Crime Library: Criminal Minds and Methods

Kevin Neal, Convicted of Murder by Forensic Entomology

Bug Collecting

It was time for Selvaggio to move the heart of his case — using forensic entomology to establish when the children had died, and by inference, that Kevin Neal was still free when they were murdered. Selvaggio could not prove the means by which the children were murdered because of their advanced state of decay, but he could establish motive — revenge — and with the help of flies and maggots, he would prove opportunity.

The fate of a fly is essentially determined by temperature. Anyone who has ever encountered a fly buzzing against a warm window on a bright, but cold, winter day, has witnessed the influence heat has on the insect. It may be barely above freezing outside, but the sunlight passing through a window has awakened a dormant fly by raising the temperature to such a point that the fly is fooled into "thinking" it is safe to emerge from stasis and become active.

Under normal circumstances, as with other cold-blooded arthropods whose body temperatures are dependent on the environment, flies can only thrive if the temperature is between two extremes. Too cold and the fly dies or goes dormant, depending on where it stands in its life cycle. Too warm, and it rushes through its developmental stages too quickly and cannot reproduce. But if the temperature is just right, the life cycle of a fly is amazingly predictable — and useful to criminalists.

Life cycle of a fly.
Life cycle of a fly.

Generally speaking, flies pass through six stages during their lives. Some of these stages can be further divided based on the activity of the insect. Flies start out as eggs, and hatch into first instar maggots — the beginning of the larval stage. Because maggots have a rigid exoskeleton, as the maggot feeds, it eventually must molt its smaller shell and thus passes into a second instar maggot. How quickly the maggot passes from first to second instar is — assuming there is food for it to eat — regulated by heat. As the maggot continues to eat, it grows, and again must molt, entering the third instar phase. This developmental phase can be divided into "feeding" and "migrating," because after the maggot eats its fill, it begins looking for a dry place, safe from predators, where it can pupate, forming the cocoon that will allow it to transform into its adult phase. Maggots will travel hundreds of feet from their birthplace and feeding spot in order to find an acceptable place to pupate. Once the third-instar maggot finds this location, it develops a hard shell similar to a caterpillar cocoon and makes the final transition to adult fly. After it emerges from the cocoon, the fly spends several hours in the sun, inflating its wings, developing its coloring and hardening its final exoskeleton. If the adult fly is a female, after it is able to fly it begins the search for carrion to start the cycle all over again.

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