Information about the development and appearance of some common forensically important insects.

Insects have existed on earth for about 250 million years; comparatively humans have existed for about 300,000 years.  Such an enormous amount of time has allowed insects to attain a wide diversity in both form and development.  There are currently about 700,000 described species and it is estimated that there may be more than 10 million species of insects yet to be described.  Some insects have evolved a gradual or “paurometabolous” development in which there is an egg that hatches into an immature or “nymph”, which resembles the adult form, but is smaller and lacks wings.  In the forensically important insects, this is best represented by the cockroaches.  However, most forensically important insects undergo a complete or “holometabolous” development. There is an egg stage (except for a few insects such as the flesh flies that deposit living larvae) which hatches into a larval form and undergoes incremental growth in various life stages.  In this type of life cycle, the insect undergoes successive molts (shedding of the outer skin that has become too small) that the larva must undergo before it finally enters the inactive pupal stage.  The puparium is simply the hardened outer skin of the last larval stage that encloses the pupa.  The adult form will develop inside of this protective covering.

Blow flies

In the insects that undergo complete development, the larval stages appear quite different from the adult form.  The larvae of flies (order Diptera) that are commonly recovered from decomposing human remains lack functional legs, and the body of many species appears cream colored, soft-bodied, and quite “maggot-like”.  It is important for the forensic investigator to recognize the various species of fly larvae, or maggots, commonly found on human remains.  Once the larva is through feeding it will migrate away from the corpse in order to find a suitable site to form the pupal stage.  The pupae of blow flies are often overlooked, as they closely resemble rat droppings or the egg case of cockroaches.  The pupal stage is an extremely important stage to the forensic entomologist and a thorough search should be made for the presence of pupae at any death scene.  If the adult insect has not emerged, the pupa will appear featureless and rounded on both ends.  If the adult insect has emerged, one end will appear as if it has been cut off, and the hollow interior will be revealed.  The adult fly will emerge from one end of the puparium.  Most adult blow flies appear a metallic green or blue and are easily distinguished from other fly species.  Two of the most common species of blowflies found in the southeastern United States are in direct competition for the resources needed for species survival.

Beetles

The beetles (order Coleoptera) are one of the largest groups of animals and they also undergo complete development.  Because of their development the larvae appear very different from the adult form.  Although the larvae or “maggots” of a large number of blow fly species may look almost identical; the larvae of beetles may look very different from one species to the next.  Beetle larvae recovered from corpses can be easily differentiated from maggots as they have 3 pairs of legs and the maggots found on decomposing remains will not have any legs.  Once a larva as been identified as that of a beetle, further field identification can be accomplished because of the wide diversity of larval forms.  The bodies of beetle larvae may range from almost white, robust, and hairless to dark brown, slender, and quite hairy.  Others may appear almost black and have armored plates on their back.  In general, there are 4 distinct beetle larvae body types.   Although the number and appearance of adult beetles that can be found on human remains is much too diverse to show even a representative sample, two of the most common species are easily recognized.

 

 

Dr. J. H. Byrd
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