Insects, arachnids, centipedes, millipedes and crustaceans: forensic arthropod biology associates these creepy crawly critters with death investigations; drawing together the human world after death and the dark, misunderstood but fascinating world of these animals (characterised by their common exoskeletons). In life, we’re desperately trying to draw the boundaries between these invertebrates and ourselves (exterminators and bug sprays), but there’s no separating these worlds when it’s too late.
In practice, entomology has three potential applications: urban, stored-product and medico-criminal. We’ll be focusing on the last of these, which covers evidence collected through arthropod studies at the crime scene. Fauna at the scene is highly variable — and any number of factors can affect what and how much is found there, including moisture levels, a cadaver’s proximity to bodies of water, sun and air exposure, as well as climate and geography.
Just how does a cutting-edge forensic entomologist control for these? Several new ground-breaking techniques have been developed to more accurately gather evidence or to give old data sets a new life. Read on to find out just how they work.
Through the body scanner: electron microscopy
It’s a basic fact: larvae and eggs in any insect’s life cycle are utilised to determine the post-mortem interval (or the time since death). How is an expert, then, supposed to figure out which specimens are forensically important? One technique involves the electron microscope, which creates images of a given sample by focusing a beam of electrons over it that an entomologist can examine. Using this device in conjunction with another technique dramatically improves a scientist’s chance at locating a sample with a distinct story to tell.
Up close, this method provides a look at morphological (features having to with the structure of an insect’s body) that can used to identify intriguing eggs and larvae. One downside? A microscope of this kind constitutes expensive equipment, and it can take a great deal of time to derive conclusions once the images are made available.
Premium potash: Potassium permanganate staining
Need a faster, low-cost solution? It might be slightly less accurate, but staining collected eggs with a salt referred to as Condy’s crystals is a viable method. Once the arthropod samples have been collected, the specimens are rinsed with saline (a salty) solution and placed in a glass petri dish. The dish is treated with a low-concentration solution containing the crystals for one minute, and then transferred onto a slide to observe.
They’re now visible, and can be checked under a light microscope to compare and examine various structural features. It’s here that a trained eye can determine which species are significant.
On your mother’s side: detecting mitochondrial DNA
In all species, mother’s play a special part in genetics — especially when you consider that there’s a type of DNA which only inherited from the maternal lineage, located in a cell’s mitochondria (the energy powerhouse). Studying this DNA helps entomologists differentiate easily between different blow-fly species. In cases where specimens do not have specialised morphologies at particular life stages, extracting and evaluating mitochondrial DNA helps determine what they are.
Making it up: mock crime scenes
Ever been party to a pig head burial? We have, more than once. In actual fact, it’s one of the most important tools forensic entomologists use to determine insect succession patterns on something that resembles human flesh closely enough. These days, it’s pretty common for experts without access to a fully tricked-out body farm facility to perform studies on pig carcasses that help them establish what decomposition ought to look like in their particular environments and in certain situations.
Another king of profiling: Gene expression studies
As in cases where using mitochondrial DNA might be important, there are issues which arise when physical characteristics are slightly difficult to pin down at certain life stages. Extracting a bit of DNA from specimens can allow genes to be sequenced, so that it’s simple for the the expert to determine what they’re analysing and to more easily specify the post-mortem interval.
Your Turn: Is there another technique within this field that we’re missing? Want to propose another method of evaluating arthropod evidence? Let us known in the comments. We’d like to hear from you.