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).................
		.........................................M. atribiseta Gonzalez & Brown
S	Ovipositor lacking enlarged apical setae.................................13A
13A.	Ovipositor laterally flattened and deep apically (Fig. 84)..................
		...............................................M. nigricorpus Borgmeier
S	Ovipositor more rounded, narrow at apex (Fig. 63).........M. anaticula Brown
14.	Bases of some frontal setae dark brown to black, at least in dorsal fronto-
		orbital pair (as in Fig. 27).........................................15
-	Without black setal bases on frons........................................18
15.	Ovipositor laterally or posteriorly with prominent setae (Figs. 67, 79,
		90)..................................................................16
-	Ovipositor without prominent setae (Fig. 49)......M. spatula Brown (in part)
16.	Longest setae of ovipositor are apical pair (Fig. 67).......................
		..............................................M. biseta Brown (in part)
S	Longest setae of ovipositor are lateral (Figs. 79, 90)....................17
17.	Apex of ovipositor in dorsal view broadly rounded; lateral setae of oviposi-
		tor relatively thick (Fig. 79); palpus with only one apical, black
		setula as well as several yellow setulae; frons often with dark spots
		on middle and lower row of setae as well as upper row..................
		...................................................M. maculifrons Brown
-	Apex of ovipositor narrowed in dorsal view; lateral setae relatively thin
		(Fig. 90); palpus with more than one black setula; black spots present
		only on upper row......................................M. villosa Brown
18.	R2+3 present, at least as basal remnant...................................19
S	R2+3 completely absent....................................................22
19.	R2+3 complete; venter of ovipositor consists of pair of large, ventrally
		directed hooks (Figs. 46-47); ovipositor with thick, long ventral setae
		.....................................................................20
S	R2+3 basally present; venter of ovipositor without ventral hooks or large,
		thick, setae (Fig. 66).................................M. basella Brown
20.	Ventral setae organized into clumped groups; apex of dorsal lobe sharply
		curved ventrally (Fig. 47)..............................M. hamata Brown
S	Ventral setae more scattered; apex of dorsal lobe gradually curved ventrally
		(Fig. 46).........................................M. mexicana Borgmeier
22.	Ovipositor extremely deep, laterally compressed, shiny (Figs. 68, 72).......
		................................................M. compressicauda Brown
-	Ovipositor more tubular in shape..........................................23
23.	Abdominal segment 6 with large, yellow lobe; ovipositor densely setose
		(Figs. 78, 85)..........................................M. lobata Brown
S	Abdominal segment 6 without large lobe; ovipositor various................24
24.	Ovipositor, in lateral view only slightly curved or curved only at apex
		(Figs. 45, 53, 54, 55, 65, 81, 82, 86, 92 93, 95). Note: if there is
		some question about the curvature of an ovipositor, try both leads from
		this couplet.........................................................25
-	Ovipositor strongly and evenly curved in lateral view (Figs. 74, 75, 87, 97)
		.....................................................................37
25.	Venter of ovipositor with long, black, well-developed setae from near base
		to near apex (Fig. 93); apex of ovipositor without larger setae (Fig.
		88)..............................................M. simillima Borgmeier
-	Venter of ovipositor with at most fine, long or short setulae, or setae
		restricted to base or apical one-third; some species with larger setae
		at apex of ovipositor................................................26
26.	Ovipositor in dorsal view with long, thick setae laterally (Figs. 44, 77)...
		.....................................................................27
-	Ovipositor in dorsal view with only minute or fine setae on lateral margins
		.....................................................................28
27.	Ovipositor with lateral setae strong along entire length, only slightly and
		gradually smaller near apex (Fig. 77)...........M. hirticauda Borgmeier
S	Ovipositor with thicker setae near base and thinner, sparser setae near apex
		(Fig. 44)..............................................M. spicula Brown
28.	Apical tarsomere of foreleg dark brown, in contrast to other, yellowish-
		brown tarsomeres (note that, in air-dried specimens, yellowish tarso-
		meres can sometimes appear darkened).................................29
-	Apical tarsomere of foreleg yellowish-brown, like other tarsomeres........30
29.	Ovipositor in dorsal view expanded posterior to mid-point (Fig. 49).........
		.............................................M. spatula Brown (in part)
S	Ovipositor in dorsal view parallel-sided (Fig. 50)..........M. vargasi Brown
30.	Apex of ovipositor with pair of hornlike processes (GB Figs. 6, 13).........
		........................................M. corniculata Gonzalez & Brown
S	Apex of ovipositor without pair of hornlike processes....................30A
30A.	In lateral view, ovipositor with small, ventral, preapical swelling, and
		apical point, making apex somewhat triangular (Fig. 95)................
