1	Palpus yellow (apex infuscated in A.apfelbecki). Face white to light brown, rarely brown. First flagellomere usually light yellow, sometimes with outer surface lightly infuscated	2
–	Palpus brown or darker. Face light to dark brown. First flagellomere orange to dark brown	8
2	At least 6 dorsocentral setae and 5 fronto-orbital setae. Wing length at least 4.0 mm. Epistoma large, distinct	A.apfelbecki Strobl
–	No more than 4 large dorsocentrals and four fronto-orbital setae. Wing length 3.4 mm or less. Epistoma narrow	3
3	Body, excluding most of head and legs, yellow. Thorax shiny. Mid tibia with 2 or 3 posteromedial setae. First flagellomere with apical patch of slightly longer, paler hairs. Wing length 2.7–3.4 mm. Eye 6.1–9.7 × higher than gena. Surstylus indistinct, appearing as broadly rounded ventral margin of epandrium; with tubercle-like setae (Fig. 180). Cercus only with unmodified setae. Distiphallus terminating in a large, dark bulb (Figs 181, 182). Basiphallus consisting of 1 pair of lateral bands	A.diversa Johnson
–	Body, excluding most of head, dark brown. Thorax densely to lightly pruinose. Mid tibia without posteromedial setae. First flagellomere with all hairs similar. Wing length usually 1.7–2.4 mm; 3.4 mm in A.deserta. Eye 1.6–3.9 × higher than gena. Surstylus ending in triangular point; without tubercle-like setae. Cercus with tubercle-like setae along inner face. Distiphallus a flat, curved sclerite. Basiphallus consisting of a single band	4
4	Thorax with dense grey pruinosity. Calypter margin and hairs white. Legs light yellow. Parafacial visible laterally, continuing under eye as distinct cheek. Ca. 8 rows of acrostichal setulae. Eye 1.6–2.7 × higher than gena. Length of ultimate section of vein M4 divided by penultimate section: 0.6–0.9. Mesophallus several ×longer than distiphallus (males unknown for A.pallidiseta). Ejaculatory apodeme minute and subcylindrical	5
–	Thorax with light dusting of pruinosity. Calypter margin and hairs brown. Legs dark brown with tarsi and knees paler. Parafacial not visible laterally; cheek not strongly delimited. Four rows of acrostichal setulae. Eye 3.5–3.9 × higher than gena. Ultimate and penultimate sections of vein M4 of equal length. Mesophallus shorter than distiphallus. Ejaculatory apodeme large and fan-shaped	7
5	Setae yellow. Ocellar seta 1/2 length of postocellar	A.pallidiseta Malloch
–	Setae dark brown. Ocellar seta as long, or nearly as long as postocellar	6
6	Wing length 2.0–2.4 mm. Legs yellow with mid and hind coxae at least partially brown (Fig. 37). Presutural dorsocentral not much longer than surrounding setulae. Distiphallus straight (Figs 165, 166)	A.aristata Malloch
–	Wing length 3.4 mm. Legs brown with apex of femora and base of tibiae yellow. Presutural dorsocentral distinct. Distiphallus curved	A.deserta (Patton)
7	First flagellomere entirely light yellow, or with dark dorsobasal spot. Mid and hind tibiae dark brown with base yellow, and fore tibia yellow with centre brownish. Male cercus thin along length (Fig. 237)	A.varifrons Coquillett
–	First flagellomere yellow with outer face lightly infuscated on distal 2/3. Tibiae dark brown with only base of fore tibia yellowish. Male cercus broad and V-shaped (Figs 186, 187)	A.fission Eiseman & Lonsdale
8	Five or 6 fronto-orbital setae, with only one ors (some A.virginiensis with posterior ori appearing as ors). Fronto-orbital plate and parafacial not projecting. Centre of frons always brownish orange. Distiphallus long, thin, tubular, and bifid	9
–	Four fronto-orbital setae on one or both sides of frons, with two ors, but if 5 fronto-orbitals present (A.aprilina), fronto-orbital plate and parafacial projecting. Centre of frons usually dark brown (pale in A.aprilina). Distiphallus undivided	10
9	At least apical 1/2 of first flagellomere brown. Anterior margin of buccal cavity sharply angulate. Distiphallus strongly curved, forming a semicircle (Figs 147, 148)	A.ambrosivora Spencer
