Stenus canaliculatus Gyllenhal 1827

Redefinition of the canaliculatus group Diagnosis Body size moderate; body length of the known species: 2.8 to 4.7 mm. Head fairly small to fairly broad in comparison to elytra and pronotum. Upper surface between eyes feebly evenly convex, nearly flat, to slightly concave as a whole; with longitudinal elevations and impressions nearly absent to well developed. Antennae rather short to long, with the club segments more or less elongated. Internal tooth of each mandible placed a little before the middle, directed only slightly dorsally of the main plane. Maxillar palpi with yellow and not shortened basal segment. A considerable reduction of the adhesion capture apparatus may be supposed for all the members of the group (for more details see the Remarks section below). Pronotum with long and sharp longitudinal median furrow more or less deep, less often very feeble, stroke-shaped. Elytra nearly rectangular, large, with short rounded humeri, to shortened, slightly trapezoid with humeri less developed. Legs moderately long; the segment 4 of tarsi without emargination; segment 1 of metatarsi about the same length as to a bit longer than segment 5 but distinctly shorter than segments 2 to 4 together. Abdomen moderately convex, with paratergites evident on abdominal segments 3 to 7; four anterior visible tergites each with four short but evident longitudinal keels at basal part; posterior margin of the tergite 7 with well-developed fine membranous fringe. Puncturation of pronotum and elytra diverse, but always non-rugose. Ground sculpture cellular or net-shaped to entirely absent. Pubescence moderate, contiguous to fairly long, outstanding. MALE. Legs without specific features; posterior margin of abdominal sternites 6–7 shallowly emarginated to about straight;abdominal sternite 8with a broad but not deep, rounded or angularly rounded emargination of posterior margin; abdominal sternite 9 with large posterolateral teeth not incurved inwards; ventral depressions of sternites 6–8, if present, flat and feeble, never flanked by sharp keels. Aedeagus relatively large; median lobe archetypically lanceolate, with apical sclerotized part to a variable extent angular to angularly rounded; endophallus with paired medial bands (optionally, an unpaired bar may also be distinct), small but visible lateral parts, and H-shaped expulsion clasp producing lateral portions fairly broad; basal tube of diverse structure, more or less sclerotized. Parameres cylindrical, with apical broadening, if present, very feeble, rather vague; apico-internal setiferous surface differentiated poorly to moderately, setae fairly uniform. FEMALE. Posterior margin of abdominal sternite 8 broadly rounded to angularly rounded; each valvifer with posterolateral tooth directed backwards. Spermatheca sclerotized to a greater or lesser extent. In the structure of abdomen, the male genitalia, the abdominal sternite 9 of both males and females (valvifera), the group under consideration is most closely related to the boops and palposus groups of the same subgenus Nestus. It differs from these by the structure of the mandibles, by the adhesion capture apparatus more or less shortened, by the pronotum with long and sharp, line-shaped, longitudinal median furrow, by the evidently narrower aedeagus, by the endophallus with the expulsion clasp’s transversal bridge adjoining to the lateral pieces near their middle, by the abdominal sternite 9 with posterolateral teeth not being curved inwards, by the male legs without peculiar features (not all the species of the named groups have such features well developed); from the palposus group it can be easily distinguished by the yellow basal segment of the maxillar palpi. The differences from S. (N.) melanopus Marsham, 1802, similar externally to some species of the canaliculatus group, are given in the diagnosis of the melanopus group and in the key (see below). In many characters of the structure of abdomen and male genitalia, the canaliculatus group also resembles the aggregate including the large and diverse atratulus -fuscipes -cautus -crassus complex and some isolated American members, as S. (N.) mendosus Puthz, 1971 (1971d) and S. (N.) sordidus Puthz, 1988, but phylogenetic relations within the named aggregate remain poorly understood; therefore a thorough analysis seems to be premature. In external morphology, the canaliculatus group can easily be distinguished from all the species of the mentioned complex by the presence of the long and sharp, line-shaped, longitudinal median furrow of the pronotum. Taxa included S. (N.) alopex sp. nov., S. (N.) brivioi Puthz, 1972, S. (N.) canaliculatus Gyllenhal, S. (N.) canalis sp. nov., 1827, S. (N.) canosus sp. nov., S. (N.) caseyi Puthz, 1972, S. (N.) confusus J. Sahlberg, 1876, S. (N.) delitor sp. nov., S. (N.) dolosus Casey, 1884, S. (N.) geminorum Puthz, 1973 * (USA: Washington; Canada: BC, see Puthz 1973b: 208), S. (N.) idoneus Sharp, 1887 * (Mexico, see Sharp 1887: 