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Hi

I don't know leucoptera because it occurs outside the area of the Central European key but crassipennis and oblonga can be split using this key thus:

1. Haustellum long (3.5 - 5.5x as long as its diameter). Females: sternite 7 bent forwards (fig. 228). Males (typical specimens): abdomen not or hardly longer than wide, colouring yellow with a black central longitudinal stripe, which is at least as wide as 1/5 of the abdomen (fig. 171); sternite 3 at least as wide as the ventral membrane bordering it; thorax before the scutellum often blackish, but ? densely dusted; abdomen occasionally completely black......crassipennis F.

- Haustellum shorter (2.8 - 5.2x as long as its diameter). Females: sternite 7 bent backwards a little at the end (fig. 227). Males (typical specimens): abdomen clearly longer than wide, colouring red with a black central longitudinal stripe (maximum 1/8 of abdomen width), which is sometimes missing completely; sternite 3 narrower than the ventral membrane (fig. 183); thorax before the scutellum shiny black, at this spot seldom with a little dusting; gynaecomorphous specimens with evenly dense abdominal dusting seem to occur almost only in this species; darkening of the abdomen is rare.......oblonga R.D.

Hope that helps :)