No. 45: May-Jun 1986
Bats are divided into megas and micros. The "megabats," represented by the fruit bats, possess an "advanced" connection between their eyes and midbrains. No other mammals except the primates possess this type of advanced visual organization. In contrast, the "microbats," the common echo-locating insect-eaters have a "primitive" eyebrain connection. This deep division in the bat family -- mega/micro, vegetarian/carnivorous, sight-dependent/echolocating -- suggests that mammal flight has developed at least twice.
(Pettigrew, John D.; "Flying Primates? Megabats Have the Advanced Pathway from Eye to Midbrain," Science, 231: 1304, 1986.)
Reference. The problems of bat evolution can be found at BME1 in our catalog Biological Anomalies: Mammals II. Ordering information here.