species, with few variations on the anterior and posterior head, Th. II m4 complex and p series, plus central Abd. IV. Further characteristics may be shared among Falcomurus species, such as the presence of only two enlarged distal mac on the anterior face of the ventral tube [15] (p. 81, Figure 11, and the remarks we present on F. chilikaensis) and elements of the lateral chaetotaxy of the posterior head and trunk segments, which were omitted in [15,17]. On the other hand, the Ant. II undivided described for F. chilikaensis, D. litoreus and D. halophilus is Further notes on the morphology of Falcomurus are presented in the first discussion topic. Mandal, 2018 Figures 2A and 3A. Falcomurus chilikaensis Mandal, 2018 [15] (pp. 77-83, Figures 1-17), India, Odisha state, Ganjam district, Rambha town, nearby Sabulia village (original description).
Remarks: It is quite probable that Mandal's description of F. chilikaensis includes misinterpretations and omissions regarding the dorsal chaetotaxy and other morphological aspects, which we were able to track and correct after reviewing the other species. In this sense, it is worth noting we did not examine the F. chilikaensis type series.
For the dorsal head macrochaetotaxy [15] (p. 79, Figure 6), the common group of A0, A2-3, A5 mac seen in several Heteromurinae and all other Falcomurus species were not clearly represented, and the marked A0 actually belongs to the antennal series. So we considered in the anterior region the antennal series with three mac (originally represented only by one side), but quite possibly with more chaetae near the eyes, as we observed in the other taxa, and the original three anterior chaetae as misinterpretations of A0, A2-3, while M3 is actually A6. Regarding the M and S series, the original drawing is not entirely symmetrical, so it is not clear if there are polymorphic chaetae, absent as mac in one of the sides. Since the author omitted mac on both sides in this drawing, we considered the absent mac of these series only as omissions and not as polymorphic chaetae, and such series were entirely represented in the left side of the original drawing. Concerning the posterior head, we believe the most posterior region was disregarded by the author, possibly due to the absence or reduction of mac, and the original drawing represents Pa5, almost universal to Entomobryoidea, Pm3 and Pp3, seen in all other Falcomurus species, plus Ps2, exclusive of F. chilikaensis. It is also quite possible that F. chilikaensis has interocular chaetae, such as the other species and most, if not all, Entomobryoidea; but, they were overseen due to the presence of dark pigments over the eyepatches [15] (p. 79, Figure 3). On the labrum, Mandal's representation of a1 chaeta clearly marks it as enlarged on the left side, as described by [17] for D. litoreus and D. halophilus, while a2 is apparently similar to the others [15] (p. 80, Figure 8).
For the dorsal trunk chaetotaxy [15] (p. 80, Figure 7), we considered the Th. II main chaetotaxy as it was presented by the author, since it fits the a and m series of the other species (but with m4p), while the p series shows some secondary multiples of p1-2 mac, differently from other Falcomurus taxa but as seen in part of Dicranocentrus and some other Orchesellidae [6][7][8]29,32,33]. However, to Th. III until Abd. IV, Mandal's representation of the dorsal mac encompasses chaetae from the left and right sides at the same time, being more complete on the right side. On the Th. III there are six posterior mac represented on the original drawing, but they are likely the left and right sets of p1-2e, seen in all other species of the genus. So, the two superior mac on the right side represent p3 and a5, also seen in all other species of Falcomurus. The upper a5 mac marked by the author could represent the a1 mac seen in scaleless Orchesellidae, like Orchesella [32] (p. 152). However, since the a1 mic in the new species occurs near to the pseudopore, next to m1, a feature seen in several other Heteromurini [3,8,27,33,34], we disregarded Mandal's representation of this mac, considering it as a doubtful feature. On the Abd. II, Mandal's description shows 3 bothriotricha; however, the Entomobryoidea present 2 + 2 bothriotricha on this segment [1,3,32]. In this segment, the author actually represents, from the left to the right: the left m3 mac, left m2 bothriotrichum, right m2 bothriotrichum, right m3-3e mac, right a5 bothriotrichum and right m5 mac, all also seen in the other species of the genus. A similar mistake was seen in the Abd. III, which originally represents, from the left to the right: the left m3 mac (left m2 bothriotrichum was overseen, but its alveolus is smaller than the mac ones), right m2 bothriotrichum, right m3 mac, right a5 and m5 bothriotricha and right pm6 mac, similar to its congeners. In this segment, it is quite possible that the p6 mac is also present, and it was not represented since it did not fit the drawing. The Abd. IV interpretation was quite puzzling, since the original drawing was different from the expected disposition of Dicranocentrus and Heteromurus central mac [29,33,37]. Falcomurus halophilus comb. nov. also has three central mac on Abd. IV, A3, A6 and B5, while the other species of the genus have A6 as a mes, so we believe the original drawing of F. chilikaensis shows the chaetae inverted, and they are actually the same three. Once again, the left side is incomplete, as for the most trunk segments, so we disregarded a possible polymorphic state of A6 and considered the Abd. IV with three internal mac.