799; Puthz 1968: 11 ), S. (N.) illotulus Puthz, 1972, S. (N.) illusor Ryvkin, 1987, S. (N.) immigratus Puthz, 2006 * (China: Taiwan: Ilan Hsien: Shen Mi Lake, 1100 m, see Puthz, 2006: 187), S. (N.) labilis Erichson, 1840, S. (N.) latipennis J. Sahlberg, 1880, S. (N.) milleporus Casey, 1884, S. (N.) nitens Stephens, 1833, S. (N.) raddei Ryvkin, 1987, S. (N.) shogun Puthz, 1987 * (Japan: Nishigo Uzan, see Puthz 1987: 45), S. (N.) sphaerops Casey, 1884, S. (N.) vinnulus Casey, 1884. Notes on comparative morphology and evolution As most other polytypic groups in Steninae, the canaliculatus group reveals diverse evolutionary trends quite evident at a comparison between species of the same lineage. A concise review of the mentioned trends is given below. A. Upper surface of head between eyes tends either to become flattened, at most slightly convex (S. shogun, S. nitens, S. illotulus, S. raddei, S. caseyi, S. canaliculatus, S. canalis, S. canosus, S. alopex) or to produce distinct longitudinal impressions with median elevation in between (S. labilis, S. idoneus, S. vinnulus, S. brivioi, S. confusus, S. delitor, S. illusor, S. latipennis, S. immigratus, S. geminorum, S. sphaerops, S. dolosus, S. milleporus). The latter tendency seems to arise and develop independently, at least within the complexes labilis-confusus-delitor-illusor-latipennis-sphaerops and dolosus - milleporus, in spite of the fact that the latter is closely related in the shape of the male genitalia to the ‘ canaliculatus s.str. ’ complex characterised by a flat front. B. For three species studied by him (S. canaliculatus, S. nitens, S. labilis), Betz (1996) has shown a considerable reduction of the adhesion capture apparatus. It is expressed both in the degeneration of sticky cushions of paraglossae (decreasing of their surface and number of adhesive setae etc.) and in a shortening of the labium. No data on labium length have been provided by the named author but, as it follows from the text, S. milleporus (as sectilifer Casey, 1884), S. caseyi, S. vinnulus, as well as S. melanopus, which is regarded as a member of the same group, have also been studied in this respect. I have measured the relative length of the prementum (eulabium after Weinreich 1968) for six species of the canaliculatus group: S. canaliculatus (I: 17, 0.53; II: 16, 0.53), S. nitens (33, 0.87), S. labilis (37, 1.12), S. illotulus (26, 0.84), S. raddei (35, 1.09), S. latipennis (31, 0.79); and for three species of other groups of the subgenus Nestus Rey, 1884: S. ruralis Erichson, 1840 (47, 1.52) from the palposus group, S. argus Gravenhorst, 1806 (47, 1.74) from the fuscipes group, and S. melanopus Marsham, 1802 (41, 1.46) presumably representing a monotypic group. In each case above, the first number in brackets means length of prementum whereas the second one is a rounded ratio between the former and the length of the head (two specimens have been measured for S. canaliculatus). One can see that the data split the examined species into three clusters: a) S. canaliculatus; b) the remaining species of the canaliculatus group; c) the species of other groups; both total and relative length of prementum increases from category a) to c). It can be supposed that the reduction of the eulabium is a characteristic feature of all the members of the canaliculatus group, but it must be verified by a specific study. The shortening of the adhesion capture apparatus seems to correlate with some other modifications of the mouth parts, namely the more proximal position of the internal tooth of mandibles as well as its lesser deflection off the main plane of the mandible. The small median notch of the labrum is a specific character of S. canaliculatus and the most closely related Palaearctic species (S. canalis, S. canosus, S. alopex). Though the functional explanation of this feature is obscure as yet, one can suppose it to correlate with the extreme reduction of the adhesion capture apparatus in this species complex. The Nearctic members of the same complex (S. milleporus and S. dolosus) have only a broad and shallow emargination on the anterior margin of the labrum; the margin is slightly sinuate at the middle in S. nitens, S. illotulus, S. raddei, S. caseyi, S. brivioi, S. vinnulus; lastly, the species of the labilis-confusus-delitor-illusor-latipennis-sphaerops line have the margin nearly straight. Betz (1996) supposed the reduction of the adhesion capture apparatus in the canaliculatus group to be a result of miniaturisation, but this explanation does not stand up to any criticism. Indeed, the smallest specimens of the species within the group may be considered among the medium-sized representatives of the genus, while even much smaller members of both the pusillus and the pumilio groups have the adhesion capture apparatus well developed (S. (N.) pusillus Stephens, 1833 has been studied by Betz (1996) himself). I believe that the modification of the mouth parts should be

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