Larvae ofBembidiini ( Coleóptera : Carabidae ) : Subtribes Tachyina and Xystosomina

Larvae of 13 species of the bembidiine subtribes Tachyina (the genera Tachyta Kirby, Tachys Dejean, Polyderis Motschulsky, Elaphropus Motschulsky, Sphaerotachys G. Müller, Paratachys Casey, Porotachys Netolitzky) and Xystosomina (the genus Mioptachys Bates) were studied. Larvae of all studied taxa are described, diagnosed and illustrated. A key to genera is pro­ vided separately for the firstand older-instar larvae. Monophyly and phylogenetic position of the subtribes and genera are discussed on the basis of presumably apomorphic character states of larvae. Based on at least six synapomorphic character states the subtribes Tachyina and Xystosomina appear to form a monophyletic group. On the other hand, there are no synapomorphies to propose a sister-group relationship between Tachyina and Xystosomina within the supertribe Trechitae.


INTRODUCTION
Among the smallest ground beetles are the minute members of the subtribe Tachyina of Bembidiini, as well as members of the subtribe Xystosomina, a group recently removed from the Tachyina (Erwin, 1994).They are rarely longer than 5 mm, and some adults are as small as 0.7 mm (Erwin, 1974a).There are several hundred spe cies within these two groups, living on the ground in saline habitats, other riparian environments, xeric areas, as well as under bark of logs, in trees, and with ants (Erwin, 1974a;Baehr, 1995).
The recent papers by Erwin (1973Erwin ( , 1974aErwin ( , b, 1975Erwin ( , 1978Erwin ( , 1994) ) for Neotropical and Baehr (1986Baehr ( , 1987Baehr ( , 1989Baehr ( , 1990Baehr ( , 1995) ) for Australian regions demonstrate that both subtribes are extremely diverse in the shape, size, and color of adults.To date, larvae have not been as thor oughly studied.In this paper, we describe in detail the external structure of known tachyine and xystosomine lar vae, as part of a long-term study of larval diversity within the carabid supertribe Trechitae.
In the present article we provide descriptions, diagnoses and illustrations of reared larvae of the bembidiine sub tribes Tachyina and Xystosomina belonging to eight genera and 13 species.Also, we offer keys for first-and older-instar larvae to all genera studied.Finally, we dis cuss phylogenetic aspects of the studied taxa on the basis of shared larval apomorphies.
Larvae were mounted on microscope slides in Hoyer's medium and studied with a compound microscope MBI-1 at magnifications up to 900x.Morphological drawings were pre pared using Reichert camera lucida.The notation of sensilla in first-instar larvae follows Bousquet & Goulet (1984), that in older-instar larvae follows Bousquet (1985), that on hypopha rynx follows Makarov (1996).The term "egg-bursters" for frontal structures in first-instar larvae is used to distinguish sym metrical lines of large spines in Mioptachys and Tachyta larvae (Figs 3, 4) from groups of microscopically small spines in remaining larvae  which are referred as "microspines".All measurements have been made using a micrometre.The following abbreviations are used: HWmaximum width of head; HL -length of head along midline; L1, L2, and L3 -first, second, and third instar respectively.

SUBTRIBES TACHYINA AND XYSTOSOMINA
Diagnosis.First-instar larvae of Tachyina and Xysto somina differ from those of other taxa of the supertribe Trechitae by the very short or absent coronal suture ; if present, its length is less than half the width of the proximal antennomere.Second-and third-instar

Description
Width of cephalic capsule in Li varies from 0.18 to 0.29 mm, in L2 from 0.28 to 0.42 mm, in L3 from 0.40 to 0.58 mm.Body white with lightly sclerotized sclerites (Figs 1,2,61).
Second-and third-instar larvae.Length of coronal suture about 0.5-1.0*that of proximal antennomere ; dorsal surface of cephalic capsule without ocular (except Tachyta, Fig. 49) and cervical grooves .Mandible without microspines on dorsal surface .