		..................................................M. triangularis Brown
S	In lateral view, ovipositor not so pointed................................32
32.	Ovipositor laterally compressed at apex, so that apex is deeper than broad
		(Fig. 86)..............................................M. parkeri Brown
S	Ovipositor not laterally compressed at apex...............................33
33.	Ovipositor strongly dorsoventrally flattened and pointed at apex, slightly
		downturned; without apically enlarged setae (Fig. 48)..................
		.....................................................M. horologia Brown
S	Ovipositor, if dorsoventrally flattened, only slightly so, not pointed at
		apex; other characters various.......................................34
34.	Apex of ovipositor, in dorsal view, narrowed to point (Figs. 59, 76)......35
-	Apex of ovipositor, in dorsal view, truncate, broad (Figs. 67, 80)........36
35.	Apex of ovipositor with larger, thicker, pairs of setae dorsoapically and
		subapicolaterally (Fig. 76)............................M. hansoni Brown
-	Apex of ovipositor without larger, thicker setae (Fig. 59)..................
		.......................................................M. apicula Brown
36.	Apex of ovipositor without large setae; ovipositor narrowed through mid-
		length (GB Fig. 9).........................M. stenotes Gonzalez & Brown
S	Apex of ovipositor with large setae at apex; not narrowed at midlength...36A
36A.	Dorsum of apex of ovipositor with two posterolateral pairs of longer, curved
		setae (Fig. 80)...................................M. piliapex Borgmeier
S	Dorsum of apex of ovipositor with single pair of relatively straight
		posterolateral seta (Fig. 67).................M. biseta Brown (in part)
37.	Apex of ovipositor drawn out into elongate, narrow, dorsally curved tip
		(GB Figs. 8, 15).......................M. schiaffinoae Gonzalez & Brown
S	Apex of ovipositor broader, not curved dorsally..........................37A
37A.	Ovipositor in lateral view with extremely short setae, thus appearing almost
		bare (Fig. 87, 97)...................................................38
S	Ovipositor with longer, thin setae (Fig. 74, 75)..........................39
38.	Ovipositor broad at base; slightly dorsoventrally flattened (Figs. 91, 97)..
		.........................................................M. woodi Brown
S	Ovipositor narrow at base; tubular in shape (Fig. 87)....M. ronnai Borgmeier
39.	Ovipositor laterally emarginate (GB Fig. 10).........M. altobenia n. sp. 135
S	Ovipositor smooth-sided laterally........................................39B
39B.	Ovipositor in dorsal view narrowed abruptly near apex (GB Fig. 7); oviposi-
		tor slightly dorsoventrally flattened..................................
		.........................................M. sarmientoi Gonzalez & Brown
S	Ovipositor in dorsal view evenly narrowed; ovipositor slightly laterally
		flattened...........................................................39C
39C.	Ovipositor short, truncate apically in dorsal view (Fig. 70)................
		........................................................M. exigua Brown
S	Ovipositor longer, narrower in dorsal view (Fig. 71).......................
		.............................................M. falcata Brown (in part)
140.	Ovipositor with dorsal concavity that is either broad (Figs. 35, 39) or
		narrow and confined to apex of ovipositor...........................142
S	Ovipositor convex dorsally...............................................148
142.	Ovipositor broad, concavity shallow and across entire ovipositor (Figs. 35,
		39).......................................................M. trua Brown
S	Ovipositor narrowed near midlength in dorsal view (GB Figs. 1, 2); concavity
		located near apex of ovipositor.....................................144
144.	Exoskeleton of concavity of ovipositor black, shiny, thick (GB Fig. 2)......
		.........................................M. cucharella Gonzalez & Brown
S	Exoskeleton of concavity of ovipositor whitish, opaque, thin (GB Fig. 1)....
		...............................M. concavella Gonzalez & Brown [Bolivia]
148.	Ovipositor relatively straight (Fig. 64)...............M. angustifrons Brown
S	Ovipositor ventrally curved (Figs. 40-43)................................152
152.	In lateral view, ovipositor not deeper apically; ovipositor with long
		ventral setae (Fig. 75)......................M. falcata Brown (in part)
S	In lateral view, ovipositor deeper apically (Figs. 40, 41, 43), or if not
		greatly deepened (Fig. 42), then ovipositor ventrally without long
		setae...............................................................154
154.	Ovipositor basally deep (Fig. 43)...........................M. valeria Brown
S	Ovipositor shallow basally, deepening apically (Figs. 40-42).............156
156.	Ovipositor abruptly deeper at apex, broadly truncate (Fig. 40)..............
		..................................................M. colossia Borgmeier
-	Ovipositor gradually deepening, apically pointed (Figs. 41-42)...........158
158.	Ovipositor with prominent longitudinal striations; ovipositor apically
		deeper; lateral setae of ovipositor sparse, thin, and short (Figs. 36,
		41)....................................................M. rhampha Brown
S	Ovipositor without distinct striations; ovipositor apically only slightly
		deepened; lateral setae of ovipositor denser, thicker, and longer (Fig.
		42)...................................................M. rostrata Brown