DIXIDAE

Sinclair, Bradley J., Borkent, Art & Wood, D. Monty, 2007, The male genital tract and aedeagal components of the Diptera with a discussion of their phylogenetic significance, Zoological Journal of the Linnean Society 150 (4), pp. 711-742 : 730-731

DIXIDAE

Description ( Figs 5B, C , 6A , 8C, D )

Testis: Each testis is spindle-shaped and partially pigmented, tapered narrowly anteriorly, and nearly twothirds as long as the accessory gland complex. In freshly collected swarming males, the internal zones of the testis are apparent, corresponding to the growth and maturation of the spermatozoa.

Epididymis: Not differentiated.

Vas deferens: The vasa deferentia are long and slen- der, extended to the posterior margin of the united accessory gland complex, where the ducts join medially and extend anteriorly as twin ducts closely abutting each other along the midline of the glands. The ducts enter the accessory gland complex prior to the anterior margin.

Accessory gland and seminal vesicle: In wholemounted specimens of Dixidae , the accessory gland complex comprises three pairs of chambers, united along the midline. The anterior chamber is rounded, as observed in Austrothaumalea , with the vasa deferentia entering near the midlength of its corresponding side of this chamber. The middle chamber is nearly square in dorsoventral view. The third chamber appears cylindrical and narrow, with a small oval pocket at the anterior end, which appears narrowly separated from the remaining chamber. The anterior two chambers are thick walled.

Ejaculatory duct: The accessory gland complex empties posteriorly into short, paired ejaculatory ducts, which fuse medially just prior to the external genitalia, forming a large atrium. The ducts are strongly sinuous at the base of the accessory gland complex, curving sharply ventrally and then dorsally prior to entering the genitalia.

Ejaculatory apodeme, sperm pump, and aedeagus: The ejaculatory apodeme is absent and the ventral plate (= prosophallus) is present, lying between the gonocoxites ( Fig. 8C, D ). The aedeagus is membranous (see the discussion of its homology in ‘Remarks’). The parameres are complex structures, often with long slender aedeagal guides.

Remarks: The Dixidae are considered to represent the sister group to the remaining Culicoidea, but despite its important phylogenetic position, the male genital tract has not been previously described and illustrated. Its configuration (based on four species) is extremely similar to that observed in the Chironomoidea and very unlike that of the remaining Culicoidea. Given the structure of the accessory gland complex, the formation of a preformed spermatophore is predicted in Dixidae .

As suggested by Belkin (1968), the ‘aedeagus’ of Dixidae ( Fig. 8C, D ) and Culicidae is probably a secondary subdivision of the paramere (= phallus sensu Belkin, 1968), and hence a sclerotized aedeagus is considered absent in all Culicomorpha ( Sinclair, 2000) . Downes (1968) viewed the slender ‘aedeagal’ filaments in southern Chilean Dixidae ( Edwards, 1930: fig. 13) as representing a rigid-walled duct, but this was not observed in specimens examined in this study. These long filaments are probably homologous to the elongate structure labelled ‘aedeagus’ and ventral plate (= prosophallus) of Dixella californica (Johannsen) by Wood & Borkent (1982: fig. 9) and Wood (1991: fig. 11). This long, filamentous structure in this species is not tubular, but merely a slender plate that possibly functions as an aedeagal guide. These slender processes are interpreted here as parameral in origin.

The ventral plate in Ceratopogonidae is used in the alignment of the spermatophore ( Linley & Adams, 1971), where the tip of this plate (‘aedeagus’) is applied to the dorsal wall of the female spermathecal duct. The long filamentous paramere and the ventral plate (prosophallus) in some dixids could function in a similar manner, but also inserted into the correspondingly long female duct to ensure the passage of spermatozoa. The spermatophores of these species possibly possess a long neck, similar to that of Culicoides melleus (Coquillett) (see Linley & Adams, 1971: fig. 1) and the filamentous paramere and the ventral plate act as guides for correct insertion and positioning of the spermatophore.

Figures

Figure 5. Male internal reproductive organs (dorsal view). A, Trichocera sp. (Trichoceridae). B, Dixa yamatoma (Dixidae). C, Dixa yamatoma (Dixidae). D, Austrothaumalea zentae (Thaumaleidae). E, Austrosimulium furiosum group (Simuliidae). F, Chironomus plumosus (Chironomidae). acc gl cplx, accessory gland complex; ej dt, ejaculatory duct; tes, testis; vas df, vas deferens. Scale bars = 0.1 mm.

Figure 6. Male internal reproductive organs (dorsal view). A, Dixa yamatoma (Dixidae) (based on the specimen photographed in Fig. 5B). B, Corethrella sp. (Corethrellidae) [modified from McKeever (1985)]. C, Anopheles quadrimaculatus (Culicidae) [modified from Hodapp & Jones (1961)]. D, Ptychoptera nr japonica (Ptychopteridae) (scale bar = 0.1 mm). E, Ptychoptera lacustris (Ptychopteridae) [modified from Just (1973)]. acc gl, accessory gland; acc gl cplx, accessory gland complex; ej dt, ejaculatory duct; sem ves, seminal vesicle; tes, testis; vas df, vas deferens.

Figure 8. Male external terminalia. A, Androprosopa americana (Thaumaleidae) (posterior view). B, Androprosopa americana (Thaumaleidae) (posterior view). C, Dixella nicholsoni (Dixidae) (lateral view). D, Dixella nicholsoni (Dixidae) (posterior view; gonostyli removed). aed, aedeagus; cerc, cercus; gst, gonostylus; hyprct, hypoproct; pm, paramere; v pl, ventral plate. Scale bars = 0.1 mm, except in B scale bar = 0.05 mm.