Largusoperla crassus Chen, sp. nov.

Figs. 1–5.

Description. Macropterous; body length (excluding antennae and cerci) ca. 11.0 mm, plump, generally brown (Figs. 1, 2).

Head (Figs. 1, 2): Head oval, with a dark stigma covering the three ocelli and the anterior half, compound eyes dark. Left antennae dark, with at least 38 segments; right antennae brown, with at least 39 segments. Maxillary palp slender, with a shortened apical segment, slightly longer than the three-segmented labial palp.

Pronotum (Figs. 1, 2): Pronotum about as wide as head, near trapeZoidal with obtuse corners, mostly brown except for the pale median part. Mesothorax and metathorax sclerotiZed, nearly as wide as pronotum. Legs brown; two tibia spurs present; first two tarsal segments shortest with conspicuous euplantulae.

Wings (Figs. 1, 3): Wings hyaline, veins brown. Forewing length ca. 12.0 mm; ScP reaching RA beyond ra-rp in left forewing; a subcostal crossvein present between ScP and RA in right forewing; a crossvein present in area between anterior margin and RA in left forewing; 14 crossveins present between ScP and the anterior margin, crossing with ScP; RP originating from basal ¼ of RA and with three branches; M forked at near half of the wing length; ra-rp not connecting with the opposite crossvein rp-ma; CuA forked basal to the fork of M, with three branches; CuP simple; area between M and CuA with four crossveins; area between CuA and CuP with three crossveins in left forewing, five in right forewing; AA1 simple, AA2 with three branches. Hind wings not clearly visible.

Abdomen (Figs. 1-2, 4-5): Abdomen enlarged, shorter than half the length of the body, with dark lateral surfaces; posterolateral margins of each segment not extended. Median part of tergum 9 with two patches of long hairs. Posterior half of tergum 10 strongly cleft, posteromedially with a V-shaped, dark epiproct sclerite. Sternum 9 extended backwards, posteromedially with an elliptical hammer. Paraprocts asymmetrical, subtriangular and slightly upcurved, mostly sclerotiZed except for the paler bases; left paraproct narrow, right paraproct expanded; apex blunt, subapically with several folds. Cerci hairy, 11-segmented, each segment with several long bristles.

Type material. Holotype male, No. CZT-PLE-MA5. This specimen is mostly preserved except for the apex of antennae. Wings are overlapped so only the forewings are visible.

Etymology. The specific epithet means “fat” in Latin, which refers to the enlarged abdomen of the new species.

Syninclusions. Plant remains and other unidentified insects.

Remarks. The general body color, hammer on sternum 9 and the enlarged paraprocts of L. crassus are typical for the genus Largusoperla. However, in L. crassus, the body, especially the abdomen is enlarged; wing venation is not identical between left and right wings, and the left and right paraprocts are asymmetrical. The asymmetry of paraprocts has also been reported in L. difformitatem (Chen 2018a). In L. difformitatem, the paraprocts appear less sclerotiZed, left paraproct is wider than the right one, and the apex is acute (see Figs. 6–8 in Chen 2018a). In L. crassus, the paraprocts are mostly sclerotiZed, the left paraproct is narrower than the right one, and the apex is blunt (Figs. 4, 5).

Typically in stoneflies adults, males are often more slender without an obviously enlarged abdomen, apparent with the male of L. crassus. The inconsistent wing venation between left and right wings have also been found in other Perlidae (Béthoux et al. 2011). Until further material of this enigmatic species becomes available for study, wing venation as a diagnostic character in this genus should be used with caution. Finally, the second discovery of asymmetrical paraprocts in Largusoperla suggests that the asymmetry of paraprocts is not aberrancy.