Forensic entomologists are mostly called upon to analyze insect larvae in cases where they are feeding and growing inside decomposing human tissue. Insects enter a corpse through openings in the body - eyes, ears, nose, mouth, anus, and genitals - to lay their eggs. If they enter through another area, chances are this indicates a wound area.
As these eggs hatch and feed, they attract other predators and parasites, such as flies, beetles, and wasps. Some of these insects have an appetite for human flesh, some preferring a "fresh kill", others the rotting odor. Flies are the most common insect attracted to the odor of rotting flesh, but they will only be active around such flesh when the outdoor temperature is fifty-two degrees or higher.
Insects are remarkable creatures. They're the oldest and most dominant life form on Earth, outnumbering animals by three to one. Compared with humans, they might be said to be inside-out because their skeletons are on the outside and their spinal cords are upside-down. They have no lungs, but breathe through tiny holes behind their heads, distributed throughout their bodies in a system of tiny tubes.
Most have two sets of eyes, two or three simple eyes, and a pair of compound eyes. Hearing is accomplished with eardrums (for some), or by antennae (for most). Most of them have keen camouflage abilities. They can mimic their environment by changing color, or mimic other insects by changing their behavior. Most of them can be frozen at many degrees below zero, and still be revived.
Also, they can handle extreme differences in vacuum or air pressure quite easily. Their most comfortable temperature is the 100-120 degree range. Their social organization, sex control, and egg laying abilities seem to show an intelligence far greater than humans. They seem to be constantly seeking out new, curious, habitats, moving by sense of smell (pheromones), temperature, light, gravity, and air or water currents.
They are not affected by tactile stimulation. Their reproductive process is unique. Their eggs hatch into larvae, then enter a pupae stage, and grow into adults from maggots or grubs (metamorphosis). Insects feed on just about anything. A small number are vegetarian (plant feeders), but most are carnivorous. Some are predatory, some are parasites, and others are blood-sucking.
Most have an appetite for decaying and decomposing material. Their mouths contain very powerful jaws and needlelike parts that function as teeth. In chewing insects, the mandibles move sideways. Sucking insects have a long proboscis and require liquid food. Most do not require water, as their bodies either conserve it or metabolize it from food.
Their digestive systems are not unlike humans. Insect sex is also not unlike humans. In a moist, insect-friendly environment, over 300 species of insects will visit a decomposing human body from death through the skeletal stage.
All living things are classified into a Phylum - Class - Order - Family - Genus - Species system. Phylum refers to how "high" or "low" the life form is on the evolutionary scale. Class usually distinguishes between structural characteristics (insects, for example, belong to the Class Insecta in the Phylum Arthropods).
The Order Diptera is that group of insects where the word "fly" is used as a second word, as in "blue fly" as opposed to dragonfly where "fly" is part of the word. It's often said that Diptera refers to the true "flies". Orders are further subdivided into families, and families into genera, and genera into species.
New "types" or species are always evolving, but a principal rule of zoology is that you cannot have 2 species in the same genus with the same name. For present purposes, we'll concern ourselves with the Order Diptera and certain families of it that prey on humans. Diptera have two sets of wings (front and hind), especially sharp mouth parts, larvae called "maggots", and undergo their metamorphosis either in excrement or animal flesh.
They are politely regarded as "nature's scavengers" since, if given time, they will remove all traces of dead carcass from an area. However, where there's large numbers of dead carcasses and Diptera, there's also usually outbreaks of dysentery. Some Diptera are parasitic, which means that, if they can accomplish it, they will try to find a living animal host to plant their eggs in (horses, for example, are especially sought out for this, with the eggs layed on the horse's mouth, which are then swallowed, allowing maggots to grow in the stomach).
A Family of Diptera exist (shown in the photos) which are predatory. They seek out, hunt, and prey on animals in hopes of killing them in order to lay their eggs. This is the group we are concerned with, and they love nothing more than to find a human carcass. Blow flies are found practically everywhere, and look like an ordinary house fly, or slightly larger. Many are metallic blue or green.
They have especially strong rear legs. Each female lays about 100 or more eggs. A carcass teeming with maggots is usually densely populated by the larvae of blow flies. Flesh flies are also quite common, similar in color, but shaped differently than blow flies (smaller legs, for example). Their larvae tend to be smaller than blow flies, both in size and number.
Their preference is to be parasitic, attaching their eggs to a living host, if possible. One species, Wohlfahrtia, only parasitizes humans. Screwworm flies are rather short, fat, and round, existing in numerous species. They prefer to lay their eggs on the skin of a living host. As the eggs hatch, the larvae burrow down under the skin until they find an internal organ. Some parasitic species are so clever as to lay their eggs on a foodstuff right before the animal eats it.