Regarding other characteristics, in Ant. IV we disregarded the presence of the antennal bulb, as noted by [8], but considered the left side structure as a pin projection (see [15] (p. 79, Figure 5) and [27] (p. 4, Figure 2B)). Mandal's description noted his species had all legs scaled; however, the author did not remark on the presence or absence of scales on any particular segment of the legs. In his depiction of the tibiotarsi, the superior (proximal) region has "spine-like" structures, which most likely represent the scales alveoli, as they are somewhat pointed [15] (p. 81, Figure 12). The habitus photograph of F. chilikaensis also suggest such scales are present, as seen in the right fore and the left hind legs [15] (p. 78, Figure 1). In addition, all specimens of Falcomurus we revised do not have short spines on the tibiotarsi. Because of this, we considered the tibiotarsi of F. chilikaensis proximally scaled, as we observed in F. litoreus comb. nov. and F. hilli sp. nov. Finally, the drawing of the ventral tube corpus in Mandal's description [15] (p. 81, Figure 11) possibly represents the lateral region, which is devoid of chaetae in many Entomobryoidea and also in other Falcomurus species. So, the right side of the drawing possibly represents the two distal macrochaetae (as in [17] (p. 314, Figure 78)) and the left side, one of the outer posterior ciliate chaetae.
After our reinterpretation of the original F. chilikaensis description, we believe the type material should be re-examined and the taxon redescribed, to confirm its morphology is similar to other Heteromurinae taxa as herein discussed. In this sense, our diagnosis, notes and drawings of this species should be taken as provisional, until further data based on the type specimens are published. Diagnosis: Dark blue/violet pigments over all antennal segments, tibiotarsi, dorsoanterior head and around the mouth, light blue/violet uniformly distributed over trunk, other leg segments, ventral tube corpus and manubrium. Ant. IV without pin projection. Ant. II undivided, without scales. Labral a1-2 chaetae enlarged, distal labrum with two distinct pointed papillae. Dorsal head with Pm1, Pp1, Pp5 and Pe3? mac, without An1a and Ps2 mac, interocular area with five ciliate chaetae or four ciliate chaetae and one scale ( Figure 2B). Mandible apex curved to the midline. Post-labial quadrangle without smooth chaetae, anterior post-labial region with a pair of smooth chaetae near the labium. Th. II with two m4 and four p mac, respectively ( Figure 3B). Abd. IV with three central mac (A3, A6 and B5) ( Figure 3B). Unguiculi with the outer tooth. Dorsal manubrium without smooth chaetae (adapted from [17]).
Remarks: Dicranocentrus halophilus is herein transferred to Falcomurus since it shares characteristics with other species of the genus, especially the absence of A1, M3-4, S0 and S2 mac on dorsal head, the overall reduction of main dorsal macrochaetotaxy on trunk, including the Abd. I lacking mac, ungues short and thick, internally with only the basal paired teeth, unguiculi excavated, dens lacking spines, proximally with a falcate ciliate mac, and mucro lacking the mucronal spine.
