Stenomastigus Leleup

Paweł Jałoszyński, 2018, World genera of Mastigitae: review of morphological structures and new ecological data (Coleoptera: Staphylinidae: Scydmaeninae), Zootaxa 4453 (1), pp. 1-119 : 95-103

Stenomastigus Leleup

Stenomastigus Leleup, 1968 : 55. Type species: Stenomastigus franzi Leleup, 1968 (des. orig.).

Acanthostigus Leleup, 1968: 95 (as subgenus of Stenomastigus ). Type species: Stenomastigus basilewskyi Leleup, 1968 (des. orig.). Synonymized by Jałoszyński (2012d).

Diagnosis. Adults of Stenomastigus differ from remaining Mastigini in extremely elongate, conspicuously slender aedeagus with one paramere extremely long, much longer than copulatory piece, and the other paramere either completely obliterated or vestigial; in microsculptured, granulose surface of elytra ( Fig. 235 ); and in elytra in females strongly elevated along suture in posterior half, forming a steeply pitched gable roof-like structure ( Fig. 162 ). Mature larva: antennae about 3–4 times as long as head capsule ( Fig. 236 ).

Characteristics. Adults. Body ( Figs 157–158 , 161–162 ) moderately large, 3.10̄ 4.90 mm in length, brown to nearly black, in some cases with elytra lighter than head and prothorax, testaceous or reddish-brown; body strongly convex, dorsally densely but finely setose, setae very short and recumbent, unmodified except for long and thick bristles on scape and pedicel.

Head capsule ( Figs 221–224 ) divided into large and exposed anterior part and much smaller, subcylindrical 'neck' region retracted into prothorax and demarcated by distinct occipital constriction; 'neck' region much broader than half width of head. Anterior part of head flattened, subequal in width with prothorax, distinctly elongate, broadest near middle. Composite eyes dorsolateral, moderately large, composed of numerous small ommatidia, not projecting or weakly projecting from the silhouette of the head, broadly separated from mandibular bases. Vertex and frons divided by a distinct median longitudinal groove ( Fig. 223 ; mg); vertex strongly transverse, convex at sides, with posterior margin nearly straight or slightly concave. Tempora much longer than eyes, weakly rounded. Frons between antennal insertions not forming a demarcated 'platform', weakly convex or flattened, anteriorly demarcated by a deep and very short frontoclypeal groove largely obliterated at sides. Clypeus very short and broad, broadly subtrapezoidal, with nearly straight sides, which are slightly convergent anterad or parallel. Antennal insertions ( Fig. 223 ; ia) located anterodorsally, relatively narrowly separated. Gular plate ( Fig. 224 ; gp) lacking sutures, indistinctly transversely reticulate; posterior tentorial pits ( Fig. 224 ; ptp) typically circular, rarely oval, in front of broad and diffuse transverse impression demarcating 'neck' region ventrally; hypostomal ridges ( Fig. 224 ; hr) arcuate, posteriorly reaching middle between anterior submental margin and posterior tentorial pits. Head finely to strongly punctate, densely setose ( Fig. 223 ).

Antennae ( Figs 157–158 , 161–162 , 221–222 ) very long and slender, subequal to body length or distinctly longer; scape ( Fig. 222 ; sc) 6–8 times as long as broad, thickened, much longer than head, with lateroventral (more ventral than lateral) emargination; pedicel ( Fig. 222 ; pd) enlarged, slightly to much shorter and narrower than scape, typically 5–8 times as long as broad, broadening from narrow base to submedian or subapical region. Both scape and pedicel with two ventral longitudinal rows of several long and thick bristles with papillate insertions, area between bristles with variously densely distributed porous fields; antennomeres III–XI distinctly narrower than pedicel, strongly elongate, each indistinctly thickened distad, basal stalks not exposed in intact beetles, basal rings absent or indistinct; antennomere XI elongate and indistinctly asymmetrical. Antennomeres covered with variously dense, long setae; surface of antennomeres smooth.

