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A new approach to estimating the developmental stage of maggots turning into flies promises to help forensic entomologists with their criminal investigations.
The new method, devised by a team including researchers from the Museum, is less destructive, faster and more flexible than current techniques. It involves CT scanning the puparium - the hardened, opaque cover that encloses the pupa. For this approach, X-rays are used to build a 3D picture of otherwise hidden internal structures.
'We've shown that the images allow us to estimate the age of a blow fly to within a 10% time interval of the total period they spend in the puparium,' says Museum researcher Dr Daniel Martín-Vega, who worked on the study.
This means that for flies that spend ten days in a puparium, scientists can now calculate their age to within 24 hours. The current most popular method gives a less precise age and is more prone to subjective errors.
Blow flies such as bluebottles (eg Calliphora vicina) and greenbottles (eg Lucilia sericata) are used in criminal forensic investigations to help estimate the time of death, or more accurately the minimum post-mortem interval. These flies are among the first organisms to find a dead body, which they then lay their eggs on.
As the resulting maggots and pupae develop at predictable rates according to temperature, entomologists can calculate back to the time when the flies first discovered the body.
It takes a bluebottle roughly 19 days, depending on temperature, to develop from egg to adult fly. Over half that time can be spent within the puparium. Because the puparium is opaque brown, investigators can find it difficult to accurately estimate the developmental stage of the insect inside and thus its total age.
Current approaches are to break open the puparium and either visually inspect the specimen for general changes or take carefully prepared slices of the insect and examine them under a microscope. As well as being destructive, both methods have other disadvantages. The first is fast and cheap but not that accurate, while the second is more accurate but is tricky and takes a significant amount of time.
The new method offers a non-destructive way to look inside the puparium. A micro-CT scanner, similar to a hospital CT scanner, is used to build a 3D image of the developing insect.
As an insect changes from maggot to fly inside the puparium, it goes through clearly defined stages, such as developing a head, legs and wings. For a given temperature and species, the changes will always take place at the expected time.
The time-lapse video below shows the most dramatic stage of a bluebottle blow fly's metamorphosis inside the puparium. The video has been sped up 300 times and the counter shows the minutes passing.
To build a table of observable changes that can be used to estimate the age of a pupa, the researchers scanned groups of puparia that had been left to develop for different amounts of time.
Each group corresponded to a time slot between 0% (just formed the puparium) to 100% (about to emerge as a fly) of the total development time within the puparium.
The scientists were able to identify a set of characteristic changes that could be seen from the CT images, marking every step of development from 0-90% (differences were too small to see for the final 10%). For example, when the fly is 10-20% developed within the puparium, its head becomes visible.
This clear set of changes can be used to estimate the development stage of a blow fly pupa found at the scene of the crime, and thus the age of the insect, provided the temperature is known.
As the micro-CT scan is made up of a set of 3,000 images that build up to the 3D picture, specimens can also be virtually dissected. This allows investigators to accurately measure changes in the relative volumes of parts of the fly as it develops.
For example, the volume of the fly's wing muscles compared to its midgut alters in a predictable way over the course of development. This can be used as an additional quantitative check on the age of the pupa.
The researchers point out that the new method offers a number of advantages over the traditional techniques. Because the method is non-destructive, specimens can be scanned then used for other techniques. The method is also fast, particularly as a number of puparia can be scanned at the same time.
Although the researchers acknowledge that currently not all investigators have access to micro-CT scanners, this will likely change as more forensic uses are found for the technology.
'Our results confirm micro-CT as an emerging, powerful tool in medico-legal investigations,' concludes Dr Martín-Vega.