Monophyly and phylogenetic relationship
Larval features of Tachyina and Xystosomina support their inclusion into the supertribe Trechitae.Larvae of these groups share with those of the remaining Trechitae groups the absence of pores PRc, PRe, PRi, PRj on protergum, pores MEd, MEe on meso-and metaterga, seta ES1 on metathorax, pore TEb on abdominal terga 1-8, and setae TA3, TA4, TA5, TA6 on tarsus.These structures are part of the ground plan of the family Carabidae (Bousquet & Goulet, 1984) and their absence in Trechitae larvae is very likely an apomorphic condition.
To determine the relationships of tachyine and xystosomine species to other trechites, we needed to determine which of the larval features observed are apomorphic.To do this, we chose as an outgroup, and thus as an indicator of the plesiomorphic condition, the supertribe Patrobitae.Based upon adult and larval structure, cytogenetic charac teristics, and molecular data (reviewed in Maddison et al., 1999), patrobites are the sister group to Trechitae.
Larval features suggest that the subtribes Tachyina + Xystosomina may represent a monophyletic group.The known larvae possess 10 character states which were never, or rarely, discovered for larvae of other Trechitae taxa and most of them are likely apomorphic within the supertribe.
Four of these character states provide weak evidence of the monophyly of Tachyina + Xystosomina.The presence of six long setae on urogomphi in second and third instar in Tachyina + Xystosomina larvae (character 01), instead of seven in the remaining Trechitae groups (or nine and more in some species of the bembidiine subgenus Bracteon, see Maddison, 1993: 279), appears to be a character state of uncertain phylogenetic value, since larvae of Patrobitae, the taxon chosen as an outgroup, have both six and seven long setae on urogomphi in older instars (Luff, 1993: 79;Zamotajlov, 1994;Bousquet & Grebennikov, 1999).Three other apomorphic character states of tachyines and xystosomines, not found in the outgroup, are also present in a few other trechites.While we consider these states in other trechites to be convergent (as dis cussed below), their existence weakens the evidence they provide for the monophyly of Tachyina + Xystosomina.These three states are: (02) Group gMX consists of no more than 6 (usually 5) setae.A similar number of setae in first-instar larvae had been noted in the genera Perileptus and Thalassophilus of the tribe Trechini (Greben nikov, 1996;Grebennikov & Luff, 1998).( 03) First-instar larvae with sensilla EM1 on prothorax, EM1 and ES1 on mesothorax, EM1 on metathorax, and EP1 on ninth abdominal segment modified as pore-like structures, instead of being trichoid setae.Similar modifications of some of these setae were noted for species of the trechine genera Epaphius, Perileptus and Thalassophilus (unpub lished).( 04) Second-and third-instar larvae with 1-7 (usually 4-6) secondary pore-like structures on anterior ventrites on meso-, metathorax, and all abdominal seg ments.A similar character state was found in Perileptus larvae (Grebennikov & Luff, unpubl.).Characters 02-04 alone might suggest that Perileptus is the sister to Tachyina + Xystosomina, with Thalassophilus as their sister, and then with Epaphius as the sister to these four groups combined.However, other larval characteristics indicate that Epaphius is a member of Trechina (based on three synapomorphies), that Perileptus and Thalasso philus are sister groups (with three synapomorphies), and that Tachyina + Xystosomina belong to a clade including all trechites except Perileptus and Thalassophilus (this clade is united by the derived presence of only one larval claw) (Grebennikov, unpubl.).Thus it is more parsimo nious to presume that character states 02-04 are derived characteristics of Tachyina + Xystosomina, convergent to the states found in the unrelated to them genera Perileptus, Thalassophilus, or Epaphius.
Six other traits of Tachyina + Xystosomina provide compelling evidence for monophyly of that group.These states are unique within Trechitae and are not found in the outgroup Patrobitae, and thus they are clear synapomor phies of Tachyina + Xystosomina.These character states are as follows: (05) Seta FR6 on frontale is removed mediad instead of being located near lateral margin of frontale, as is shown in generalized larval chaetotaxy by Bousquet & Goulet (1984).( 06) Seta LA5 on ligula is absent.(07) Stipes with one or more teeth at base.( 08) Coronal suture in first-instar larvae is very short or absent.(09) First-instar larvae with some microspines on dorsal surface of coxa, instead of being smooth.(10) Se cond-and third-instar larvae with only one secondary seta on lateral sides of stipes and labium, and mandibles without secondary setae.The plesiomorphic condition for trechites is more than one (usually two or more) secon dary seta on each lateral side of stipes and labium, and with at least one secondary seta on mandible.
On the other hand, we were unable to find synapomorphic character states that indicate the sister group of Tachyina and Xystosomina combined.Studied larvae exhibit only a few similarities with those of some (but not all) members of the tribe Trechini, which have been noted above, but these similarities are considered convergent.Tachyine and xystosomine larvae do not show clear apomorphies with members of Bembidiina, the subtribe to which they are most often associated as members of the tribe Bembidiini.The distinctiveness of Tachyina + Xys tosomina larvae, and their lack of clear relationship with other taxa, including bembidiines, provides some evi dence that they could be regarded as a separate tribe Tachyini, consistent with the views of Kryzhanovskij (1983: 236).The tribe Tachyini (sensu Kryzhanovskij) includes also two more subtribes Anillina and Lymnastina, larvae of which are still unknown.Erwin (1982) considers Anillina and Lymnastina as highly derived Tachyina, not as separate clades.The question about the sister group of Tachyina + Xystosomina will not be resolved until larvae of Lymnastina, Anillina, and more taxa of Bembidiina and other Trechitae are described.

