Each time a larva molts to the next instar phase it becomes a lot bigger and as such the existence of these four instar phases is one of the reasons why the size of bloodworms varies throughout the year as explained here.
Before starting to pupate the instar 4 larvae secrete a silky material covering their body; these are the ‘white-skinned’ bloodworms you might occasionally notice to be present. Although maybe unsightly, these white-skinned bloodworms can and will be eaten by your fish, aquatic amphibians, invertebrates and reptiles without a problem.
But as most people don’t like to see these white-skinned bloodworms, we utilize a mechanical separation step as one of the cleaning steps during the processing of the harvested bloodworms before they are to be packaged and frozen. However, in case too many harvested bloodworms are found to be preparing for entering the pupa life stage (as for example pictured below), the batch is not processed further but sold off as live food to fish, shrimp and crab breeders.