Our diagnosis of F. halophilus comb. nov. was based on the original description, since we did not revise the type specimens. As Mari-Mutt's description of this species noted mostly its dissimilarities with F. litoreus comb. nov., we used some data of the latter species to complement the diagnosis of F. halophilus comb. nov. [17] (pp. 319-320). On the other hand, the exact number of trochanteral spines and the presence of scales on the tibiotarsi and ventral tube anterior face, seen in our revision of F. litoreus comb. nov. (see the next species), were not considered for F. halophilus comb. nov., as they were not mentioned by the author. Concerning the interocular chaetotaxy of this species, the author described it with one outer and three inner chaetae plus a scale, similarly to F. litoreus comb. nov. However, in his drawing [17] (p. 315, Figure 80), there are only three interocular chaetae and the scale. We believe both Mari-Mutt species have a similar interocular chaetotaxy. Even so, the author regarded the interocular scale as a chaeta in one specimen [17] (p. 320).    Trunk dorsal chaetotaxy ( Figures 5 and 6). Tergal S-chaetotaxy from Th. II to Abd. V as 2,2|1,3,3,+,3 for sens, and 1,0|1,0,1,0,0 for ms. Th. II, excluding the anterior collar, with one anterior (a5), three medial (m4-4i and a multiple of m7p row far from the collar) and seven posterior (p1-3, p6-6e) mac ( Figure 5A). Th. III with two anterior (a5-6), two medial (m6-7) and four posterior (p1-3) mac, m1 mic not seen but possibly present ( Figure  5B). Abd. I lacking mac, p5 mic present ( Figure 5C). Abd. II with three medial (m3-3e, m5)   oped. Ventral tube anteriorly without smooth chaetae, with scales from the basis to the apex, with 12-17 ciliate chaetae of different sizes plus the distal two mac ( Figure 7A). Posterior face densely covered by ciliate chaetae and some central smooth chaetae, not entirely clear in the analysed specimens. Lateral flaps with about six smooth and 26 ciliate chaetae each, four distal smooth chaetae larger than the others ( Figure 7B). Manubrial plate with 11-13 ciliate chaetae and five small pseudopores ( Figure 7C). Remarks: Dicranocentrus litoreus is herein transferred to Falcomurus as it shares with other species mainly the following combination of characters: absence of A1, M3-4, S0 and S2 mac on the dorsal head; the reduction of the main dorsal macrochaetotaxy on the trunk, as described in the genus diagnosis; a foot complex with a short unguis holding internally only the proximal paired teeth; unguiculi excavated; dens devoid of spines, with the typical proximal falcate mac seen in the genus; and the mucronal spine absent.
Our revision of F. litoreus comb. nov. type material confirmed most of the data provided by [17], with few corrections: the antennal series could present one more mac and the posterior head could also present Pp1 as mac (seen only in one side of one specimen), while the ventral tube anterior region does not hold smooth chaetae [17] (p. 314, Figure  78). We also could confirm the Ant. II undivided and the interocular element v as a scale, as noted by the author [17] (p. 318). We disagree with the original description of the mandible apex, described as elongated. In this case, it is quite similar to F. pulukokos sp. nov. in length, but it is somewhat curved internally. Examined material: Holotype (sex unknown) and 4 paratypes (one female, others unknown). The Philippines, Mindoro, Puerto Galera, on the beach, 27-29.xii.1979. All deposited now at MVMA.
Diagnosis: Dark blue/violet pigments over distal Ant. Ib, II and entire III (IV unknown), dorso-anterior head and tibiotarsi, light blue/violet pigments on Th. I, coxae, trochanters and distal ventral tube corpus; trunk devoid of pigments or brownish, with a median unpigmented line from Th. II to Abd. III. Ant. II undivided, with scales. Labral a1-2 chaetae enlarged, m2 normal, two distal labrum papillae pointed but weakly developed. Dorsal head with Pp1 (also as mic) and Pe3? mac, without An1a, Ps2, Pm1 and Pp5 mac, interocular area with four ciliate chaetae and one scale or three ciliate chaetae and two scales. Mandible apex curved to the midline. Post-labial quadrangle without smooth chaetae, anterior post-labial region with a pair of smooth chaetae near the labium. Th. II with two m4 and four central p mac, respectively. Abd. IV with two central mac (A3 and B5). Ventral tube densely scaled, lateral flap with about six smooth and 26 ciliate chaetae. Trochanteral organ with 27-32 chaetae. Tibiotarsi proximally scaled. Unguiculi with the outer tooth. Dorsal manubrium without short smooth chaetae, manubrial plate with 11-13 chaetae.
