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Video reveals rapid changes in fly metamorphosis

An X-ray film of a fly pupa undergoing metamorphosis has, for the first time, captured minute-by-minute details of the most dramatic part of the reorganisation process.

The research by Museum scientists examines the metamorphosis of the bluebottle blow fly Calliphora vicina. This can't usually be seen because it occurs inside an opaque brown structure, the puparium.

The paper was published today in the Royal Society journal Open Science.

Watch a time-lapse video of the bluebottle blow fly's metamorphosis beginning at 28 hours after the puparium formed. It has been sped up 300 times and the counter shows minutes passing.

Lead author and forensic entomologist Dr Martin Hall says, 'The fly's astonishing transformation has never before been captured at such short time intervals.

'We have been able to visualise the timing of the major shape changes that the fly undergoes as it transforms from larval to adult form during the pupal stage. It is surprisingly quick.

'This new information is part of a wider study of the whole period of metamorphosis within the puparium. This will be useful in the forensic analysis of crime scenes and other situations where it is important to accurately determine a person's time of death from insect evidence.'

Female blow fly settled on a leaf. Female blow flies are quick to find dead bodies and lay their eggs on them.

Rapid transformation

Although the whole period spent in the puparium lasts about 10 days in the summer, the changes that visibly transform the organism from legless maggot to an adult with legs, wings and a head occur within just 1.25 hours.

In the first 24 hours of metamorphosis, much of the maggot's larval structures degenerate. The dramatic change then begins when an air bubble escapes from the pupa into the puparium, allowing space for the head to appear and for the adult framework to erupt from special cells (imaginal discs) within the insect.

'This very dramatic change shows the moment the framework for the fly is produced,' says Dr Hall. 'The rest of the pupation period is then spent fitting it out.'

Imaging the pupa

The video was produced in the Museum's Imaging and Analysis Centre. Using a micro-CT scanner, the researchers took X-rays of the pupa at one- and two-minute intervals, capturing the process in greater detail than had been done previously.

3D virtual reconstruction of a CT-scanned blow fly pupa

In total, seven pupae were imaged in this scientific paper and each showed comparable rates of transformation.

The technique could also be applied to other insects that undergo complete metamorphosis through a pupal stage - including butterflies, moths, beetles, bees and wasps.

Forensic entomology benefits

Forensic entomologists such as Dr Hall are interested in the lifecycle of C. vicina because they can help estimate a person's time of death.

Like other flies, Calliphora vicina goes through multiple life stages. The female adult lays eggs, which hatch into maggots (larvae) that pass through two moults. When mature, the larval skin (cuticle) contracts and hardens into a shiny brown puparium. Metamorphosis to adult fly occurs within this puparium.

The flies are one of the first organisms to find a dead body, which they then lay their eggs on. The resulting maggots and pupae develop at predictable rates according to the temperature, so the time when the flies first discovered the body can be calculated. This gives a minimum time since death.

The forensic entomology team at the Museum have been involved in more than 200 forensic cases to date. Calliphora vicina was the primary blow fly species recovered in most of these.

The new research is part of a wider study that will help determine the age of pupae found at a crime scene more accurately, thus improving the estimated time of death.