Blowfly Larvae Photo by Leon Higley, UNL Entomology
Yummm… necrophagous larvae, or maggots, of the Blue Blow-Fly,Cynomyopsis cadaverina (Robineau-Desvoidy). The larvae of many species of fly feed on dead tissue. This is highly useful as an ecosystem service as they very effectively clean up any carrion that is missed or left over from any larger scavengers.
Some species maggots feed only on necrotic tissue leaving surrounding healthy tissue completely alone. These maggots clean out the dead tissue of large wounds in the wild and have been reintroduced to modern western medicine recently in a procedure called maggot debridement therapy. The therapy wass reintroduced by a Dr. R. Sherman from the Long Beach Veterans Hospital. He set up breeding of the Green Bottle Fly (Phaenicia sericata). These “Medical Maggots” are disinfected and introduced into wounds with non-healing necrotic tissues. The larvae dissolve and eat only the necrotic tissue, at the same time cleaning the wound of bacterial infection and the combined action of the necrotic tissue removal and secretions released in feeding significantly stimulate new healing of the wound.
C. cadaverina is also an important maggot, but it is used more in forensics. Different flies lay their eggs (oviposit) on carrion at different times and have different larval growth rates. By examining the maggots and eggs on a body and measuring the length and developmental stages of the maggots, forensic entomologists can determine roughly when death occurred. C. cadaverina oviposit 1.5 days after death according to Byrd & Castner’s Forensic Entomology. Of course th question can arise, how we know that a maggot arose from an egg laid on the body in question and whether it has actually fed upon the body. One way to find out ould be to examine the contents of the maggots gut. Of course the contents of the maggot’s gut aren’t like looking at the contents of a fish gut.
Jeffrey Wells and his colleagues needed just such a way of confirming that the maggots actually developed on the body for forensic investigations. They had three cases that come up in forensic investigations they wanted to be able to address:
- Investigators discover maggots but no body of any type, suspect possible foul play.
- Maggots are discovered, but not directly on a corpse, and alternative food sources are near.
- Maggots are found on a corpse, but may have come from somewhere else.
In the first case, if there are necrophagous maggots present there must have been a carcass nearby recently. If it was human then there may need to be an investigation. In the second case, because larva can crawl they may leave a corpse if disturbed or if the soft tissue is gone before they have grown enough to enter the pupal stage. If there are other food sources nearby the issue is to determine which food source the maggots originated on. Similarly in the third case it may be that the maggots found on a corpse may not have originated on that corpse as the maggots can crawl several meters and with the presence of other food sources may have crawled from a different source. If so those particular maggots should not be used to calculate the time of death.
Wells et al. raised eggs to the third instar maggots on human liver from a liver transplant patient. Since the eggs were collected from the wild, they allowed some maggots to pupate and were positively identified as C. cadaverina. They dissected the crops out of the maggots and adult flies and then extracted the DNA from the crops and their contents. The team performed two PCR reactions for each DNA extract. One PCR reaction used primers for fly cytochrome oxidase subunit one (COI), the other used primers for a region of human mtDNA. All the maggot and fly samples successfully amplified the fly COI fragment, and the human blood did not. The adult fly’s crops did not show human mtDNA as expected. They were able to successfully extract human mtDNA from the maggot crops that had fed on the donated liver as well as the blood sample donated by the liver transplant patient. The sequence from the maggot crops differed slightly (two position) from the blood sample. So they were able to conclusively identify maggots which had fed on human tissue, advancing the ability to use maggots in forensic investigations.
- Diptera (Flies)
- Calliphoridae (Blow-flies)
- Cynomyopsis cadaverina
J.H. Byrd and J.L. Castner, Editors, Forensic Entomology: The Utility of Arthropods in Legal Investigations, CRC Press, Boca Raton (2001)
J Wells, F Introna, G Di Vella, C Campobasso (2001). Human and Insect Mitochondrial DNA Analysis from Maggots Journal of Forensic Sciences, 46 (3)