Additions to the original description: Ventral head and anterior face of the ventral tube densely covered by scales, tibiotarsi scaled proximally.
Trunk appendages (Figure 7). Ungues lateral paired teeth present and underdeveloped. Ventral tube anteriorly without smooth chaetae, with scales from the basis to the apex, with 12-17 ciliate chaetae of different sizes plus the distal two mac ( Figure 7A). Posterior face densely covered by ciliate chaetae and some central smooth chaetae, not entirely clear in the analysed specimens. Lateral flaps with about six smooth and 26 ciliate chaetae each, four distal smooth chaetae larger than the others ( Figure 7B). Manubrial plate with 11-13 ciliate chaetae and five small pseudopores ( Figure 7C).
Remarks: Dicranocentrus litoreus is herein transferred to Falcomurus as it shares with other species mainly the following combination of characters: absence of A1, M3-4, S0 and S2 mac on the dorsal head; the reduction of the main dorsal macrochaetotaxy on the trunk, as described in the genus diagnosis; a foot complex with a short unguis holding internally only the proximal paired teeth; unguiculi excavated; dens devoid of spines, with the typical proximal falcate mac seen in the genus; and the mucronal spine absent.
Our revision of F. litoreus comb. nov. type material confirmed most of the data provided by [17], with few corrections: the antennal series could present one more mac and the posterior head could also present Pp1 as mac (seen only in one side of one specimen), while the ventral tube anterior region does not hold smooth chaetae [17] (p. 314, Figure 78). We also could confirm the Ant. II undivided and the interocular element v as a scale, as noted by the author [17] (p. 318). We disagree with the original description of the mandible apex, described as elongated. In this case, it is quite similar to F. pulukokos sp. nov. in length, but it is somewhat curved internally.
Trunk appendages (Figure 11). Trochanteral organ unclear, with at least 20 spinelike smooth chaetae. Femurs and tibiotarsi lacking extra smooth chaetae. Tenent-hairs capitate, slightly shorter than ungues; tibiotarsus III smooth inner distal chaeta present; anterior and posterior pretarsal chaetae present and well developed; ungues with a pair of lateral underdeveloped teeth on proximal 1/3, inner side with only the two basal paired teeth on proximal 1/3, internal lamellae only merging at the apex, dorsal tooth present, almost in the middle of the unguis; unguiculi excavated, all lamellae smooth, except for the postero-external with an external tooth in its distal half ( Figure 11A). Ventral tube anteriorly without smooth chaetae, with two distal mac (the inner larger than the outer one), plus 15-16 ciliate chaetae of different sizes ( Figure 11B). Posterior face unclear, but apparently with several lateral ciliate chaetae and few centro-distal smooth chaetae. Lateral flaps with about 22 smooth chaetae each, three of them larger than the others ( Figure 11C). Tenaculum corpus with a single clearly ciliate chaeta, each ramus with four teeth. Manubrium and dens lacking smooth chaetae of any size. Manubrial plate with 7-8 chaetae, the two internal slightly larger than the others, plus at least three small pseudopores, possibly more ( Figure 11D). Ventro-distal manubrium with two inner ciliate chaetae plus about eight scales ( Figure 11E). Dorso-proximal dens with a typical sinuous ciliate mac ( Figure 11F), mucro apical tooth larger than the basal one, mucronal spine typically absent ( Figure 11G).
Trunk appendages ( Figure 15). Trochanteral organ with 14-17 spine-like smooth chaetae ( Figure 15A). Foot complex as in F. pulukokos sp. nov. (Figure 11A). Ventral tube anteriorly without smooth chaetae, with two distal mac (the inner larger than the outer one), plus 9-12 ciliate chaetae of different sizes ( Figure 15B). Posterior face not entirely clear, with at least 16 lateral ciliate chaetae and a pair of centro-distal long smooth chaetae. Lateral flaps with about nine ciliate and six smooth chaetae each, three smooth chaetae larger than the others ( Figure 15C). Tenaculum corpus with a single clearly ciliate chaeta, each ramus with four teeth. Manubrium and dens lacking smooth chaetae of any size. Manubrial plate with six chaetae, the two internal slightly larger than the others, plus five small pseudopores ( Figure 15D). Ventro-distal manubrium with two inner ciliate chaetae plus about 10 scales ( Figure 15E). Dorso-proximal dens with a typical sinuous ciliate mac ( Figure 11F), mucro apical tooth larger than the basal one, mucronal spine typically absent ( Figure 11G).
