Fig. 3. Crystal cell mediated stress responses.

When larva gets a sterile wound, crystal cells migrate towards the wound site, and rupture to initiate the melanization cascade (left). This promotes the formation of a scab at the wound site leading to a subsequent healing process. Upon wasp infestation, wasps lay eggs inside Drosophila larva that trigger innate immune responses including differentiation of lamellocytes. Lamellocytes encapsulate wasp eggs and activate PPO3 to neutralize them (right). During encapsulation, crystal cells co-opt PPO1 and PPO2 to facilitate melanization. In melanization, rupturing of crystal cells is coupled with the activation of Serine proteinase cascade which triggers the conversion of PPOs into POs initiating non-enzymatic melanin formation (middle).