Embryonic hemocytes are derived from head mesoderm and enter the hemolymph as circulating cells.
Specifically, they are released during the pupa stage in order to prepare the fly for the transition into an adult and the massive associated tissue reorganization that must occur.
There are four basic types of hemocytes found in fruit flies: secretory, plasmatocytes, crystal cells, and lamellocytes.
[1] In mosquitoes, hemocytes are functionally divided into three populations: granulocytes, oenocytoids and prohemocytes.
Prohemocytes are small cells of unknown function, which may result from the asymmetric mitosis of granulocytes.