Etymology: The species honours the entomologist Lionel Hill, who collected the type material.
Remarks: The main useful features to separate Falcomurus species are the subdivision of Ant. II, mandibles apex, labral papillae morphology, posterior dorsal head chaetotaxy, presence of smooth chaetae in the post-labial region, Th. II central chaetotaxy, presence of A6 mac on Abd. IV, presence of the outer tooth on the unguiculi and manubrial dorsal chaetotaxy. In that regard, F. chilikaensis is unique due the presence of Ps2 mac on the dorsal head and Th. II with a complex central macrochaetotaxy, with three m4 mac and seven mac on the p1-3 group, unguiculi without the outer tooth and dorsal manubrium with short smooth chaetae [15]. This species also combines an undivided Ant. II, post-labial quadrangle with smooth chaetae and A6 mac present on Abd. IV. Unfortunately, Mandal's description did not provide data on the mandibles' apex and labral papillae.
As noted by [17], the main characteristics to distinguish F. halophilus comb. nov. from F. litoreus comb. nov. are the degree of labral papillae development (fully developed in F. halophilus comb. nov. vs. weakly developed in F. litoreus comb. nov.), posterior head macrochaetotaxy (with Pm1 and Pp5 mac in F. halophilus comb. nov. vs. without in F. litoreus comb. nov.) and Abd. IV central macrochaetotaxy (with three mac in F. halophilus comb. nov. vs. two in F. litoreus comb. nov.). The author also remarked on the presence of scales on Ant. II in the latter species (vs. absence in F. halophilus comb. nov.); however, our analysis of the new species showed the number of scales in this segment is reduced, which could be easily overseen due to their fall. Some unknown or unclear features of F. halophilus comb. nov., such as the presence of a ventral tube and tibiotarsi scales, ventral tube lateral flap's chaetotaxy, the exact number of trochanteral organ smooth chaetae, among other characteristics, could be useful to better compare this species with its other congeners.
Falcomurus pulukokos sp. nov. and F. hilli sp. nov. are the only species in the genus with the Ant. II subdivided [15,17]. They also differ from Mari-Mutt's species by their normal mandible apex (curved in F. halophilus comb. nov. and F. litoreus comb. nov.) and interocular region with four elements, three chaetae and one scale (five in F. halophilus comb. nov. and F. litoreus comb. nov.). The new species differ from each other by their Ant. III sensory rods (swollen in F. pulukokos sp. nov., normal in F. hilli sp. nov.); labral papillae (four in F. hilli sp. nov. and two in F. pulukokos sp. nov.); m1-0 labral chaetae enlarged and dorsal head Pp5 as mac only in F. hilli sp. nov.; post-labial chaetotaxy without smooth chaetae on F. hilli sp. nov. (with four in F. pulukokos sp. nov.); and ventral tube's lateral flaps without ciliate chaetae only in F. pulukokos sp. nov.

On Falcomurus Species Morphology
The overall morphology of Falcomurus species is well conserved in its five species. Regarding the three species we could analyse in detail, namely, F. litoreus comb. nov., F. pulukokos sp. nov. and F. hilli sp. nov., not only their dorsal macrochaetotaxy is remarkably similar, but also other elements of the idio and sensillar chaetotaxy as well, with the chaetae in similar positions in the three species. Some mac on the Abd. IV and V interchange to mes in different species, such as A6 and D2 on Abd. IV and m5a and a6 on Abd. V, differences we could better track studying the alveoli of the chaetae. Nevertheless, such differences are minor considering the general dorsal chaetotaxy of the genus. Other important features shared by the three species that may be constant within Falcomurus are the sublobal plate with four chaetae-like appendages; labial papillae guard chaetae formula; labial basal fields scaleless, with a variable set of ciliate chaetae on the labial triangle; foot complex overall morphology (with the exception of the unguicular outer tooth absent in F. chilikaensis); and the presence of two pairs of ciliate chaetae on the ventral manubrium (unclear in F. litoreus comb. nov.).
