Determining the order in which insects colonize a dead body is a crucial aspect of forensic entomology. Forensic entomologists can estimate a postmortem interval (PMI) – the time elapsed since death – based on insect evidence. Knowing which insects arrive first provides clues about how long the body has been dead.
What are the first insects to arrive at a corpse?
Blow flies in the family Calliphoridae are often the first insects to detect, colonize and lay eggs on a dead body. Common blow fly species include the green bottle fly (Lucilia sericata) and the blue bottle fly (Calliphora vicina). Their keen sense of smell allows them to detect a corpse from over a mile away. Just minutes after death, blow flies are attracted to the body to lay hundreds of eggs in natural openings and wounds. Within hours, blow fly larvae (maggots) emerge and begin feeding on the body.
Other flies are also early colonizers. Flesh flies (Sarcophagidae), house flies (Muscidae) and cheese skipper flies (Piophilidae) may arrive shortly after blow flies. Meanwhile, predatory insects like wasps (Vespidae) and beetles (Silphidae and Staphylinidae families) may come to feed on the fly larvae.
Why do blow flies arrive so quickly?
Blow flies have specialized sensory organs and behaviors that enable them to rapidly locate and colonize carrion. Their antennae contain receptors to detect gases emitted at death, like cadaverine and putrescine. They can smell a corpse from over a mile away. Blow flies are also strongly attracted to the dark silhouette of a body. Once they land on the remains, blow flies use taste receptors on their feet to confirm it is a suitable oviposition site.
Female blow flies are programmed to lay their eggs on carrion. They can begin laying hundreds of eggs within minutes of finding a body. The eggs hatch in as little as 8 hours, giving blow flies a significant head start over later colonizers.
What factors affect colonization order and rate?
Several key factors affect the order and speed of insect succession on a corpse:
- Accessibility – How easy is it for insects to reach the remains? An exposed body will be colonized faster.
- Season – Blow flies and other flies are more abundant in warm months.
- Location – Urban areas provide earlier access than remote locations.
- Manner of death – Traumatic deaths attract insects sooner.
- Weather conditions – Hot, humid climates accelerate insect activity.
What is the general order of insect colonization?
While variation exists across regions and seasons, the generalized succession of insects on a corpse in temperate regions is:
- Blow flies – Within minutes if accessible
- Flesh flies – Within hours
- Wasps and beetles – Within hours or days
- Mites – After a few days
- Butterflies, moths and beetles – After 1-2 weeks
- Ants – After 2-6 weeks
Later colonizers feed on the corpse itself, the fly larvae, fungi, and predators and parasites of earlier insects. The diversity and succession of insects provides a timeline of the stages of decomposition.
How does colonization differ in aquatic environments?
On bodies submerged in water, blow flies are replaced by aquatic insects as early colonizers. Midges (Chironomidae) and caddisflies (Trichoptera) may arrive within hours to lay eggs. Mosquito larvae (Culicidae) and predacious diving beetles (Dytiscidae) follow shortly after. Aquatic insect colonization proceeds rapidly in warm, shallow water. In deep, cold water, insect activity is delayed by days or weeks.
How are insects used to estimate time since death?
Forensic entomologists use patterns of insect colonization and development to estimate a postmortem interval. Key evidence includes:
- Species and life stages present – Earlier colonizers indicate shorter PMI
- Degree of decomposition – Extensive feeding by larvae indicates longer PMI
- Larval development – Older larvae and pupae indicate longer PMI
- Seasonality – Presence of seasonal insects provides timeframe
These insect clues provide a timeline of the stages of decomposition and minimum PMI estimates. However, many factors can complicate estimates, and forensic entomology evidence is considered along with other findings.
What are some limitations of insect evidence?
Limitations of using insects to estimate PMI include:
- Influence of weather conditions on insect activity
- Lack of insect evidence in cold weather or if remains were concealed
- Difficulty determining colonization order if remains were moved
- Unknown variables affecting decomposition rate
- Errors in estimating larval development rate
Despite these limitations, analysis of insect colonization remains a valuable tool for estimating PMI. When used properly alongside other forensic techniques, insect evidence can provide important clues about time since death.
Conclusion
Blow flies are typically the first insects to colonize human remains, arriving within minutes after death if conditions allow access. Their rapid arrival provides entomologists with key evidence for estimating a minimum postmortem interval. While many factors affect insect activity, understanding colonization order provides useful clues about the timeline of decomposition. When integrated thoughtfully with other evidence, forensic entomology can yield valuable insights into the circumstances of death.
