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I asked Verner Michelsen who sent me this identification key:

Males
1 Vein C with a few v-setulae immediately distal to insertion of vein R1. Fore tibia with four apical setae in d, pd, p and pv position. Longest pv-setae of hind femur slightly exceeding depth of femur Alliopsis silvestris
– Vein C with scattered v-setulae reaching far distal to insertion of vein R1. Fore tibia with three apical setae in d, p and pv position. Longest pv-setae of hind femur shorter than depth of femur 2
2 Longest aristal pubescense distinctly longer than basal diameter of arista. Male: Frons at narrowest point not exceeding width of ocellar tubercle Alliopsis silvatica
– Longest aristal pubescense equal to basal diameter of arista. Male: Frons at narrowest point wider than ocellar tubercle Alliopsis maculifrons
Females
1 Frons at narrowest point less than 1.5 times as broad as ocellar tubercle, with 0–1 pair of orbital setae. Fore tibia with 4 apical setae in d-, pd-, p- and pv-positions. Vein C with v-setulae barely extended distal to insertion of vein R1 Alliopsis silvestris
– Frons at narrowest point at least 2 times as broad as ocellar tubercle, with 2 pairs of orbital setae. Fore tibia with only 3 apical setae, seta in pd-position absent. Vein C with v-setulae extended far beyond insertion of vein R1 2
2 Aristal pubescence distinctly longer than greatest basal diameter of arista. Fore tarsomeres 2–3 moderately expanded Alliopsis silvatica
– Aristal pubescence no longer than greatest basal diameter of arista. Fore tarsomeres 2–3 strongly expanded Alliopsis maculifrons