Genus Mioptachys Bates, 1882
Diagnosis.First-instar larvae of this genus can be rec ognised by the presence of egg-bursters on the frontale in combination with absence of microsculpture on the dorsal surface of the parietale.
Description.First instar.Cephalic capsule flat, later ally rounded (Fig. 3); frontal sutures U-shaped posteriad; frontale with egg-bursters consisting of two longitudinal rows of spinules, without microspines at base; parietale smooth, without microspines laterad of seta PA3; pore PAb present at level of seta PA2; seta FR4 as short as diameter of seta FR2; seta FR6 removed proximad and closer to FRf than to FRe; seta FR9 normal, about 3-5x longer than FR5; sensorium on antennomere III larger than in Tachyta (Figs 3,12), but smaller than in remaining taxa; dorsal surface of mandible near pore MNb without microspines (Fig. 3); terebra without serration; group gMX on stipes with 6 setae (Fig. 40); second galeomere about 5 times longer than first; pore MXc in distal fourth of ventral surface of stipes; distal seta of group gMX on stipes anteriad of level of seta MX5; pore PRh on protergum present; setae PR13, ME14, and TE11 about 3-5* longer than proximal diameter of setae PR12, ME13, and TE10 respectively; seta on claw as long as a half of proximal diameter of claw.Second and third instars.Unknown to us, briefly described by Thompson (1979).According to his drawings, cephalic capsule later ally rounded.
Monophyly and phylogenetic relationships.Studied larvae of the genus Mioptachys possess only one unique, probably apomorphic morphological character state: the presence of six setae in group gMX on the stipes in first instar larvae instead of five setae.Beside that, Mioptachys larvae exhibit some similarities with those of the genus Tachyta, which we regard as convergences; these char acter states are discussed under Tachyta.Because second and third instars of Mioptachys are unknown, the relation ships of the genus are unclear.
Geographical distribution and diversity.The genus Mioptachys includes about 12 described species which are distributed from southern Canada to northern Argen tina including Hispaniola (Erwin, 1994).

