The Life Cycle of a Fly

We all know that many species of flies are attracted to decomposing bodies. But many authors may be unaware of what happens after the eggs are laid. While there are a number of families of flies (Diptera) that may be encountered on a corpse I will focus on the three most common families; the Blow Flies (Calliphoridae), the House Fly (Muscidae), and the Flesh Fly (Sarcophagidae). The Blow flies and House flies will lay eggs on the corpse while the Flesh fly eggs develop inside the female and she deposits the maggots onto the corpse.

Following the egg stage is the first of three larval stages called instars. During each instar the larvae will feed and grow to the physical limits of their cuticle (skin). Maggots skin will not continue to expand like a mammal and so they must shed this skin (molt) in a manner similar to a snake. Each instar will be larger than the previous one as the maggot continues to feed and grow. So a third instar larvae will be much larger than a first instar larvae. The third instar is the last stage of maggot development. When the maggot reaches the end of this stage they will “wander” from the food source to find a place to pupate.

The process of pupation is the metamorphosis of the maggot into the adult fly. This process takes place in the pupa (cocoon) of the maggot where it spends about 2/3 of it’s immature life span. These pupae go through a light to dark color change and during their final days have the appearance of rat droppings. Once the maggot fully transforms into the adult fly it will break out of one end of the casing and emerge as an adult to start the process all over again.

The total time of development varies for each species and under different temperatures. As a general rule of thumb, maggots will develop more quickly under hotter temperatures than they would under colder temperatures. Although far from accurate, I used to tell detectives that they could use a 24 hour time table for each stage as a rough estimate until the analysis was complete. So a body with second instar larvae would be around 3 days old (egg, 1st instar, 2nd instar). Obviously, this ignores a plethora of conditions that could severely alter that final estimate but it is a place to start until the entomologist can complete their analysis.