Insects are ubiquitous in nature
so they occur all around us all the time, even if we don’t see them
and therefore they're very likely to be involved in a crime scene
whether just accidentally or actually because they have moved there
in response to some element of the crime scene.
Looking at the body itself it will be the pathologist
who would look at the changes in the body and try to determine
from the body what the post-mortem interval is.
But as an entomologist I can help the pathologist
by looking, not at the body, but at the insects that are feeding on and around the body.
More precisely we are determining a minimum post-mortem interval,
that's basically the period that the first insects found the body after death.
Ordinarily in the summer months in the UK with a body outdoors
it would be colonised by insects within 12 or 24 hours.
At the crime scene the main thing is recovery of evidence
and I’ll do that at the scene and then go back to the post-mortem
and carry on there if necessary.
We're interested not just on the insects on the body
but on insects that have been feeding on the body
and may already have dispersed away,
because it makes good sense for these insects, once they finish feeding,
not to hang around on the body because some other scavenger
may come and consume them as well as the body,
so they tend to disperse away.
The gold standard forensic indicators are blowflies and these are the ones that come in first,
immediately after the body's dead and starts to decompose.
They have an incredible sense of smell
and will arrive very, very quickly after death.
The female fly will lay between 150 to 200 eggs on the suitable substrate,
and a human body is absolutely ideal for that.
Within about 24 hours in the UK summer
those eggs will hatch and they'll hatch into first stage larvae,
and these larvae are tiny, 1.5mm long.
Those larvae, though, are very hungry,
they will crawl into the recesses of the body and start feeding.
Within 24 hours they themselves moult into the second stage maggot
and then about another 24 hours later,
those will moult into the final, third stage maggot.
And that will feed for about three or four days.
When it's finished feeding it disperses away from the body
and looks for somewhere to pupate,
usually somewhere dark
It begins to harden itself and darken itself until it forms a rugby ball shaped pupariam,
and inside that pupa the larval tissues are reassembled
through metamorphosis into an adult fly,
it’s a quite remarkable process that amazes me even today.
After about 10 days in the UK summer the adult fly is now ready to emerge.
And so the life cycle continues.
When blowflies respond to the odours of decomposition
they tend to navigate their way upwind through the odour plume
until they arrive at the scene.
They will then use their visual cues to land on the body
and to move around and look for the most suitable egg-laying site.
Eggs are very susceptible to desiccation so they tend to choose areas
where the humidity is higher so that tends to be the body orifices,
the eyes, the nose, the mouth,
and the genital/anal orifices if those are exposed.
And of course the humidity is maintained in those environments
but it also affords the larvae the opportunity to crawl into the body
and feed on the internal tissues,
which are less well protected from the insects' enzyme activity
than the hard, tough, outer skin that normally covers our bodies.
So the focus of blowfly colonisation tends to be on the head orifices.
In this image just coming up here we have a stillborn piglet.
All you can see at the moment is the hotspot and you can see a fly that’s just landed on it there.
Surprisingly, although they’re coldblooded, flies themselves generate quite a bit of heat
through their flight activity and muscle activity.
The pig outline now is formed because we’ve adjusted the temperature range
and you can see there’s a couple of flies moving around there.
We know they’re not the first flies to have found this piglet
because that hotspot there is in fact where there is already a small maggot mass developing.
And what you can do actually is you can take a series of images over a timescale from,
in this case it was from day one to fourteen, so a couple of weeks period.
In the first six days or so, not a lot’s happening because it takes a while for these first flies
that have come in on day one to lay their eggs, for the eggs to hatch.
- this was during a winter period.
But on day six the temperature starts to go up in the mouth of the pig
and then over successive days that temperature hotspot moves down through the body of the piglet
as its being consumed basically by a larval mass
which is just rolling from the front end to the back end of the piglet.
And then after two weeks the piglet is now completely cold and empty.
All of the maggots have finished feeding and they’ve moved away.
So one of the accessibility issues we’ve been exploring at the museum just in the last few months
is the impact of putting a body in a suitcase.
Is it possible for flies to find a body in a suitcase?
And if so, how quickly?
Well, we found that using pig heads as surrogate bodies from a butchers shop,
just pop them into a normal, carry-on suitcase
and they delay the arrival of the flies by one to three days or more,
depending on the temperature.
However, the flies will arrive at the suitcase,
and they know that there’s a body in there.
And they will actually oviposit through the zip,
which most suitcases are sealed with these days.
The eggs will be laid into the suitcase
and in fact if they're just scattered around the zip
the larvae themselves when they hatch can crawl and enter the suitcase.
So even hiding a body in a suitcase won’t stop flies finding it
and once they’ve found it they will feed on the body in there.
And in fact they will then try to actually exit the suitcase as well,
and they can do this often through the holes in the frame
where the wheels of the suitcase are found.
So there's a whole range of factors out there in the environment
and even in the human factors of the environment
that can complicate the situation
and make it a real challenge for a forensic entomologist
to engage in any investigation.