The Insect Life Cycle

Reproduction in most insects involves two sexes. The female reproductive system is responsible for producing and storing eggs, providing eggs with nutrition, receiving and storing the sperm, fertilizing the eggs, and depositing the eggs into the environment. The male reproductive system is responsible for producing sperm and presenting it to the female for egg fertilization (Nation, 2011).

The primary forensically important insects are the Diptera, or the true flies. Since Diptera are generally the first insect to colonize animal remains, it is the dipteran life cycle that is most commonly used in forensic cases. The dipteran life cycle is therefore considered the most important life cycle to understand (Smith, 1986; Catts & Haskell, 2008; Nation, 2011). All flies start out as eggs that are usually laid in the environment, although some eggs remain in the body of the female and are placed in the environment as larvae (Mullen & Durden, 2002). The adult flies arrive at a dead animal and lay their eggs or larvae onto the natural bodily openings and wounds of that animal (Nation, 2011). The resulting larvae feed on the body until they are ready to pupate. Flies have what is called a complete life cycle, or metamorphosis: they begin as eggs, hatch into larvae, pupate, and eclose from the pupae into the adult stage. The larvae go through three stages, or instars, before they finish feeding on the body and move into the dispersal or wandering stage (Mullen & Durden, 2002). This dispersal stage takes the larvae 15-20 feet away from the carrion and to a protected place in which to pupate (Catts & Haskell, 2008). The dispersing larvae will crawl under things in the environment, or dig a few centimeters down into the soil, where the outer cuticle of the larva hardens into a protective shell, or pupal case. Inside this pupal casing enzymes transform the larvae into an adult (Gullan & Cranston, 2009).

Use of Entomological Evidence

The evaluation of entomological evidence at a scene has the potential to lend invaluable information about the scene and surrounding circumstances (Anderson et al., 1984; Braack & Retief, 1986;

Benecke, 2001; Watson & Carlton, 2005; Velasquez, 2008). The interpretation of entomological evidence can inform an investigator about many different aspects of the scene, including time of colonization (and by extension, time of death), season of colonization, location of colonization and potential movement or storage of the remains after death, evidence of neglect, sites of trauma on the remains, and presence of chemicals in the remains (Smith, 1986; Catts & Haskell, 2008; Byrd & Castner, 2010; Barnes & Gennard, 2011). This information may then inform several stages of the investigation.

Time of Colonization

On the morning of July 2, 2014, a family went into their backyard and noted their 6-month-old Labrador retriever puppy missing. The puppy had spent the night in a doghouse within the fenced yard. There was not apparent damage to the fencing, and no obvious means of escape. The body of the animal was found 2 days later, on July 4, 2014 at approximately 5:00 PM, located in a vacant lot near the home. The dog had been dismembered postmortem. A large number of fly larvae were present on the remains and were observed to be traversing the entire body. Fly larvae were identified as third instar Lucilia sericata. This species is common in the area where the animal was found and is expected during the summer months. The age of the collected larvae was estimated at 2 days, indicating that the larvae had colonized the body of the animal sometime on July 2, 2014. Since the puppy was observed as healthy the night before it disappeared, the flies likely colonized the dog just after death, putting the animal’s time of death sometime on July 2, the day it disappeared.

Insects reveal a great deal about a scene to a knowledgeable investigator. First and most sought after is the time of colonization (TOC) estimation, which is often associated with the postmortem interval (PMI), or time of death (Byrd & Castner, 2010). Many insects important in forensics are in the ecological group of decomposers—those organisms that utilize the nutrients bound up in dead matter (Price, 1997). Decomposers efficiently find and exploit dead matter, and those that rely upon ephemeral resources such as animal carcasses excel at resource location (Price, 1997; Cain et al., 2008). Insects may arrive at a newly dead animal within minutes after death (Hall, 1995; Catts & Haskell, 2008), and either feed upon the carcass directly or colonize the carcass through egg oviposition (Price, 1997; Byrd & Castner, 2010). Because of the efficiency of resource location, it may be assumed that an insect with unfettered access to a carcass arrived and colonized that carcass within minutes of animal death. Therefore, estimation of the age of the insects colonizing the animal body (TOC estimation) may be used as an indicator of how long that carcass has been available for insect colonization, and, by extension, how long that animal has been dead (PMI) (Byrd & Castner, 2010; Tomberlin et al., 2011).

Insect age estimation relies on their basic physiology. Insects are poikilothermic, meaning their growth and development is affected by ambient temperature (115). Lower ambient temperatures lead to slower insect growth, while higher ambient temperatures lead to faster insect growth. Each insect species has an upper and lower threshold, above or below which the insect no longer grows (Yang et al., 1995; Price, 1997). We can describe and predict this growth based on ambient temperature by using a mathematical formula (see section Calculating Time of Colonization) (Higley et al., 1986; Byrd & Castner, 2010; Michaud & Moreau, 2011).

Investigators are able to use the ability of insects to quickly locate and colonize carrion, and the entomologist’s ability to mathematically estimate insect age given ambient temperature, to estimate a PMI. However, there are some assumptions that must be made. If we assume that the insects arrived very soon after death and that their growth was dictated by ambient temperature, then we can calculate how long it would take an observed insect to reach an observed developmental stage at a given ambient temperature. This calculated time is the minimum PMI (Smith, 1986; Catts & Haskell, 2008; Byrd & Castner, 2010). If the exact arrival time of insects at carrion is unknown, however (i.e., before or after death the animal was blocked from insect activity, etc.), this calculated time is the time of colonization interval (Byrd & Castner, 2010; Tomberlin et al., 2011).