On the cracking surface of a rotting opuntia branch, a large male cactus fly (Odontoloxozus longicornis
) chases away a small male that has just dismounted a female. The large male guards the female by standing over her while she is feeding in crevices on the cactus surface. The large male then goes on to feed, while the female begins to oviposit into a crevice. Undetected by the large male, the small male approaches the female again and mates with her. When other males approach the mating pair, the large male defends the female, who has started ovipositing again while the small male crouches behind her, apparently hiding from the large male. Even as he is standing above the pair, the large male does not seem to notice the small male who swiftly slips away. The large male establishes genital contact, repeatedly stroking the female's ovipositor with his aedeagus, then dismounts. Tucson, Pima County, Arizona, USA. 16 November 2008.