Genus Helophilus Meigen (1822).

The genus Helophilus may be characterized as follows: Eyes bare and dichoptic in both sexes, though approximated in the male; face concave below antennae, but not dished, thence evenly convex; oral margin from the cheeks strongly descending (fig. 77); body not densely but rather inconspicuously haired; posterior femora thickened but not unusually so, and sometimes with a tooth below near the base; cell R₁ of wing open; vein R_{4 + 5} distinotly curved into cell R₅; cross-vein r-m beyond middle of cell 1st M₂. Species usually large and robust, including the largest of New Zealand Diptera; metallic in colour or blue and black with yellow stripes and spots.

Of the ten species already recorded from New Zealand, one has been herein placed in the genus Mallota; another, described by the writer in 1910 as new, is now found to be a synonym of one of Walker's species. Further, after a careful examination of Hutton's syntypes of vincinus, all of which are females, it is apparent that this species is a variety intermediate between the male and female of Schiner's antipodus. H. antipodus and H. trilineatus resemble each other closely in both sexes, and since only the male of antipodus and the female of trilineatus have until now been described—the former by Schiner and the latter by Fabricius—the males and females of both species have been grouped by Hutton and others under antipodus and trilineatus respectively. From the original descriptions by Schiner and Fabricius the sexes and species can, however, readily be separated, the main character being the presence in trilineatus (♂ and ♀) of an inferior tooth near the base of the posterior femora—“(femoribus) posticis unidentatis” (Fabr.)—the presence or absence of which was not noted by later authors.

• Fig. 43.—Melanostoma fasciatum: genitalia of male.

• Fig. 44.—Platycheirus lignudus n. sp.: antenna.

• Fig. 45.—P. lignudus n. sp.: anterior tibia and tarsus.

• Fig. 46.—P. lignudus n. sp.: diagram of abdomen of female.

• Fig. 47.—P. atkinsoni n. sp.: outline of head in profile.

• Fig. 48.—Xylota montana n. sp.: outline of head in profile.

• Fig. 49.—Platycheirus clarkei n. sp.: anterior tibia and tarsus.

• Fig. 50.—Tropidia bilineata: dorsal view of head of male.

• Fig. 51.—T. bilineata: dorsal view of head of female, showing colour-pattern.

• Fig. 52.—T. bilineata: outline of head in profile.

• Fig. 53.—Xylota montana n. sp.: genitalia of male.

• Fig. 54.—Tropidia bilineata: diagram of thoracio dorsum, showing colour-pattern.

• Fig. 55.—Xylota montana n. sp.: posterior femur.

• Fig. 56.—Tropidia bilineata: onychotarsus and appendages

• Fig. 57.—Mallota cingulata: dorsal view of head of female, showing colour-pattern.