Mouthparts. Labrum ( Figs 223 , 225 ; lbr) strongly transverse, with lateral margins slightly divergent anterad or parallel and weakly rounded, and with anterior margin weakly concave, with a pair of broad and short sublateral teeth broadly separated at middle by a very shallow emargination, and with two partly irregular transverse rows of long and short setae. Mandibles ( Figs 225–226 ) symmetrical, subtriangular and robust, each with one dorsal mesal tooth and a group of 2–3 ventral mesal teeth, setose prostheca present and long. Maxilla ( Fig. 229 ) with large, moderately long cardo ( Fig. 229 ; cd); basistipes ( Fig. 229 ; bst) subtriangular and elongate; mediostipes ( Fig. 229 ; mst) large and sharply demarcated from lacinia and galea, which are both elongate and each with conspicuously dense group of thin distal setae; palpifer ( Fig. 229 ; ppf) broad and elongate; maxillary palp slightly to much longer than head capsule, composed of minute palpomere I ( Fig. 229 ; mxp1), slender, curved, distinctly but only slightly broadening distad palpomere II, palpomere III strongly elongate, strongly and gradually broadened distad, with transverse or (rarely) slightly obtuse distal margin, palpomere IV ( Fig. 227 ; mxp4) subequal in length to III, suboval or indistinctly subtriangular and broadening from base to distal third, with rounded apex, elongate. Surface of palpomere IV, and sometimes also III, with sparsely distributed porous fields ( Fig. 188 ; pf). Palpomeres III and IV slightly (sometimes indistinctly) flattened, II–IV covered with relatively long and dense setae. Labium ( Figs 224 , 229 ) with broad and short submentum ( Fig. 224 ; smn) posteriorly not demarcated from gular region, densely setose and lacking an outstanding pair of anterior or subanterior lateral setae; mentum ( Fig. 229 ; mn) subtrapezoidal and strongly transverse, with anterior margin very deeply emarginate, so that anterolateral corners form triangular and usually pointed lobes projecting anterad; prementum ( Fig. 229 ; pm) long, subtrapezoidal, broadest distally, lacking demarcated ligula, with several pairs of submedian anterior setae, with broadly separated bases of labial palps; lateral hypopharyngeal lobes moderately large, with conspicuously sparse, thick mesal setae; labial palp composed of three palpomeres: palpomere I ( Fig. 229 ; lp1) small, elongate, strongly broadening distad, palpomere II ( Fig. 229 ; lp2) largest, conspicuously enlarged, long and broad, approximately barrel-shaped, palpomere III ( Fig. 229 ; lp3) very small and narrow in relation to II, about half length of II, pointed.

Prothorax ( Figs 157–158 , 161–162 , 230–231 ) strongly elongate, strongly convex but usually with flattened dorsum ( Fig. 231 ), broadest near anterior third. Pronotum with anterior and posterior margins arcuate (anterior margin sometimes nearly straight), sides rounded in anterior half and sinuate in posterior half; anterior and posterior corners obtuse-angled or broadly rounded; pronotal base lacking pits and groove. Prosternum ( Fig. 230 ) with basisternal part ( Fig. 230 ; bstr) subequal in length to coxal part ( Fig. 230 ; cxst). Prosternum laterally completely fused with hypomera. Coxal region anteriorly and laterally without marginal carina; postcoxal hypomeral lobes ( Fig. 230 ; pchl) conspicuously large, rounded and strongly projecting mesad or anteromesad and overlapping with (but not fused to) posterolateral lobes of prosternum, so that procoxal cavities are not open, but entirely delimited posterioly by hypomeral lobes. Prosternal intercoxal process developed as a diffuse, barely discernible longitudinal elevation, which in intact beetles is hidden between procoxae. Ventral surface of prothorax densely setose ( Fig. 230 ).

Mesoventrite ( Figs 232–233 ) subtrapezoidal, broadening posteriorly. Prepecti ( Fig. 232 ; pre) moderately long and together with anteromedian mesoventral area forming a relatively short 'collar', which has a narrow transverse groove just behind its anterior ridge; setose impression absent or very shallow and diffuse. Mesoventral intercoxal process ( Fig. 232 ; msvp) reaching middle of mesocoxal cavities; short and very broad, subtriangular with pointed or narrowly rounded apex, weakly convex, broadly separating mesocoxae, posteriorly separated from metaventrite. Mesanepisterna ( Fig. 233 ; aest2) relatively narrow and strongly elongate, demarcated from median part of mesoventrite by a distinct ridge and from mesepimera by complete suture; mesepimera ( Fig. 233 ; epm2) elongate, indistinctly demarcated from metepimera, not exposed in ventral view.

Mesonotum with cordiform, broad mesoscutellum with pointed apex, in intact specimens not visible between elytral bases, or only its very tip discernible; scutoscutellar suture absent.

Metanotum partly reduced, with lightly sclerotized mesoscutum, but only slightly shortened alacristae; hind wings absent.

Metaventrite ( Figs 232–233 ) short, subrectangular and strongly transverse, with lateral margins rounded; mesocoxal cavities with all margins non-carinate; posterior margin of metaventrite deeply bisinuate laterally (in front of each metacoxa) and with a broad metaventral intercoxal process ( Fig. 232 ; mtvp) with shallowly concave or straight posterior margin; anteriorly metaventrite forming a short anterior metaventral process ( Fig. 232 ; amvp) similar in shape to mesoventral intercoxal process or with its tip concave (when the tip of mesoventral process is rounded, as in Fig. 232 ) and meeting the latter at middle of mesocoxal cavities; metaventral foveae absent. External admetacoxal part of posterior metaventral margin lacking adcoxal carinae, but with adcoxal expansions ( Fig. 232 ; ade). Metanepisterna ( Fig. 233 ; aest3) relatively narrow, partly visible in ventral view, narrowing posteriorly; metepimera ( Fig. 233 ; epm3) 2–3 times as broad as metanepisterna, with not demarcated inner and outer components, posteriorly extending far behind metacoxae.

