Bessierus Thomas &Orth

Domínguez, Eduardo, Cuezzo, María Gabriela & Clavier, Simón, 2019, The phylogenetic relationships of the two-winged South American Leptophlebiidae genera revisited with first description of the male imago of Bessierus Thomas & Orth (Insecta: Ephemeroptera), Zootaxa 4674 (3), pp. 375-385 : 377-379

Genus Bessierus Thomas &Orth

Thomas & Orth, in Thomas et al., 2000: 49 (nymph). (Type-species: Bessierus doloris Thomas & Orth , original designation). Domínguez, Molineri, Pescador, Hubbard & Nieto, 2006: 356; Domínguez, 2009: 70; Nascimento, Barcelos-Silva & Salles, 2011: 191.

Diagnosis: The male imago of Bessierus can be separated from all other genera of Leptophlebiidae by the following combination of characters: (1) fork of vein MA of forewing asymmetrical ( Fig. 2A ); (2) vein ICu1 attached to vein CuA by a cross vein, and vein ICu2 attached at base to vein ICu1 by a cross vein; (3) hind wings absent; (4) tarsal claws of a pair dissimilar, one apically hooked, the other blunt ( Fig. 2B ); (5) forceps sockets fused ( Figs 2 C-D); penes widely divided in apical 2/3, apex of each penis lobe with a laterally orientated broad projection ( Figs 2 C-D); (7) forceps segment 1 smoothly roundly, broadened in basal 1/3 ( Fig.2C ).

Description of the male imago (in alcohol). Length of body 2.9 – 3.0 mm, forewing length 3.0- 3.1 mm. Head ( Figs 3 A-B): Eyes meet on meson of head ( Fig. 3A ), lower portion of eyes approximately 4/5 length of upper portion ( Fig. 3B ).

Wings ( Fig. 1A ). Maximum width of forewing is 1/3 of maximum length of forewing; vein Rs of forewing forked 1/5 of distance from base to margin; vein MA forked a little more 1/2 of distance from base to margin, fork asymmetrical, distal portion of vein MA not sagged posteriorly; vein MP forked 3/8 of distance from base to margin, fork symmetrical, distal portion of vein not sagged posteriorly; vein ICu1 attached to vein CuA by a cross vein, and vein ICu2 attached at base to vein ICu1 by a cross vein. Hind wings absent.

Legs. Ratio of segments of male forelegs, 0.42:1.0 (0.96 mm): 0.06:0.23:0.19:0.08: 0.05. Claws on each leg dissimilar, with one apically hooked and one blunt, pad-like ( Fig. 2B ). Male genitalia ( Figs 2 C-D): segment II of forceps subequal in length to segment III, segment II of forceps 1/6 length of segment I; forceps segment I smoothly roundly broadened in basal 1/3 ( Fig. 2C ); forceps sockets fused, forming a single cavity ( Fig. 2D ); length of styliger plate along median line 1/3 maximum width, posterolateral corners slightly developed; penes widely divided in apical 2/3, apex of each penis lobe with a laterally orientated broad projection ( Figs 2 C-D).

Comments. Among the diagnostic characteristics of Bessierus (also shared with Perissophlebiodes ), the fused forceps sockets are very unusual. Until recently this has been considered an exclusive characteristic of Thraulodes Ulmer. Mariano (2010) also described this character for two species of Simothraulopsis Demoulin from Brazil. More recently, Nascimento et al. (2011), in their revision of this genus, stated the fusion of the forceps socket as a diagnostic character. According to Domínguez (2009), Thraulodes is not closely related neither to Bessierus nor Perissophlebiodes . Simothraulopsis , on the other hand, together with Homothraulus Demoulin , was the sister group of Perissophlebiode s and Bessierus . In that analysis, the adult of Bessierus and Perissophlebiodes remained unknown, hence it was impossible to know the state of this character.

Figures

FIGURE 1. Map of distribution showing collecting sites in French Guiana and present distribution of Bessierus doloris. Round- ed sites were plotted according the indications of Thomas et al., 2000.

FIGURE 2. Bessierus doloris, male imago. A, fore wing; B, tarsal claws; C, genitalia, ventral view; D, genitalia, ventral view, forceps socket, detail.

FIGURE 3. Bessierus doloris, male imago. A, head, d.v.; B, head, l.v.; C, habitus, d.v.; D, habitus, l.v. d.v.= dorsal view; l.v.=lateral view.