SUBTRIBE TACHYINA
Genus Tachyta Kirby, 1837 Diagnosis.First-instar larvae of this genus are unique in the presence of egg-bursters in combination with the presence of meshed microsculpture dorsolaterad on the parietale.Second and third instars of Tachyta may be immediately recognised by at least two unique features: the galea is shorter than two proximal maxillary palpomeres combined, and the secondary seta at middle of lateral side of ninth abdominal tergum is absent.
Monophyly and phylogenetic relationships.The single Tachyta species studied has six presumable larval autapomorphies: (1) the claw seta long and thin, as long as proximal width of claw; (2) antennomere III with small sensorium; (3) first-instar larvae with only four setae in group gMX on stipes instead of five; (4) second and third instars with very short galea, which is shorter than two proximal maxillary palpomeres combined; (5) secondand third-instar larvae with only a few secondary setae on terga; (6) lateral side of ninth abdominal tergum in second and third instars without secondary seta at middle.
Second and third instars of the genus Tachyta also have a notable plesiomorphy: the presence of an ocular groove on the cephalic capsule, a feature unique within Tachyina + Xystosomina.
It should be mentioned that Tachyta larvae share some character states with those of Mioptachys.However, we do not use them to link the genera as closely related taxa.We consider two of these characters to be convergent similarities due to similar way of life under bark of logs.This derived strategy is unique within the discussed genera of Tachyina and Xystosomina.These characters are: (1) cephalic capsule depressed and laterally rounded (Figs 3, 4) and ( 2) frontal sutures U-shaped posteriad (Figs 3, 4).Adaptive value of the depressed cranium is obvious for larvae living under bark.We propose that the wider and laterally rounded cranium, as well as the shape of the base of frontale, are connected to the depressed cranium.Consequently, we believe that the discussed characters have been acquired independently as a result of subcortical larval habits.
The remaining two characters, namely (3) frontale in first-instar larvae with egg-bursters and (4) dorsal surface of mandible near pore MNb in first-instar larvae without microspines, are considered as being of uncertain phylo genetic value.Absence of egg-bursters in first instar is generally considered as a derived character state in carabid larvae, but egg-bursters are lacking in at least one Patrobitae species (Bousquet & Grebennikov, 1999).However, the presence of markedly developed eggbursters on frontale in first-instar larvae, similar to those of Mioptachys and Tachyta, was also recorded for mem bers of some other Trechitae taxa: the genus Asaphidion (by several authors, SEM picture and earlier references in Maddison, 1993) and the subgenus Metallina Motschulsky, 1850 of the genus Bembidion (our data, unpubl.).In addition to this, markedly developed egg-bursters were found not only on frontale but also on parietale of first instar larvae of two rather unrelated groups within Trechi-tae: some Bembidion species (Maddison, 1993) and the monobasic pogonine genus Thalassotrechus (Greben nikov & Bousquet, unpubl.).An even more complicated pattern was found for the presence or absence of micro spines on mandible in first-instar Trechitae (our data, unpubl.);this character is poorly known outside the supertribe.We therefore prefer not to use the above char acters for phylogenetic purposes.
Geographical distribution and diversity.The genus Tachyta, according to Erwin's review (1975), is distrib uted in all zoogeographical regions of the World and includes 19 species arranged in two subgenera.Recently Baehr (1986) described a new species of Tachyta from Australia, and Erwin and Kavanaugh (unpubl.)discov ered another from China.
Remarks.Our material represents the nominotypical Palaearctic subspecies, Tachyta nana nana.We could not find any significant larval morphological differences between the nominotypical subspecies and the Nearctic subspecies Tachyta nana inornata described by Erwin (1975).

Genus Tachys Dejean, 1829
Diagnosis.All instars of Tachys have one unique fea ture, namely the lack of pore PRh on protergum (Fig. 58).First-instar larvae can be recognised from other Trechitae taxa (except Polyderis) by the combination of smooth terebra and absence of egg-bursters on frontale.Second and third instars of Tachys bear the highest number of secondary setae on terga and have a characteristic shape of urogomphi (Fig. 58).
(3) Relatively high number of secondary setae on thoracic and abdominal sclerites in second and third instars.
Larvae of the genus Tachys share with those of Polyderis one apomorphic character state: pore MXc on ventral surface of stipes removed proximad and located at middle of stipes (Fig. 45) instead of distal quarter.

Geographical distribution and diversity. The genus
Tachys (which is considered as a subgenus Tachys s. str. of a more-inclusive genus Tachys by Kryzhanovskij, 1983 andKryzhanovskij et al., 1995) has not been recently revised, so we repeat Kryzhanovskij's (1983) remark that it has up to 50 species, mainly in the Holarctic Region, but with some in Africa and one in Argentine.
Tachys scutellaris (Stephens, 1829) (Figs 5,14,25,26,42,45,50,58) Diagnosis.Larvae of this species differ from those of T. centriustatus by the shape of nasale and by a smaller cephalic capsule; from those of T. halophilus -by the shape of the mandible.We could not find differences between larvae of T. scutellaris and T. vittatus.

Tachys vittatus Motschulsky, 1850
Diagnosis.We did not find structural differences between larvae of this species and T. scutellaris.

