Note that this is a working key, not a final product. Don’t worry about couplet numbers, which I made high to allow for insertion of further species.

With this key, you should be able to identify all described species, including those in Brown (2004) and Gonzalez & Brown (2004). Figure numbers in the key refer to Brown (2004), unless they state, for example “GB Fig. 1”, which refers to the Gonzalez & Brown paper. PDF files of both papers are available on this site. Please let me know <bbrown@nhm.org> of any mistakes or exceptions that you note in this key.  

1.	Ovipositor with dorsomedial carina (Figs. 31-33)...........................2
-	Ovipositor lacking dorsomedial carina......................................4
2.	Thorax and abdomen almost completely yellow, except for dark posterior
		markings on tergites (Fig. 24)........................................3
S	Thorax and abdomen mostly dark in color (Fig. 23)...........................
		.................................................. M. brevicarina Brown
3.	Ovipositor with numerous large setae (Fig. 30)...........M. premordica Brown
S	Ovipositor nearly bare (Fig. 29)...........................M. carinata Brown
4.	Lateroventral one-half of abdominal tergites with orange-yellow coloring
		(Fig. 25).............................................................5
S	At least abdominal tergites 1-5 dark-brown to black laterally (Fig. 22).....
		......................................................................7
5.	Ovipositor broad in dorsal view, with lateral emargination (Figs. 56-57);
		tergite 6 mostly orange dorsally......................................6
-	Ovipositor narrow in dorsal view (Fig. 34), without lateral emargination;
		tergite 6 mostly dark brown dorsally.................M. flavilata Brown
6.	Ovipositor with short, peg-like process at dorsal apex (Figs. 57, 62).......
		.......................................................M. paxilla Brown
S	Ovipositor without process, somewhat concave at apex (Figs. 56, 61).........
		........................................................M. aprica Brown
7.	Dorsal interfrontal setae present near ventral ocellus (Figs. 26, 27)......8
S	Dorsal interfrontal setae absent.........................................140
8.	Frons mostly dark brown to black...........................................9
S	Frons mostly orange to yellow, although setal bases might be black (as in
		Fig. 27).............................................................14
9.	Ovipositor laterally with subapical curved hooks (GB Figs. 4, 11)...........
		...........................................M. ancistra Gonzalez & Brown
S	Ovipositor without such hooks.............................................9A
9A.	Ovipositor with three posterior projections: rounded central projection, and
		two longer, lateral projections (Fig. 69).......M. deinocerca Borgmeier
S		Ovipositor with single posterior projection..........................10
10.	Ovipositor sinuous in lateral view, with long ventral setae basally (Fig.
		94)....................................................M. sinuosa Brown
S	Ovipositor smoothly curved ventrally, ventral setae extremely short.......11
11.	Tarsomeres 4 and 5 of foreleg dark brown, in contrast to rest of yellowish-
		brown leg; frons relatively narrow...................................12
-	Foreleg entirely yellowish-brown; frons various...........................13
12.	Only tarsomeres 4 and 5 of fore leg dark brown; ovipositor in lateral view
		deeper (Fig. 51)......................................M. individa Brown
S	Tarsomeres 3-5 of fore leg dark brown; ovipositor not as deep (Fig. 52).....
		.....................................................M. rhypopoda Brown
13.	Ovipositor with pair of enlarged apical setae (Figs. 5, 12).................
		.........