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Helophilus is one of the few genera where males have the eyes separated like the females (though they stay a little closer to eachother than in females). Yes, the shape of the abdomen is the next best character.
Sorry for my omission: view the tip of the femur 3 and base of tibia 3: hybridus femur with only a very small area yellow, the rest black. Base of tibia is about 1/4 yellow in hybridus, instead of 2/3.