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I do not have the time to go into detail now, 21392, but I would like to mention the dance flies. The females of many dance flies need proteins and, thus, eat other insects, while the males just visit flowers. For many species the male catches a fly and gives it to the female before copulation. It is quite a common sight in Spring to see "trio's" in nature: male, female and prey.
Dance flies developed in the middle of the Mesozoic. In the same period the first real flowers developed. You can imagine easily that from pure carnivore dance flies flies developed, which began to frequent flowers for alternative food. The females, however, often needed, nevertheless, protein from prey for laying eggs.