Metendosternite (metafurca) with stem much broader than long and broadly separated, with divergent lateral furcal arms, lacking median longitudinal projection.

Legs ( Figs 157–158 , 161–162 , 221 , 232 ) extremely, strikingly long and slender. Pro- and mesocoxa short subconical, metacoxa with nearly hemispherical basal part and subconical distal part. Mesocoxa lacking coxal bristles. All trochanters short and subtriangular; femora weakly clavate; tibiae slender; tarsi long and slender, nearly subcylindrical, tarsomeres I–V reducing in length, tarsomere V strongly elongate, with curved and slender claws lacking elongate costae; empodial region was not studied.

Elytra ( Figs 157–158 , 161–162 , 234–235 ) oval, strongly convex, lacking humeral calli and basal impressions, with rounded or pointed apices; elytral disc with superficial, very indistinct and incomplete longitudinal rows of fine punctures obscured by coarse granulose microscultpure ( Figs 234–235 ). Elytra densely setose, setae short and nearly recumbent.

Abdomen with sternite III firmly fused with metaventrite (so that during disarticulation it is almost impossible to separate intact abdomen), much longer than sternite IV, but shorter than IV–VI together; sternite VIII in male distinctly, deeply emarginate.

Aedeagus (illustrated in Jałoszyński et al. (2018)) strongly, often extremely elongate, with asymmetrical median lobe and asymmetrical parameres, one paramere extremely long, much longer than copulatory piece, the other one obliterated or (rarely) present but vestigial; flagellum very long and forming several coils. Ejaculatory duct with elongate and narrow sperm pump lacking funnel-like structures. Aedeagus with elongate copulatory piece with membranous endophallus permanently everted, and only inflated during copulation, with distal end of flagellum permanently fixed, so that flagellum is not extricable. Aedeagus in repose positioned asymmetrically within abdomen, with basal orifice lateral or dorsolateral; in species with extremely long paramere aedeagus cannot be completely retracted into abdomen and in repose a short apical portion of paramere is permanently outside.

Spermatheca subglobose, with relatively large accessory gland.

Sexual dimorphism distinct, females distinctly larger and with much broader elytra, which have different shape than those in males, with a pair of variously large and deep impressions in anterior third (to receive protrochanters of males during mating) and posteriorly steeply elevated along suture, forming a 'gable roof', which, through a gap between separately rounded elytral apices, receives the long paramere during copulation. Slender males have evenly convex elytra and the apical portion of each protibia curved and/or provided with a variously developed tooth, in some species protrochanters are modified, angulate or with variously long ventral projection.

Larva. Very similar to larva of Palaeostigus , but lightly pigmented ( Fig. 236 ), orange when alive, and with extremely elongate antennae, which in mature larvae are more than three times as long as head capsule.

Composition and distribution. Stenomastigus comprises 17 species and 6 subspecies distributed in eastern part of South Africa ( Fig. 165 ).

Natural history. Species of Stenomastigus live in large groups on relatively restricted areas in South Africa; adults are diurnal and they can be found along streams, rivers or ditches with trees, bushes and long grasses ( Figs 237–239 ) providing some shade and surface for beetles to climb; or in forests, also close to water ( Figs 241–242 ) or in swampy areas. They seem to be more active, or at least can be encountered in greater numbers, during cloudy or even rainy days. Beetles can be found on ground, on mosses and among low vegetation ( Figs 234–246 ), but they also climb tall grasses, bushes and trees ( Fig. 240 ) and can be collected by using an entomological umbrella and beating twigs at 2 m and higher. Larvae live in moist leaf litter close to water.

During short field observations and a few days of keeping living adults of two species in plastic containers, I was not able to observe any feeding-related behavior.

An unusual way of copulation in South African Stenomastigus was described (Jałoszyński et al. 2015); males have extremely long aedeagi, in some cases nearly reaching 3/4 of the body length, and they mount females in such a way that both individuals face the same direction, with the tip of copulatory piece inserted into the female's gonopore, and the long paramere inserted into the female's subelytral space, the apical portion of paramere is positioned along suture between anterior impressions of female's elytra, which provides an additional stabilization for the aedeagus. Retracting the aedeagus after copulation takes some time, during which male lays on its back twisting and positioning its enormously long aedeagus. Not only the copulation is remarkably different that that in externally similar Palaeostigus ; the male terminal abdominal segments in Stenomastigus have an additional, asymmetrical group of muscle fibers, presumably to assist the rotation of the aedeagus during its retraction (Jałoszyński et al. 2015). It was hypothesized (Jałoszyński et al. 2015) that the elongation of the aedeagus during evolution was possible because of an already existing preadaptation, a broadly separated arms of the metendosternite, between which the median lobe of aedeagus can be placed, and in extreme cases its base can reach the mesoventrite. In genera with Y-shaped metendosternite and especially in those with a long metafurcal stem, this structure forms a structural obstacle between the abdomen and thorax, and the aedeagus is always shorter than the abdomen.

