forensic entomology Flashcards
forensic biology:
use of biological knowledge in legal investigations.
considering variety of living organisms, niches occupied and potential for conflict with mankind, biologists are often involved in cases involving organisms.
foremost amongst them are invertebrates and their study
when:
if known when person died one can include/exclude witnesses to a person’s death (either because they claimed to have seen it or are thought responsible for it).
eg presence/ development stage of maggots.
where:
did person die where body found or was body transported from elsewhere.
eg presence specific species of invertebrates.
when where the wounds caused:
emphasises need to distinguish between wounds that caused death and those caused by natural decay.
development stage of invertebrates can be related to stage of decay- did they arrive before or after the person died.
wound characteristics can determine whether or not they were caused by invertebrates. damage caused by silphid beetles can resemble gun shot wounds.
history:
any discussion of forensic entomology almost always mentions chinese tract washing away of wrongs. written by sung tzu in 1235. murderer identified by flies buzzing around the sickle he used to murder his victim.
over following centuries, sporadic cases in which insects mentioned as source of evidence but not until late 1900s forensic entomology became a major focus for research.
blowflies:
many arrive within minutes of death.
most commonly used organisms in forensic entomology.
many authors identify 5 stages of post mortem decomposition associated with bodies.
these stages associated with waves of colonisation by insects.
calculating the PMI from species composition:
problem 1- no stage has fixed duration and different regions of body may decay at different rates.
problem 2- season and weather affect sequence of colonisation. many blowflies not active during winter period in UK and northern europe. therefore may not be found on a dead body during this time.
stage 1- fresh:
from moment of death to the body starting to bloat.
first insects to arrive are usually nlowflies such as calliphora vicina and lucilia secricata.
stage 2- bloat:
gases released by bacteria cause body to swell. some gasses attract blowflies.
predators that feed on blowfly eggs also attracted (they may also feed on flesh).
eg staphy;indid beetles (rove beetles) and silphid beetles (burying beetles) as well as parasitoids that lay eggs in blowfly larvae.
stage 3: active decay
skin splits and sloughs off body and body deflates. body undergoes putrefaction- butanoic acid and caseic acid produced.
as putrefaction continues, more ammoniacal fermentation products produced- these attract a different range of insects like histerid beetles and dentriivores eg muscid fly.
stage 4- post decay:
most soft tissue lost and that which remains tend to be dry.
blowfly larvae usually absent or departing by this stage. only species of fly larvae that feed on dry remains (stratiomytid fly larvae)
fauna often dominated by larvae of coleoptera (dermestid beetles)
stage 5- skeletonisation:
body reduced to constituent bones although fragments of hair and tendons may remain. as long as organic matter remains a few insect species found such as nitidulidae beetles
estimating time of death from species composition:
depends on time of year, temperature and the species of insect found on the body.
for example, in the UK, presence of blowfly eggs on a fresh body would indicate dead in 24 hours during the late spring or summer. however, absence of blowfly larvae on fresh body during winter does not indicate recent death.
presence/absence of certain insects can indicate past history of body.
minimum time since death:
reflects the least time it would take for invertebrate to reach a particular stage of development- person might have been dead for longer but very unlikely that initial infestation took place after the calculated date. even with most rigorous collection and analysis of evidence, biological processes usually involve a great deal of noise as consequence of complicating factors.
blowfly life cycle 1:
gravid female chooses natural openings and under body in which to lay eggs. batches of up to 180 eggs laid.
eggs hatch to release first instar larva. this begins feeding using its mouth hooks and enzymatic secretions, feeds on tissues and bacteria.
blowfly life cycle 2:
larva moults to second instar after 24-48 hours. larva moults to third instar after 24-48 hours.
larva continues to feed voraciously and dramatically increases in size over 3-4 days.
third instar reaches optimal size, empties gut and usually crawls away. some species move further away from food source than others.
blowfly life cycle 3:
larva burrows into soil and pupates. adult fly emerges. the body hardens and the wings expanded.
blowfly cycle 4:
adult blowfly flies off in search of food. adults usually live for several weeks.
duration of all stages hugely is affected by temperature.
calculating the minimum time since death:
compare stage of development of field maggot with time taken for a lab maggot to reach the same stage.
but lab reared maggot experienced a different temperature regime to the field maggot, there fore allow for the temperature effect.
this is relatively easy because time taken to develop increases with temperature.
ADD/ geographic origin:
calculate accumulated degree days/hours. time taken to reach each stage x temperature- base temperature.
geographic origin- many invertebrates have restricted georaphical distribution or occupy specific habitats. therefore, their presence can be used to determine the past history of a person or object- like has the body been moved.
insects can be used to determine source of drugs.