On the other hand, some variable characteristics caught our attention, especially the presence of subdivisions on Ant. II seen in F. pulukokos sp. nov. and F. hilli sp. nov. Although the subdivision of the Ant. II is a useful taxonomic character to delimit genera among the Orchesellidae, it proved to be variable within genera and even species, such as within Dicranocentrus and Nothobrya Arlé, 1961 [6,29,39]. Other variable features between the species are the scales on Ant. II, anterior ventral tube and proximal tibiotarsi. As previously remarked, the scales on Ant. II may be an unreliable feature by which to compare Falcomurus species, as they are reduced in number in F. pulukokos sp. nov. and F. hilli sp. nov., and were only seen in few specimens. The same occurs for the tibiotarsal scales, and while F. litoreus comb. nov. has several proximal scales on tibiotarsi, F. hilli sp. nov. only has a few, seen in few specimens. However, the scaled ventral tube of F. litoreus comb. nov. is clearly seen as there are several scales from the base to the apex, and if they fall, their alveoli could still be reliable to define the species.

On Falcomurus Affinities
As noted by [17] and partially by [15], the dorsal chaetotaxy of Falcomurus is remarkably similar to that of Heteromurus s. str. Among the Heteromurini, at least two other taxa show a clear reduced dorsal chaetotaxy, Heteromurtrella and Verhoeffiela, although the dorsal chaetotaxy of Dicranocentrus is variable, and some species show marked reductions in the number of mac also [8,12,29,33,37,38,40]. Excluding F. litoreus comb. nov. and F. halophilus comb. nov. from Dicranocentrus, no other species of the genus is devoid of mac on Abd. I. As pointed out in [17], the only species with a similar reduction in dorsal macrochaetotaxy in Dicranocentrus is D. singularis Mari-Mutt and Bhattacharjee, 1980 [40], from India, which is also devoid of the S0 mac on the dorsal head. Even so, this species presents 2 + 2 mac on the Abd. I [40] (pp. 168-169, Figure 33). Although Falcomurus has the Ant. II undivided in part of its species and the mucronal spine absent, such features were also recorded in Dicranocentrus taxa from India: in D. indicus Bonet, 1930 [41] sensu Yosii [42] the mucronal spine can be present or absent; while D. pilosus Mari-Mutt and Bhattacharjee 1980 [40], do not show a clear division of the Ant. II, which may occur abnormally in other species of the genus as well [29]. The sinuous dental mac seen in Falcomurus may also have a parallel within Dicranocentrus. At least D. indicus, D. janetscheki Yosii, 1971 [43], from Nepal, and D. javanus Yoshii and Suhardjono, 1989 [5], from Java, show one or two pairs of ciliate mac on the proximal dens. However, in the three species, the chaetae are erect and blunt, somewhat different from Falcomurus. Furthermore, the dorsal macrochaetotaxy of these three latter species is more complex than in the late genus.
The dorsal head Ps2 mac seen in F. chilikaensis was only observed previously in one other genus of Heteromurini, Pseudodicranocentrus [8,33]. However, this genus differs from Falcomurus by: the prelabral chaetae bifurcate, dorsal head A1 and S0 mac present, and a clearly more complex trunk macrochaetotaxy, with 4-5 mac on Abd. I [8,33]. So, a close relationship between Pseudodicranocentrus and Falcomurus is unlikely, and the Ps2 mac seen in F. chilikaensis may have arisen independently from the former genus.
Heteromurus s. str. and Verhoeffiela are closely related, and recent studies strongly support Verhoeffiela as an ingroup of Heteromurus, possibly being polyphyletic [44,45]. They will be herein be treated combined as Heteromurus s. lat., as in [8]. Accordingly, with the overall similarity of chaetotaxy of Falcomurus with Heteromurus s. lat., other features may support a closer relationship between them, like the absence of the mucronal spine (seen in part of Heteromurus s. str.), Ant. III annulated (as in Verhoeffiela), and more importantly, the sensillar pattern of Abd. V with three sens displaced anteriorly. This last character, also noted by [15], may hold a strong phylogenetic signal within the Heteromurinae, as proposed by [8], and puts Falcomurus closer to Heteromurus s. lat., possibly as an ingroup of the later.
