Which of the following is defined as each of the three different larval stages of flies?

The term that defines each of the three different larval stages of flies is "Instar." In the life cycle of insects, particularly in flies, instars refer to the stages of development between molts, where the larvae grow and change form. For example, a fly's larval stage goes through multiple instars characterized by increasing size before eventually pupating into an adult fly. This concept is essential in forensic entomology, as the number of instars present can provide valuable information about the age of a decomposing body, aiding in estimating the time of death.

Other terms mentioned do not pertain specifically to the stages of fly larvae. "Crop" refers to a part of the digestive system in some insects, while "larva" is a general term for the immature form of an insect that undergoes metamorphosis. The term "grub" typically describes the larval form of certain beetles, not flies. Thus, "Instar" is the precise term that encapsulates the specific developmental stages of fly larvae.