Describe forensic entomology and its applications.

Principles

• A body is colonised in a predictable succession of species, each arriving when the state of decomposition suits it, so the fauna present indicates how long the body has lain
• The first arrivals are blowflies (Calliphoridae) and flesh flies (Sarcophagidae), which are attracted within minutes to a few hours of death by the odour of decomposition, and lay eggs at the natural orifices and in any wound
• The presence of maggots at an unusual site therefore indicates a wound there — a finding that has revealed injuries in bodies too decomposed for the wound itself to be recognised
• Flies do not lay at night, and are inactive in cold, heavy rain and strong wind; a body concealed, buried, wrapped or submerged is colonised much later, which is itself informative

Eggs are laid in masses at orifices and wounds — within hours of death in warm conditions → Eggs hatch into first-instar larvae (maggots) in about 8 to 24 hours → second instar at about 1 to 2 days; third instar at about 2 to 3 days — the stage of maximum feeding, when maggot masses generate considerable heat and can accelerate decomposition markedly → The larvae leave the body to PUPATE in the soil nearby at about 6 to 12 days → The adult fly emerges at about 2 to 3 weeks, leaving an empty puparium — the presence of empty pupal cases indicates that at least one full cycle has been completed → Later waves follow — beetles (Dermestidae, Silphidae) feeding on dried tissue and skin, then mites, moths and finally species that consume hair and bone

• The estimate is made in two ways — by the developmental stage of the oldest larvae present, back-calculated using the accumulated degree hours for that species at the recorded local temperatures; and by the succession of species present, which is used for longer intervals
• The oldest specimens are the relevant ones, because they were laid first; collecting only the largest, most conspicuous maggots is the correct approach
• Temperature is the dominant variable, since insect development is temperature-dependent — which is why local meteorological data for the relevant period must be obtained, and why entomology in Indian conditions runs far faster than in the European tables from which many species data derive
• It gives the time since colonisation, not since death — a distinction that matters, since a body kept indoors, wrapped, refrigerated or buried may not be colonised for days or weeks

Other Uses and Collection

• Indicating a wound — maggot infestation away from the natural orifices points to an injury at that site
• Indicating movement of the body — species present that do not occur in that habitat, or an absence of species that should be there, shows the body was colonised elsewhere and moved
• Toxicology (entomotoxicology) — larvae take up drugs and poisons from the tissues they consume, and maggots can be analysed when no soft tissue remains. Drugs also alter the rate of larval development — cocaine and amphetamines accelerate it and barbiturates retard it — which must be allowed for in the time calculation
• DNA from the blood meal in the larval gut can identify the body
• Distinguishing antemortem from postmortem infestation — myiasis in a neglected wound during life indicates neglect, which is itself an offence in the elderly and in children
• Collection at the scene and at autopsy — collect the largest larvae from several sites; divide each sample, preserving half in 70 to 80% ethanol after killing in hot water (which fixes them at their present stage) and rearing the other half alive on liver or meat to the adult stage, since identification to species is far easier in the adult; collect eggs, pupae and empty puparia; search the soil beneath and around the body to a depth of 20 to 30 cm for pupae; and record the temperature at the scene, of the maggot mass, and of the soil, and obtain the local weather records afterwards

Applied Aspects

• Do not wash the maggots away; they are the evidence, and mortuary staff routinely hose them off before the body is examined
• Collect the largest larvae, and from several sites; the oldest specimens give the interval, and a sample of small maggots from a late-laid batch will underestimate it substantially
• Preserve half and rear half; a preserved third-instar larva is often unidentifiable to species, and the species determines the developmental rate on which the whole calculation depends
• Record the temperatures and obtain the weather data; without them the accumulated degree hours cannot be calculated and the specimens are of little value
• Search the surrounding soil for pupae and empty puparia; an empty puparium proves that a complete generation has elapsed and sets a minimum interval
• Say that it estimates time since colonisation, and consider whether anything delayed access — wrapping, burial, a closed room or refrigeration — which is exactly the kind of qualification that survives cross-examination