Stenomastigus is so far the only staphylinid genus, and one of a few among beetles, in which the architecture of extrinsic and intrinsic aedeagal muscles was studied in detail, with three-dimensional reconstructions of the postabdomen and also some other internal abdominal organs (Jałoszyński et al. 2015).

Remarks. Although Stenomastigus can be easily recognized on the basis of enormously long antennae and legs, the slender body form of males and the modified elytra in females, diagnostic features of this genus are relatively weak, and the classification of Mastigini may require further study.

All nominal species were treated by Franz (1984), Leleup (1968) and Jałoszyński ( 2012d; e).

Figures

FIGURE 236. Mastigini, larva of Stenomastigus longicornis in lateral view.

FIGURES 233–235. Mastigini, morphological structures, Stenomastigus varii. Pterothorax in lateral view (233); left elytral base in dorsal view (234); microsculpture of elytron (235). Abbreviations: aest, mesanepisternum; aest3, metanepisternum; cx3, metacoxa; epm2, mesepimeron; epm3, metepimeron; pre, prepectus; v3, metaventrite.

FIGURES 159–162. Mastigini, examples of extant species; lateral habitus. Palaeostigus prolongatus, male (159); Palaeostigus bifoveolatus, male (160); Stenomastigus longicornis, male (161); Stenomastigus dendrophilus, female (162).

FIGURES 153–158. Mastigini, examples of extant species; dorsal habitus. Mastigus deustus, female (153; after Jałoszyński (2012b)); Mastigus spinicornis, male (154); Palaeostigus prolongatus, male (155); Palaeostigus bifoveolatus, female (156); Stenomastigus vulgaris, female (157); Stenomastigus allaeri, male (158; after Jałoszyński (2012d)).

FIGURES 221–224. Mastigini, morphological structures, Stenomastigus longicornis (221–222) and Stenomastigus varii (223– 224). Head and prothorax in lateral view (221); head in frontal (222), dorsal (223) and ventral (224) views. Abbreviations: ai, antennal insertion; fcg, frontoclypeal groove; gp, gular plate; hr, hypostomal ridge; lbr, labrum; mg, median groove; mn, mentum; mxp2–4, maxillary palpomere II–IV; pd, pedicel; ptp, posterior tentorial pit; sc, scape; smn, submentum.

FIGURES 225–228. Mastigini, morphological structures, Stenomastigus varii. Mouthparts in anterodorsal (225) and dorsal (226) views; left maxillary palpomere IV in dorsal view (227); porous fields on maxillary palpomere IV (228). Abbreviations: cl, clypeus; fcg, frontoclypeal groove; gal, galea; lbr, labrum; lp, labial palp; mxp4, maxillary palpomere IV; pf, porous field.

FIGURES 229–232. Mastigini, morphological structures, Stenomastigus varii. Mouthparts in ventral view (229); prothorax in ventral (230) and lateral (231) views; pterothorax and abdominal base in ventral view (232). Abbreviations: ade, adcoxal expansion; amvp, anterior metaventral process; bst, basistipes; bstr, basisternal part of prosternum; cd, cardo; cxst, coxal part of prosternum; lp1–3, labial palpomere I–III; mn, mentum; mst, mediostipes; msvp, mesoventral intercoxal process; mtvp, metaventral process; mxp1, maxillary palpomere I; pchl, postcoxal hypomeral lobe; pm, prementum; ppf, palpifer; pre, prepectus; st3–4, abdominal sternite III–IV.

FIGURES 185–188. Mastigini, morphological structures, Palaeostigus ruficornis schimitscheki. Right maxilla in ventral view (185); labium in dorsal view (186); right maxillary palpomeres II–IV in dorsal view (187); porous fields on maxillary palpomere IV (188). Abbreviations: bst, basistipes; cd, cardo; gal, galea; lac, lacinia; lhl, lateral hypopharyngeal lobe; lp2–3, labial palpomere II–III; mst, mediostipes; mxp1–4, maxillary palpomere I–IV; pf, porous field; ppf, palpifer.

FIGURES 164–165. Mastigini, distribution of extant species. Mastigus (164); Stenomastigus (165).

FIGURES 237–242. Mastigini, habitats. Durban env. (237–240) and Doreen Clark Nature Reserve (241–242), RSA, habitat of Stenomastigus longicornis.