BLOW FLIES: THEIR LIFE CYCLE AND WHERE TO LOOK FOR THE VARIOUS STAGES

By far the most important insects on corpses are the metallic green or blue flies of the family Calliphoridae. Some members of this family usually arrive within a few minutes of death. This flyer is a review of their life history and where to find the various stages.

Egg. The eggs are white and sausage-shaped and are usually laid in clumps. On fresh corpses, look for these clumps in the mouth, in nasal openings, in the ears, and generally in any area where mucus membranes come into contact with the outside air. Also look for them on wounds and bruises. Don't think that because they haven't hatched yet they are not important. Eggs can be both identified and aged. Put some of the eggs into alcohol and save some for rearing.

Live Blow Fly Maggots

Live blow fly maggots on a piece of meat.

Larva. Larvae or maggots hatch from the eggs. They too are white but are shaped more like a cone than a sausage. The mouth is at the pointed end of the cone, and the maggot uses a pair of "hooks" there to attach itself to the corpse while it feeds. Maggots also use the hooks to help themselves move. Normally they move by extensions and contractions of their segmented, legless body. The photograph above is of several live blow fly maggots on a piece of meat; the one below is of a maggot mounted on a microscope slide.

Blow Fly Maggot on Microscope Slide

Blow fly maggot on microscope slide.

The blue color of the maggot on the slide is from stain, which I applied to make the hardened portions of body visible. The two dark spots on the left are the posterior spiracles, or openings to the breathing system. The hook-like structures at right are the mouth hooks and their associated internal skeleton for attachment of muscles.

Head and Mouth Hooks of Third-Instar Maggot Posterior Spiracle of Third-Instar Maggot

Left photo is of enlarged mouth hook assembly; right photo, of posterior spiracle. Both structures have been stained and mounted on a microscope slide. Both were taken from a maggot of the calliphorid Phormia reginia (Meigen)

Differences in the fine structure of the mouth hooks and spiracles can help tell a forensic entomologist both the species the maggot belongs to and how many times it has shed its skin. Maggots shed their skin three times. Each time they get a new, slightly larger and slightly different set of spiracles and mouth hooks. In the left photo above the mouth hooks form a sort of silhouette. The pattern of these silhouettes is specific to species and instar number. The right photo is of one of the posterior spiracles. Notice that it has three slits. Three slits are characteristic of third instar maggots. Second instar maggots have two. Other characters of the spiracular openings are characteristic of species. For example, notice that in this species the outer border or "peritreme" of the opening (upper left) is interrupted at the base. In other species it may be complete.

Maggots will be the most obvious life stage on corpses that have been laying out for several days to a week or so. Initially they will be concentrated in the same places as the eggs but as they grow they will wander into other areas. Most large maggots on exposed corpses will be found underneath, along the edges where there is more air and where they can indirectly take advantage of heat from the sun. When collecting these maggots be sure to get a wide range of sizes, including the largest you can find. Also be sure to set some of the maggots you collect aside for rearing.

Posterior Spiracles of Puparium

"End-on" view of posterior of calliphorid puparium. This particular puparium belongs to the fly Phormia regina (Meigen).

Puparium. The third time a maggot "sheds its skin" a remarkable thing happens. The skin contracts to a capsule-like form and becomes rigid and hardened. It is not actually shed, but remains covering the newly-molted insect inside. You can see that the puparial skin is the same as old maggot skin because it retains the outlines of the spiracles and other sclerotized parts of that stage. For example, the scanning electron micrograph above is an end-on view of the posterior of a blow fly puparium. The three slits on each side are the "mummified" traces of the three slits visible in the posterior spiracles of the maggot (to see the resemblence compare this photo with the photo on the right preceding it).

The living insect that is inside the puparium is pale white, cannot move or feed, and has rudimentary legs and wings, antennae, etc. This mummy-like form is called a pupa. The hardened skin surrounding it is called a puparium (plural puparia).

Flesh Fly Next to Puparium from which It Emerged

Flesh fly (family Sarcophagidae) next to hollow, capsule-like puparium from which it emerged. "Cap" of puparium is on left. This fly belongs to the species Sarcophaga bullata Park. Blow flies have similar puparia.

These puparia are often present at a crime scene, but not collected because no one is looking for them, or because no one is looking in the right place. They are usually found in the vicinity of the corpse, not actually on it. Folds in clothing are good places to look. They may also be found up to 30 feet away from a corpse, for example in the deep pile of some carpets or in folds in curtains etc. indoors. If puparia are present at a crime scene, it is very important to find them, because they are the oldest stage one can definitely link with the body.

Adult. The puparium is furnished with a cap-like lid that can be popped off by the emerging fly. The newly-emerged flies are at first pale in color, soft to the touch, and with crumpled, unexpanded wings. They later expand their wings and turn green or blue. The photograph below is one of the blue varieties.

Bluebottle Fly on Piece of Liver

Blow fly adult (a bluebottle fly). This species is probably Calliphora vomitoria Linnaeus. The very similar-looking Calliphoria vicina Robineau-Desvoidy is another common member of the genus.

Blow flies do not fly much for a day or two while their body is hardening. If you are lucky enough to see a fly like this be sure to get it, for it undoubtedly developed on site. The adult flies mate after they have fully hardened. Once they have mated they are among the most mobile creatures on earth (for their size), and can travel for miles to find another site to lay their eggs. Adult flies will be very obvious on most corpses. Be sure to collect a few. These will let the forensic entomologist know what flies were in the area. Do not make the mistake of thinking that because you have captured adult flies you have done your job. Because of their mobility, fully mature adults are worthless as forensic indicators.


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