Tachys halophilus Lindroth, 1966
(Fig. 19) Diagnosis.First-instar larvae of this species differ from those of remaining Tachys by shape of the mandible which is wider in proximal half than that of other species (Fig. 19).
Remarks.In the Russian carabid checklist (Kryzhanovskij et al., 1995) T. centriustatus is treated as a member of the subgenus Paratachys of the genus Tachys.Erwin (1974a) considers Paratachys as a separate genus which is supported by the present study of the larval mor phology.Larvae of T. centriustatus share all apomorphic features of the genus Tachys and, consequently, we treat it as a member of this genus, not of Paratachys.This taxonomic action should be confirmed by study of adult morphology.

Genus Polyderis Motschulsky, 1862
Diagnosis.Known larvae of this genus differ by the character states listed in the key to genera oflarvae.
Description.All instars.Cephalic capsule subcylindri cal, parallel-sided (Fig. 6); pore PAb absent or present distad of level of seta PA2; sensorium on antennomere III large; terebra without serration; pore MXc at middle of ventral surface of stipes (as on Fig. 45); distal seta of group gMX on stipes anteriad of level of seta MX5; pore PRh on protergum present; setae PR13, ME14, and TE11 about 3-5x longer than proximal diameter of setae PR12, ME13, and TE10 respectively; seta on claw as short as 1/4 of proximal diameter of claw.First instar.Frontal sutures V-shaped posteriad (Fig. 6); frontale without eggbursters but with microspines at base; parietale smooth, without microsculpture laterad of seta PA3; seta FR4 about 3-5x longer than diameter of seta FR2; seta FR6 closer to FRe than to FRf; seta FR9 normal, about 3-5x longer than FR5, or short, not longer than FR5; dorsal surface of man dible near pore MNb with microspines; group gMX on stipes with 5 setae; second galeomere about 3x longer than first.Second and third instars.Ocular groove absent (Fig. 52); group gMX on stipes with 6 setae; galea longer than two proximal maxillary palpomeres combined; terga with number of secondary setae average for larval tachyines (as in Fig. 59); lateral side of ninth abdominal tergum with secondary seta at middle.
Monophyly and phylogenetic relationships.Monophyly of this taxon based on larval characters is uncertain since we did not find any autapomorphic feature.The studied larvae of this genus share with those of Tachys a single synapomorphic character state, namely pore MXc on ventral surface of stipes removed proximad and located at middle of stipes instead of being located at distal quarter.Only two specimens of two species are available for study; they are dissimilar and each of them possesses some unique apomorphic characters.Relation ship of Polyderis will remain uncertain until more larvae are available for study.See also remarks under each spe cies treated.
Geographical distribution and diversity.The genus Polyderis has not been recently revised, but Erwin (1974a) recognised over 50 species from all zoogeo graphical regions and oceanic islands.
Remarks.Larvae of this species are known from a single specimen; additional material is needed to confirm the morphological features.
Description.First-instar larvae.Not available.Second and third instars.HW = 0.18 mm, HL = 0.17 mm (n = 1) in second instar.Nasale: Fig. 30.Seta FR9 about 3-4x longer than FR5; pore PAb on parietale absent.Third-instar larvae not available.Remarks.Like the previous Polyderis species, P. laevis is known from a single larval specimen.It has an unusual apomorphic character state: parietale without pore PAb.The character is unique for the tribe and has been noted only for larvae of Thalassophilus longicornis (Sturm, 1825), a member of the tribe Trechini (Greben nikov, 1996).Additional material is needed to confirm the described characters.

Genus Elaphropus Motschulsky, 1839
Diagnosis.Studied larvae of this genus possess no remarkable feature which might be used to separate them from those of other genera described; see key to genera.
Monophyly and phylogenetic relationships.Studied larvae of this genus share no synapomorphic feature; con sequently monophyly of this taxon is uncertain.There are also no apomorphies shared with other tachyines or xystosomines that would indicate the sister group of Elaphropus.
Geographical distribution and diversity.This widely distributed genus is accepted in the present paper according to Erwin's (1974a) limits; since the genus was never entirely revised, it is not possible to provide the precise number of species.Erwin (1974a) pointed out that the group's greatest diversity is in the Oriental Region and in Africa.Recently Baehr (1987) revised the Austra lian fauna of Elaphropus and listed 23 species of the sub genus Elaphropus and one of the subgenus Sphaerotachys (the latter taxon is regarded in the current work as a separate genus).
Elaphropus tripunctatus (Say, 1830) (Figs 8,33) Diagnosis.Larvae of this species can be distinguished from those of E. diabrachys by the length of setae FR3 and FR4 on frontale being about as long as seta FR5.

