Poulton (1906) describes the courtship behaviour from a location in Spain, where a male "flew nearer to the female. Although only three or four inches away, he did not walk but flew towards her, taking up a nearer position, in which he sometimes faced her from the side, sometimes from behind.
On one occasion he alighted only an inch behind the female. The only movements observed in the female after alighting were of the head, but the male often fluttered his wings." In contrast to this description no courtship behaviour was recorded during the present study, neither in Darmstadt nor in the laboratory.
This is not a contradiction because other studies show (Lavigne & Holland 1969, Geller-Grimm 1995) that some species are variable in their behaviour and can adapt according to the situation. The males approached the females directly on the ground or in the air after they identified them during their searching flights.

Mating was initiated after a short struggle. Sometimes the female escaped from the male's grasp and flew away. Otherwise the male curled his abdomen between his legs, his genital claspers grasped the female's ovipositor and union was accomplished. The male then relaxed his grip and fell back, the final mating position being end-to-end. The female then flew up to find a better place. She decided the direction of the flight, the male staying motionless and hanging downwards, sometimes beating his wings.
They usually landed on a stem or on grass (Fig. 4) and stayed motionless for a long period of time. If they were disturbed, they would fly away to another place or terminate the mating. The average time in copula was 55 min. (minimum: 25, maximum: 78 min.). Poulton (1906) describes a duration of only 6 min. Approximately 10 min. before mating finished, the female became somewhat restless and touched the male's abdomen with her hind legs.
Poulton (1906) also records cannibalism, but this behaviour was not observed during the present study. It was only obeserved in closed glass containers in the laboratory. It is surprising that cannibalism was not recorded because intraspecific contact, especially between different males or between the two sexes is frequent. They must have a well-developed sense for the distinguishing between prey and members of their own species in a short space of time. Robber flies have a keen optical sense and it may be significant that D. diadema is a sexually dimorphic species.
Lavigne & Holland (1969) analysed the function of wing sound for 7 species in Wyoming, but no observable reaction could be discerned in any of the tests (before mating). A meeting of two males of D. diadema in the air occurs without direct contact, so olfactory recognition is improbable.