Bloodstain pattern analysis can provide insight into a sequence of events associated with a violent crime. However, bloodstain pattern analysis can be confounded by the feeding activity of blow flies. The current study found that C. vicina and L. sericata changed bloodstain pattern morphology through feeding and defecation. They also deposited artifacts in rooms where blood was not present originally. Chemical presumptive tests (Hemastix() , phenolphthalein, leucocrystal violet, fluorescein) were not able to differentiate between insect artifacts and bloodstains. Thus, C. vicina and L. sericata can confound bloodstain pattern analysis, crime scene investigation, and reconstruction. Crime scene investigators should be aware of these fundamental behaviors, and the effects that blow flies can have on expirated and pooled bloodstain patterns. (Published Abstract)