CSI: Crime Solving Insects

Scientist Paula catching blow flies in the field using an insect net.

Scientist Paula catching blow flies in the field using an insect net.

The activities are as follows:

Most people think that maggots (larvae of blow flies) are gross, but they are important decomposers in many ecosystems. Without maggots, we would all trip over dead bodies every time we went outside! Not only do maggots break down dead animal bodies in nature, but they decompose human bodies as well!

Forensic entomology uses these amazing insects to help the criminal justice system. Remember the next time you swat away a fly, these little insects help police solve crimes! Adult blow flies are usually the first to arrive at a crime scene with a dead body. The blow flies oviposit, or lay their eggs, shortly after their arrival. These eggs hatch and become maggots that feed on the body. Scientists can use the age of the maggot to help estimate how long someone has been dead. The longer a body has been dead, the longer ago the eggs hatched and the older the maggot larvae will be.

Kristi and Paula, two forensic entomologists, were in the field on day, documenting the timing of blow fly oviposition. They noticed something unexpected! There were wasps stuck in the traps they were using to catch blow flies. The scientists wondered if these wasps would affect a blow fly’s decision to oviposit. Wasps will attack adult blow flies and will also eat their eggs. Blow flies have an incredible sense of smell and sight. Blow flies may be able to use their senses to detect if a wasp is near a body. If a wasp is detected, blow flies could choose to avoid the area or delay laying their eggs. The scientists predicted that blow flies would delay their oviposition when wasps were present near a body. If wasps cause blow flies to delay oviposition, this could change how scientist’s use maggot age to determine how long ago a body died.

Control bait cup with a large number of blow flies on the chicken liver

Control bait cup with a large number of blow flies on the chicken liver

To test their hypothesis, the scientists did 10 trials in the field. They used bait cups that contained chicken liver to simulate a dead human body. A total of 9 bait cups were used in each of the 10 trials, for a total of 90 cups. Of the 9 cups used in each trial, three contained only chicken liver, to represent a body with no wasps present. These cups were used as controls. Three cups contained chicken liver and a wasp pinned to the side of the bait cup so that there was a visual cue of the wasp. The final three cups had a crushed wasp sprinkled over chicken liver to see if blow flies could use a smell cue to tell that a wasp was present without seeing them. Researchers checked the cups every half hour for the presence of blow fly eggs. If they saw any eggs, they recorded the time of oviposition in hours after sunrise. They then brought the maggots to the lab and raised them to the third larval stage and identified them to species.

Featured scientists: Kristi Bugajski and Paula Stoller from Valparaiso University

Flesch–Kincaid Reading Grade Level = 7.6



  1. Karen Wampuszyc says:

    I would love to use this one (3.11.16 CSI: Crime Solving Insects) in my forensic science lab, but the links to the student activities are not live. Can this be fixed?


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