How do you separate Pseudonomoneura from Nemomydas? This guy was found in the same area as

and her terminalia

seem to match

But, maybe these aren't diagnostic?

That's a good question, Alice!

The short answer is: Nemomydas and Pseudonomoneura can be separated by:

  1) characters of the male terminalia (best discerned in ventral view);
  2) the relative length of the antennal club to the 1st flagellomere; and
  3) for some (but not all!) species...the length of the proboscis.

[For details on 1) and 2) above...see couplet 6) from the MND(1)...quoted in detail below.]

However, I've found no definitive criteria in the current literature for using female terminalia to separate Pseudonomoneura from Nemomydas. In both genera females have those distinctive "circlets of spines" (which are also found in many other asiloid genera...though not in subfamily Mydinae).

Hardy(1950) did state that the female genitalia show some differences in the development of the eighth segment...it being broader in comparison to the ninth segment in Nemomydas than in Pseudonomoneura. In particular, he found the eighth segment in Nemomydas to be about one and one-half times broader than the ninth, including the spines. But whether that character works for the many species described after 1950 is not clear, as subsequent references don't seem to mention it.
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Note that the circumscription of Nemomydas and Pseudonomoneura arose through a sequence of historical stages. Initially, new world species were placed within pre-existing old world genera (i.e. Leptomydas and Nomoneura)...but eventually the new world taxa were split off into newly erected genera (i.e. Nemomydas and Pseudonomoneura). What follows pertains to nearctic taxa.

In the more recent literature, greater emphasis has been put on characters of the male terminalia for separating the two genera (as well as most species within them). But earlier...in the period from 1934 when Curran erected the genus Nemomydas, until 1950 when Hardy published his paper on nearctic Nemomydas and "Nomoneura"...the length of the mouthparts was taken as the character separating the two genera.

Prior to 1934, the 9 nearctic species then known were all placed in the genus Leptomydas (which now consists solely of old world species). In 1934, Curran(2) separated out those 9 nearctic species from Leptomydas and divied them into two groups (see here):

1) Those with "Proboscis small, not extending beyond the oral opening" were assigned to his newly erected Nemomydas...consisting of species:
brachyrhynchus Osten Saken, 1886; desideratus Johnson, 1912; jonesii Johnson, 1926; pantherinus Gerstaecker, 1868; solitarius Johnson, 1926; tenuipes Loew, 1872; venosus Loew, 1866
2) Those with "Proboscis extending well beyond the oral opening", which were assigned to genus Nomoneura Bezzi, 1924...consisting of species:
concinnus Coquillett, 1905; hirta Coquillett, 1905   ← Note: concinnus is now synonymized under hirta
By 1950, there were 3 more nearctic taxa of Nemomydas (bifidus, intonsus and fumosus) and 3 more nearctic species of Nomoneura (micheneri, californica, and tinkhami), and 4 of those (bifidus, intonsus, fumosus, and tinkhami) didn't jibe well with Curran's previous "length of proboscis" character for separating the nearctic members of the two genera. In his 1950 paper, Hardy didn't provide an explicit key to separate the two genera, but in his generic descriptions he emphasized differences in genitalia of the males.

In 1961 Bequaert moved all nearctic Nomoneura to the new genus Pseudonomoneura, with Nomoneura becoming a strictly African genus.

The 1985 Manual of Nearctic Diptera(1) provided the following key break for separating the two genera (which constitute the two representatives of subfamily Leptmydinae north of Mexico):

6a) Antennal club shorter than or subequal in length to basal flagellomere (Fig. 9). Male sternite 9 divided apically; each arm with a single spur (Fig. 22). Length 9-20 mm.................Pseudonomoneura Bequaert
6b) Antennal club longer than basal flagellomere (Fig. 10). Male sternite 9 divided basally; each arm with two apical prongs (Fig. 21). Length 12-23 mm .....Nemomydas Curran

Then, in the 1990's a number of new nearctic species were described in both genera [see Kondratieff & Welsh(1990) and Fitzgerald & Kondratieff(1995 and 1997)]. At this point there were a lot more taxa than before 1950...and those with long & short mouthparts were sprinkled in each genus! So the newer circumscriptions tended to focus exclusively on male genitalia...even to the point where keys applied to males only!

So, to summarize..characters for female terminalia are not currently used to separate Pseudonomoneura and Nemomydas. The more recent generic treatments have focused on male terminalia, although diagrams are sometimes given for the female terminalia (which all have "circlets of spines"). But it almost seems that, formally, the most dependable way to ID a female (especially to species) is to capture it in copula with an identifiable male!

Since ventral characters of male genitalia are often not discernible in photos, the "antennal club length character" for separating Nemomydas and Pseudonomoneura given in the 1985 MND treatment (and also appearing in the 1990 work of Artigas & Papavero, and the 2010 Manual of Central American Diptera(3)) is often the most practical character when working with images on BugGuide. But it can be tricky to use, because it's often difficult to discern where the antennal club begins; and also where the base and apex of the 1st flagellomere are located.