Lastly, further studies on Heteromurinae phylogenetics are needed to clarify the correct position of Falcomurus within the subfamily. At this time, since no species of the genus have yet been included in molecular phylogenies, it is not clear if it is an ingroup of Dicranocentrus or Heteromurus s. lat., an extant genus from a transitional group between them, or even an independent genus of Heteromurinae separated from both.

On Falcomurus Habitat, Ecology and Distribution
The genus Falcomurus, with two new species and two transferred from Dicranocentrus, now contains five species. Their distribution, as currently known, extends from the Bay of Bengal on the east coast of the Indian subcontinent to the Coral Sea and Philippines ( Figure 16). All five species have only been found in marine littoral habitats and in the upper intertidal zone of sandy beaches, just below high tide mark [15,17]. They appear to be exclusive to this habitat. Thus, Falcomurus is the only genus of Heteromurinae entirely restricted to marine littoral environments [46].
The marine littoral habitat has been largely neglected by springtail taxonomists, with the main exception being J-M Thibaud. However, Thibaud has not been active in south-east Asia and the Pacific so we suspect the genus distribution is wider than currently known. In fact, the current genus distribution suggests it may also be present in Indonesia (Figure 16).
Marine littoral fauna can be highly diverse and comprises genera not found in any other habitat, as in the case of Falcomurus; for instance, on Barrow Island (Western Australia), in the intertidal zones of four beaches of different types, including mud flats with mangroves, coral sand, rocky shores and high-velocity, steep sandy beaches. Altogether, 14 species of springtails were found here belonging to 11 genera. It was assumed that all species were intertidal specialists, while seven of the genera were also restricted to this habitat [47]. A different suite of species was found on four types of beaches.
trus, now contains five species. Their distribution, as currently known, extends from the Bay of Bengal on the east coast of the Indian subcontinent to the Coral Sea and Philippines ( Figure 16). All five species have only been found in marine littoral habitats and in the upper intertidal zone of sandy beaches, just below high tide mark [15,17]. They appear to be exclusive to this habitat. Thus, Falcomurus is the only genus of Heteromurinae entirely restricted to marine littoral environments [46]. The marine littoral habitat has been largely neglected by springtail taxonomists, with the main exception being J-M Thibaud. However, Thibaud has not been active in south-east Asia and the Pacific so we suspect the genus distribution is wider than currently known. In fact, the current genus distribution suggests it may also be present in Indonesia ( Figure 16).
Marine littoral fauna can be highly diverse and comprises genera not found in any other habitat, as in the case of Falcomurus; for instance, on Barrow Island (Western Australia), in the intertidal zones of four beaches of different types, including mud flats with mangroves, coral sand, rocky shores and high-velocity, steep sandy beaches. Altogether, 14 species of springtails were found here belonging to 11 genera. It was assumed that all

Conclusions
After this revision, there are now five species of Falcomurus. Their morphology is mostly conserved, and some aspects of the dorsal head posterior, post-labial, Th. II and Abd. IV chaetotaxy are the most useful features to separate them from each other. The overall morphology of Falcomurus resembles both Dicranocentrus and Heteromurus s. lat., but the Abd. V sensillar chaetotaxy and Abd. I lacking mac may put the genus closer to the latter, as they differ from Dicranocentrus. On the other hand, the presence of a proximal falcate mac on dens may be exclusive of Falcomurus. With the description of two new species and the transfer of D. litoreus and D. halophilus to Falcomurus, the genus distribution has extended from the Bay of Bengal (India) to the Coral Sea (Australia) and Philippines. These new species records are consistent in habitat (the marine littoral environments) with that of the type species, F. chilikaensis.  Institutional Review Board Statement: Ethical review and approval were waived for this study, due to Brazilian laws which do not require permission from an institutional ethics committee on the use of animals for taxonomical studies with microarthropods. Data Availability Statement: All data is contained within the article. All biological material is deposited at MVMA as previously stated.