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I don't believe you can distinguish Tabanus masses from Hybomitra masses. Don't know how long they take to hatch, don't know time of day they hatch.
This batch of eggs has either hatched or has been parasitized and the parasitoids have emerged. What you are seeing are the empty egg cases. Tabanid larvae, except perhaps Chrysops are difficult to rear, They are carnivores and require living food and are also cannabalistic.

Added 25 August 2007
Here is a photo of Hybomitra lasiophthalma in the act of ovipositing. Eggs are white at first and then the chorion hardens and darkens (probably within 24 hrs.) Note the large size of the multilayered egg mass, fly is about 14mm long. Also note the tiny parasitoid wasp ovipositing in an egg.
27 July 1992, New Brunswick, Canada. From a Kodachrome slide