There are 10,000 described species of this type of fly which range from 2mm to 20mm and have a variety of sizes and shapes. They are almost as diverse as Crane Flies. There are innumerous species that are undescribed and identification is near impossible except by specialists.
The life histories of these flies are as diverse as their numbers, but in general the larvae are parasitoids on plant-eating insects, such as beetles, caterpillars, or Hemiptera. They make up over half of all the parasitoid insects. They have developed a number of ways of finding their host to attach to; they will deposit their eggs on the host or near the host or in the host’s food. They have also evolved so that almost immediately after laying the egg, it will hatch; this keeps the egg from being exposed in this vulnerable mode.
These are identified in one way by the 4 stripes on their thorax. They can be confused with Sarcophagidae who have 3 stripes. Their abdomens also have long, stout bristles.
North American Dipterists Society has lots of good info on this family