–	First flagellomere entirely orange, with dark spot around base of arista, or with distal 1/2 darker. Anterior margin of buccal cavity straight or slightly rounded, at most with midpoint slightly produced. Distiphallus nearly straight (Fig. 151)	A.virginiensis Spencer
10	First flagellomere black; elongate to slightly enlarged and with anterodorsal angle. Body dark brown to black. Calypter margin and hairs white. Distiphallus relatively short	11
–	First flagellomere dark brown or paler; circular to elongate oval, with apex broadly rounded; if flagellomere black and with anterodorsal point (some A.kincaidi), then segment not elongate, body paler, calypter margin and hairs usually yellow to brown, and distiphallus large and clavate	12
11	Costa extending just past vein R4+5. Five fronto-orbital setae. Sides of ocellar triangle straight. Anterodistal margin of epandrium deeply incised before surstylus (Figs 157, 158). Surstylus acutely triangular and directed anteriorly. Distiphallus tapered to apex, which is directed distally (Fig. 160)	A.aprilina Malloch
–	Costa extending to vein M1+2. Four fronto-orbital setae (some Canadian material with five). Sides of ocellar triangle slightly concave. Epandrium without deep emargination as above. Surstylus relatively small and lobate (Fig. 143). Distiphallus parallel-sided with apical 1/2 slightly angled dorsally (Fig. 146)	A.albipennis Meigen
12	Four strong dorsocentral setae, presutural seta nearly as long as preceding seta and nearly as long as acrostichal seta. Thorax covered with brownish grey pruinosity, strongly contrasting brown to golden-brown setae. Mid tibia with 1 posteromedial seta. Basiphallus consisting of a single sclerite. Distiphallus and separate mesophallus simple and plate-like	13
–	Three to 6 dorsocentral setae, strongly decreasing in height anteriorly (anterior setae often barely larger than surrounding setulae). Thorax usually shiny, but sometimes with light pruinose dusting. Setae usually dark brown, but sometimes brown or (rarely) yellow. Mid tibia with two posteromedial setae. Basiphallus consisting of 2 sclerites. Distiphallus (including fused mesophallus) with transverse dorsobasal band; bulb-like or long and bifid	14
13	Calypter hairs brown. Antenna entirely dark brown. Mid and hind tibiae brown with apices yellow (base sometimes also yellow). Distiphallus longer than wide, flat, and curved; short, cylindrical with longer ventral plate (Figs 248, 249)	A.vockerothi Spencer
–	Calypter hairs usually white (only brown in holotype). Base of first flagellomere paler in colour, usually distinctly orange. Mid and hind tibiae yellow, sometimes with hind tibia brownish medially. Distiphallus very short, mesophallus dense and thick apically, and with long, thin, flat “tail” (Figs 191, 192)	A.isolata Malloch
14	First flagellomere usually at least 1.3 × longer than pedicel (larger in males); shape circular (most males) or elongate oval (some males and most females); with elongate pale hairs present and restricted to distal margin in females, but often extending to basal 1/2 or basal 1/3 of segment in males (Figs 41, 42); long-haired section without sharply delimited boundary, and with hairs relatively more sparse than in “tufted” state described below. Only males can be identified further	15
–	First flagellomere not much longer than pedicel, with shape ovate; longer hairs, if present, restricted to discrete, sharply delimited apical tuft that may form a minute spot (Figs 39, 40) or encompass the distal 1/3 of the segment	17
15	Surstylus with 2 tubercle-like setae of variable size; small and lobate, directed anteriorly and barely visible laterally (Figs 170–173)	A.bispinata Spencer
–	Surstylus with 3 or more tubercle-like setae, rarely 2; subtriangular in outline and distinct laterally	16
16	Surstylus with 3–5 tubercle-like setae (rarely 2) (Figs 226–228). Margin of surstylus, seen laterally, bulging at base of tubercles	A.tacita Spencer
–	Surstylus with numerous tubercle-like setae (Figs 232, 233). Margin of surstylus, seen laterally, relatively straight	A.echinalis sp. nov.
