These are white maggots fully turned to casters, in warm conditions they will hatch into flies.
Previously we have supplied them at a late stage of development so the customer has to wait only a few days for the flies to hatch, this involved us storing them for some time to get them to this stage, it seems that this period of storage sometimes causes the hatch rate to be a lot lower than we would hope for, therefore we now send them out at an earlier stage, it may be a week or more before they will hatch but the hatch rate will be much improved. If after delivery, storing the casters in a refrigerator will again cause the hatch rate to fall.
Casters are the non moving pupae stage of the larvae (maggot). These are not suitable for fishing.