Blowfly Lifecycle Calculations

The study of this phenomenon by entomologists has lead to our ability to estimate the time since death by examining which stage of growth the blowfly is in when found on the cadaver. The forensic entomologist conducts examinations at crime scenes and examines the insect evidence found there to bring evidence to court cases involving human cadavers. To make a determination, the scientist makes visual observations, collects insect evidence, determines the stages of development, and calculations the time of initial deposit of the blowfly eggs on the cadaver. In the laboratory the scientist rears collected specimens to confirm the species and more accurately establish the stage of growth. They then compile their findings and make a “best guess estimate” as to how long the cadaver has been dead.

  1. Calculate the heat/thermal energy (Accumulated Degree Hour - ADH) required for each stage of development in the Green Bottle Fly’s life cycle.

Table 1.

From / To / Temperature / Hours / ADH
Egg / 1st instar / 70°F / 23 / 70 x 23 = 1610 ADH
70°F
70°F
70°F
70°F
  1. Determine whether each of the following is a constant or a variable in the experiment.

Table 2.

Constant / Variable
Life Cycle Stages
Temperature
Time between the life cycle stages
ADH

The time it takes to reach the different instars depends very much on climate – temperature, humidity, precipitation. The lower the temperature, the slower the blowfly will develop. The higher the temperature, the faster the blowfly will develop. Other factors include but are not limited to: indoor/outdoor exposure, sun exposure, burial or other obstructions: plastic sheets, water (things that hinder access and egg-laying by adult flys). Because of these factors, entomologists must be able to manipulate the ADH calculations to at least account for temperature.

  1. Using Table 1 as a reference, calculate the following. Show all work!
  1. For a maggot at the beginning of the second instar stage, how many hours does it take to reach the third instar if the ambient temperature is at 77°F?

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  1. If you are rearing a Green Bottle Fly pupa, at what temperature do you need to keep the pupa to have the adult fly merge in about 7 days?

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