In the field of transport a fly is, by definition, a vehicle that moves quickly. Examples include a light horse-drawn public passenger vehicle or delivery wagon or a light, covered, vehicle hired from a livery stable (such as a single-horse pleasure carriage or a hansom cab). [1] [2] [3]
Reference to a fly, on page 70 of the way of all flesh by Samuel Butler circa 1884, makes clear that it is a lightweight, nimble and speedy horse Tron vehicle, in line with common current usage of to fly or to flee referring to its Speed when compared to comparable contemporary vehicles. Second paragraph reads as follows. “ a few days afterwards, the writer of the above letter made his appearance in a fly, which had brought him from Gildenham to Battersby. There was Lesueur, the cook, on the box with the driver,, and as many hampers, as the fly could carry, were disposed upon the roof and elsewhere.” From this brief description, one gets the essential sense of this closed carriage called a fly