Genus Sphaerotachys G. Muller, 1926
Diagnosis.Larvae of this genus can be recognised by the presence of numerous (about 15) small and equal teeth on cutting edge of terebra (Figs 9,54).Larvae of the genera Paratachys and Porotachys also have teeth on terebra, but they are less numerous (not more than 10), and the shape of the teeth is different (Figs 10,11,55,56).
Monophyly and phylogenetic relationships.Larvae of the single species studied possess equal serration along the cutting edge of terebra which we consider as an apomorphic character state.We believe that teeth appear independently within the subtribe for Sphaerotachys on one hand and Porotachys + Paratachys on the other because of the different shape and number of teeth.Rela tionship of the genus is obscure since no synapomorphic character state has been found.
Remarks.Based on adult morphology, Sphaerotachys is closely related to the genus Elaphropus and is regarded by some authors as a subgenus (Baehr, 1987;Kryzhanovskij et al., 1995) or even as a synonym (Erwin, 1974a) of the latter.

Genus Paratachys Casey, 1918
Diagnosis.Larvae of all instars of this genus (as well as Porotachys) can be recognised by having the terebra with some large teeth near the retinaculum and much smaller serrations or smooth cutting edge distally (Figs 10,11,55,56).Larvae of the genus Sphaerotachys also have teeth on the terebra (Figs 9, 54), but their shape and number differ from those of Paratachys and Porotachys.For separation of these two genera see key.
Monophyly and phylogenetic relationships.Larvae of the sole species studied exhibit no apomorphies.They share with those of the genus Porotachys a single apomorphic character: the presence of some large teeth at the base of the cutting edge of the terebra.
Geographical distribution and diversity.Erwin (1974a) indicated that the widespread genus Paratachys includes about 300 species in the New World alone, and that most of them are undescribed.
Phylogenetic relationships.The presence of large teeth at the base of the cutting edge of the terebra indi cates that the genus Porotachys is related to the genus Paratachys.
Geographical distribution and diversity.This genus includes a single described species, Porotachys bisulcatus, which is widely distributed in the western part of Palaearctic Region and was accidentally introduced in North America (Kryzhanovskij, 1983).Erwin (1974a: 126) mentioned a second, apparently undescribed species of this genus from the Oriental Region.

CONCLUDING REMARKS
Larvae of Mioptachys flavicauda (the only larvae known to date of the subtribe Xystosomina) have many apomorphic features shared with those of the subtribe Tachyina (they were noted above in the discussion of monophyly of Xystosomina + Tachyina).There is, how ever, no apomorphic character state to support the monophyly of the subtribe Tachyina exclusive of Xystosomina.Since larvae of only a single Xystosomina taxon are known, we have no evidence about the monophyly of that subtribe as well.Under these circumstances it is impos sible to separate both cited taxa for phylogenetic discus sion because none has clearly supported monophyly based on larval morphology.
Based on at least six synapomorphic character states the subtribes Tachyina and Xystosomina appear to form a monophyletic group.On the other hand, no synapomor phic feature has been discovered to propose a sister group to the Tachyina + Xystosomina within the supertribe Trechitae in general and the tribe Bembidiini in particular.Tachyina and Xystosomina could be regarded as members of a separate tribe Tachyini following Kryzhanovskij (1983), but this conclusion requires a more complete cladistic analysis, which is not currently pos sible with larvae of many Trechitae taxa remaining unknown.
The genera Tachyta and Mioptachys are rather similar in larval morphology.Nevertheless, we believe that all shared similarities are convergent characters or at least of unknown phylogenetic value, and, consequently, do not reflect the true relationship between the genera treated.
Study of larvae supports the view that Paratachys is a separate genus, not a subgenus of Tachys (contrary to the opinion expressed in Kryzhanovskij, 1983, andKryzhanovskij et al., 1995).In fact, based on larval morphol ogy, these two genera are not even sister groups, with Polyderis being more closely related to Tachys, and Porotachys being more closely related to Paratachys.