What happened to Porky? Presentation developed for use with the Crime Solving Insects activity available at http://ipm.ncsu.edu/4-h/csifinal.pdf. Image: http://www.animals-in-distress.net/files/photos/coriander2.jpg T. Trimpe 2009 http://sciencespot.net/
Things to Remember The progression of insect life follows a pattern, and the developmental rates of flies are relatively predictable. The rate of insect development is influenced by temperature because insects are ectothermic ( cold blooded ), which means their body temperatures are largely dictated by the outside temperature. Only when the outside temperature warms an insect s internal body temperature to its critical level can the insect become active (and eat and grow). The postmortem interval the time between death and discovery of the corpse can be estimated using insect evidence and temperature data along with other factors, such as the presence of drugs in a corpse and conditions related to the corpse itself (wrapped in a material, in a closed room, exposed to outside conditions, etc.) Not all fly species are found everywhere, and this can provide important information also. For example, the skipper fly, Piophila nigriceps (pie-oh-feel-ah NYE-greh-cehps), is found only in urban settings. House flies, blow flies, and flesh flies can be found in both urban and rural settings.
Metamorphosis Flies, beetles, and many other insects have complete metamorphosis, which consists of four stages egg, larva, pupa, and adult. After the adults mate, the females lay eggs onto corpses - usually near natural body openings or wounds. Feeding activity is usually seen in the head region first (mouth, nostrils, eyes, ears), followed by the excretory and reproductive openings. The trunk of the body is invaded much later in the process. The length of the life cycle varies between different fly species and is dependent on temperature. http://www.cmnh.org/site/img/researchandcollections/invertzoo/124_blowflylifecyclerw.jpg
Stages of Decay 1 - Fresh Stage Begins at the moment of death and lasts until the body becomes bloated. Blow flies and flesh flies are among the first to find the body. Predatory wasps and beetles may arrive later to feed on the maggots (but not the corpse). 2 - Bloated Stage Begins when the body becomes inflated due to the production of gases from bacteria that begin to putrefy the body or cause it to decompose. House flies now join the other flies and their maggots form feeding masses that help to liquefy the tissues of the body. 3 - Decay Stage Begins when the skin breaks and the gases escape. Maggot masses are large and very active as they grow older and larger. This is the stage of decomposition that smells bad. At the end of this stage, the maggots leave the corpse in search of a place to pupate in the soil. 4 Post-Decay Stage Most of the flesh is gone from the corpse, with only cartilage, bone, and skin remaining. This stage is devoid of flies. Some beetles continue to feed on the highly desiccated or dried remains.
Case Studies For each case: 1 Review the police report and weather report. 2 Examine & document the collected evidence. Measure the length of the maggots & pupae. Record your data in the chart. Consult the Species Key and the tables on your lab page to determine the various fly species that were found on the corpse and their ages. 3 Use the information from the reports and your examination to answer the questions.
Case #1: Oh, Deer! Police Report: The body of a female deer was found behind a fence along a busy two-lane road on the edge of the city limits of Charlotte. Animal Control was called and reported no apparent wounds on the body. It was not hunting season. Weather Report: Daytime temperatures have been fairly consistent for the past three weeks, ranging from 70 to 74º F. Questions: 1. Approximately how long has this animal been dead? Species & Stage Size (mm) Age 2. Why are maggots of different ages found in the body? 3. Other than temperature, what abiotic (external to the corpse) conditions would you want to obtain from the weather station to help you to be more confident of your time of death estimation?
Case #2: Canine Caper Police Report: The body of a large pit bull terrier was found inside a walk-in basement at a home in Cary. Maggots were found concentrated in the head and region behind the shoulder. The windows were closed, although the open curtains allowed sunlight to enter, and the air conditioner was set at 72º F. Weather Report: Daytime temperatures have been variable over the past three weeks, ranging from 75 to 94º F. Skies have been sunny. Questions: 1. Approximately how long has this animal been dead? Species & Stage Size (mm) Age 2. What effect, if any, do the outside temperatures have on your estimation of time of death in this case? 3. How does the fact that the windows were closed relate to the populations of flies you observed in and around the corpse? 4. Do you suspect foul play? Explain.
Case #3: Dandy s Death Police Report: The body of a young male horse was found in a pasture in a small town near Wilmington. The autopsy from the vet school reveals that the cardiac glycoside, oleandrin (a powerful heart stimulant), was present in the body. Oleandrin is found in the oleander plant. Oleander is a common ornamental shrub in this area, but none grows within 200 feet of the pasture. Weather Report: Daytime temperatures have been unusually warm over the past three weeks, ranging from 84 to 86º F. Questions: 1. Approximately how long has this animal been dead? Species & Stage Size (mm) Age 2. What effect, if any, does oleandrin have on your estimation of time of death? 3. What effect, if any, does temperature have on your estimation of time of death in this case? 4. Do you suspect foul play? Explain.
Case #4: Porky s Peril Police Report: The body of a large pot-bellied pig was found in a dense stand of evergreen trees far from any urban area in Buncombe County. Hairs around the pig s neck were worn away in a band pattern. Weather Report: Daytime temperatures have been average over the past three weeks, ranging from 70 to 73º F. Temperatures in the woods would be approximately 5 degrees cooler due to the lack of sun in the shady environment. Questions: 1. Approximately how long has this animal been dead? Species & Stage Size (mm) Age 2. What effect, if any, does temperature have on your estimation of time of death in this case? 3. Do you suspect foul play? Explain.