It might be tachinid flies, if so your more than likely going to lose all of your remaining crickets.
Here is some basic info on them (I just cut and pasted from some random site).
Parasitic flies (Tachnidae) Tachinids - High Dispersion - greater than 1/4 mile. ID - They are hairy, large flies (usually as big or bigger than houseflies) with wings at a 45-degree angle. Identifiying characteristic is a rounded postscutellum. These flies are commonly found in houses during late winter/early spring, emerging from the hosts that crawled into your house in the fall prior. Tachinid flies lay eggs on caterpillars and adult beetles and bugs. Many are specific to certain caterpillars or bugs- e.g., Trichopoda pennipes against squash bugs; others are generalists and will lay eggs on many types of caterpillars.
In the future you will need to keep the crickets in a fly proof container, look for air-tight bins and very fine mesh screen to keep them out. The very finest screen you will commonly find is copper or brass craft screen at hobby shops or art supply shops, otherwise 30x30 mesh stainless steel screen ordered online would be the minimum I would suggest for you to use (it's not exactly a low cost material but it keeps out flies big or small, assuming the container you start with is air-tight).
It is also a good idea to establish a good cleaning routine, for crickets at least once weekly should keep the bulk of the smell down and reduce how many flies are attracted to the colony. It will not matter how secure the bins are when you open them to retrieve crickets if flies are around they are bound to get in when you have the bins open.
If you are breeding your own, keeping more than one bin going is also a good way to prevent massive die-offs. I would suggest at least six bins (one bin per week of age, you might need more if you keep the crickets cooler than optimal).
Another fly prevention tactic is to use fly traps, the particular type will depend on what works for you in your area. Sticky fly strips are easy to use and dont cost all that much, you might be surprised at what all they capture.
Lastly, use a long acting insecticide on surfaces away from the colonies where you see flies, just about any permethrin based product will work. (if you do decide to go this route, use it only as a last resort, only if you have made containers that insects can't get in or out of, never feed off a loose insect found in or around your home, and never introduce a loose insect to your colonies)
Maurice Pudlo
