Determination+of+Elapsed+Time+Since+Death

Determination of Elapsed Time since Death **

media type="custom" key="3022962" ** Two Methods of determining elapsed time of death: ** **1st Method** – based on the predictable development of larval Diptera, primarily blow flies, over time - most valuable from a few hours to several weeks after death **2nd Method** – based on the predictable, successional colonization of the body by a sequence of carrion insects -can be used from a few weeks after death until nothing but dry bones remain // *The method used depends on the age of the remains and the type of insects collected. // -Flies are attracted to a body immediately after death – the first flies to be attracted to a body are usually the blow flies -The majority of the flies attracted to the body are females searching for suitable egg laying sites -Blow flies develop from egg through first, second, and third instar stages, and the pupal stage before becoming adults – this predictable pattern is influenced by species and temperature -Blow flies are cold-blooded animals, so their development is temperature dependent -As temperature increases, they develop more rapidly -As temperature decreases, they develop more slowly -An analysis of the oldest insect stage on the body, together with knowledge of the meteorological conditions and microclimatic conditions at the scene can be used to determine how long insects have been feeding on the body, hence, how long the victim has been dead **First Instar Stage:** -Very delicate and prone to desiccation -They are unable to break skin so they rely on liquid protein for meals -Wounds are an excellent source of liquid protein – therefore they are the first site females choose to lay their eggs -In absence of wounds – eggs will be laid near the natural orifices **Second Instar Stage:** -First-instar larval cuticle and mouthparts are shed -Slightly larger, less delicate stage that can penetrate skin by using proteolytic enzymes and the rasping action of its mouthparts **Third Instar Stage:** -Shed second instar cuticle and mouthparts -Voracious feeder and frequently aggregates in large masses that can generate a tremendous amount of heat -These masses can remove a large amount of tissue in a short period of time **Nonfeeding or Wandering Stage:** -No physical change takes place at this time, however extensive internal physiological change occurs -The larva wanders away from the food source looking for a suitable pupation site, where it will not be vulnerable to predation: -Pupation sites can include: the surrounding soil, carpet, the hair or clothing of corpse, and they even might burrow down several centimeters into the soil and may crawl several meters away from the remains -Once the prepupal maggot has contracted, it begins to pupate -Does __not__ shed third instar layers – the cuticle loosens and the insect secretes and number of substances which harder and form the puparium
 * Dipteran Larval Development **

** Factors that must be known when determining time since death: ** -Must know how far insects are through the life cycle -Each species develops at a different rate therefore it is crucial to determine the species -Insects are temperature dependent, therefore it is vital to be able to determine the temperature of the crime scene -Usually determined by government weather station data, however it can be inaccurate, therefore it is best to place a portable data logger at the scene and record temperatures for a few weeks and compare to the weather station -In order to determine the age of the oldest insects, you must know the rate of development – obtained from literature that contains developmental rates of insects at different temperatures
 * 1.   **  **The oldest stage of blow fly associated with the body.**
 * 2.   **  **The species of insect.**
 * 3.   **  **Temperature data.**
 * 4.   **  **Developmental data.**

-The science of forensic entomology is based on determining the length of tenure of insects on a body, rather than the actual time of death -Death precedes insect colonization, so the insects will indicate a time elapsed since death that is less than the actual time of death
 * In most cases, forensic entomology will only determine a __minimum__ time of death.

(Entomological evidence will usually indicate that a person has been dead for at least a certain period of time - it could have been longer, but it could not have been less.)