17	If present, pale tuft of hairs on first flagellomere encompassing entire distal 1/3 of segment, or nearly as high as segment and teardrop-shaped. Scutum with light pruinosity. Four or 5 dorsocentrals, often with 1 presutural. Distiphallus cup-shaped with central tuft of hairs (strongly enlarged in A.canadensis)	18
–	If present, pale tuft of apical hairs on first flagellomere circular to ovate in outline, and ranging in size from 1/2 height of first flagellomere to only as wide as base of arista. Scutum usually shiny to subshiny. Number of dorsocentrals variable, but often with only 2 well-developed setae. Distiphallus capsule-like and usually bell-shaped in outline	19
18	Three postsutural dorsocentral setae and one sutural to presutural dorsocentral (European specimens with 1 or 2 additional dorsocentrals). Buccal cavity curving onto face. Parafacial entirely visible laterally. Frons orange-brown with fronto-orbital plate dark brown. Distiphallus as long as mesophallus, and with distal 1/2 flared (Figs 141, 142). Mesophallus with dorsal collar	A.abiens Zetterstedt
–	Four postsutural dorsocentral setae and sometimes 1 sutural to presutural dorsocentral (Fig. 10). Margin between face and buccal cavity abruptly angled. Parafacial barely visible laterally (Fig. 8). Sides and centre of frons the same colour. Distiphallus much enlarged, ring-shaped (Figs 177, 178). Mesophallus without collar	A.canadensis Malloch
19	Costa extending to vein R4+5. Wing length 3.6–4.3 mm. Distiphallus longer than basiphallus and dark (Figs 196, 197)	A.kincaidi Malloch
–	Costa extending to vein M1+2. Wing length < 2.6 mm. Distiphallus shorter than basiphallus and usually pale	20
20	Five dorsocentral setae, with presutural dorsocentral as long as preceding setae, and nearly as long as acrostichal seta. Calypter hairs white to light brown in male (female unknown). Surstylus with four spines (Fig. 222). Distiphallus broad, short, and very dark (Figs 223, 224). Dark, sclerotised hypophallus present	A.soka Eiseman & Lonsdale
–	Only 2 or 3 well-developed postsutural dorsocentrals, with third seta (if present) usually much shorter than acrostichal. Calypter hairs sometimes brown in male, usually white in female. Surstylus with > 4 spines. Distiphallus pale and bell-shaped in ventral view. Hypophallus absent	21
21	Frons and most of gena orange-brown, or at least distinctly paler than scutum; fronto-orbital plate and posterior region of frons variably dark brown (frons appearing entirely dark in poorly preserved specimens). Lunule light brown to yellow. Frons buckled anteriorly, with anterior region visible in lateral view. Distiphallus bulging subapically	22
–	Frons, gena and lunule entirely dark brown, with frons evenly curved to meet face. Sides of distiphallus relatively straight or concave subapically	23
22	Fronto-orbital plate always dark brown to level of anterior or posterior ors, and sometimes dark along most of lateral margin. Distiphallus strongly swollen distally, almost circular in shape (viewed ventrally) (Fig. 214)	A.parvicornis Loew
–	Fronto-orbital plate usually brownish orange around base of all fronto-orbital setae, but sometimes as above. Distiphallus only gradually swollen subapically, with distal section only slightly wider than basal section (viewed ventrally) (Fig. 215)	A.proxima Spencer
23	Antenna dark brown with base of first flagellomere sometimes orange. Basiphallus produced distolaterally (i.e., apex split); without membranous lateromedial lobes (Figs 219, 220). Distiphallus narrowest medially	A.pudica Spencer
–	Antenna dark brown, but if first flagellomere paler, then entirely concolourous on outer face. Basiphallus not produced distolaterally; with 1 pair of membranous lateromedial lobes. Distiphallus narrowest apically	24
24	Apical tuft of pale hairs present on apex of male first flagellomere. Calypter hairs white to brown. Mesophallus with 1 pair of small, lateral membranous lobes at midpoint. Distiphallus relatively pale, not angled dorsally and with apical section (seen ventrally) irregular in outline (Figs 203, 204)	A.parca Spencer
–	Apical tuft of hairs absent from male first flagellomere. Calypter hairs dark brown. Mesophallus produced laterally into 1 pair of large, triangular, basally sclerotised wings. Distiphallus relatively dark, slightly angled dorsally and with distal section (seen ventrally) evenly tapered (Figs 208, 209)	A.parilis Spencer