Insects make up more than half of the known animal species on our planet and they can be found in all kinds of habitat and feed on all kinds of nutrients. They can even be used in evidence in court cases. So we are talking about forensic entomology.

The work of the Forensic Entomology Service in the Department of Zoology and Animal Cell Biology at the University of the Basque Country (UPV-EHU) involves drawing up a census of insect species of forensic interest.

These are basically necrophagous diptera, i.e. flies that live on dead tissue, cadavers. Such flies detect a dead body in a question of minutes and at times at a distance of several kilometres. They colonise the cadaver and lay their eggs there. In a matter of hours the larvae are born and the new individuals begin to develop, feeding off the cadaver until they reach full size.

Fly larvae, once developed and when about to pass to the adult stage, abandon the dead body because their predators are around. This is why, by the time a crime is investigated, there may be no insect larvae. But there may be other insects in the area which can also provide clues.

After studies of development, succession studies have to be undertaken, i.e. we try to see what lands on the cadaver over the succeeding days. This is where things become quite complicated – especially in the Basque Country where there is scant knowledge about necrophagous insects that exist and that colonise dead bodies, or about what their cycle of developments are and their distribution patterns.

Colonisers of cadavers

The UPV-EHU researchers are working on identifying these colonisers of cadavers and their life and development cycles. The first task is fieldwork and setting traps that have bait to attract those insects of interest to the study (those that eat the remains of decomposing animal flesh). They place animal visceras — they smell strongly and attract a lot of these kinds of insects – as bait, mainly to attract female adults, these being the most difficult to identify. Males have features that are more evident, so the larvae from the females are allowed to develop and the resulting male individuals make the identification of the species that much more reliable.

Thus, in the case of collecting only females, they are kept in cages for a number of days until they lay eggs and these develop to produce a new generation, one which can be identified more precisely. Besides, having a population of males and females, they can be kept in order to continue to breed and lay eggs, thereby creating what is known as a breeding colony. The eggs are laid on a culture medium so that they eat and drink under conditions similar to that of a dead body, and a daily growth graph is drawn up. These growth graphs are carried out at various temperatures, given that the decomposition of a cadaver in winter is different from that in winter.

Moment of death

The team has described all the larval stages of some species. These are useful in determining the time of death of the body, given that, if the larvae are found in the body and knowing their species and corresponding growth table, a calculation of how long the person has been dead can be made. The research team is undertaking a reference collection of insects of forensic interest, including all their larval stages, and which will be a fundamental consultation source for future forensic research in the Basque Country. This is the basis of this forensic research.

With the studies carried out to date, the researchers at the UPV-EHU are getting to know which species are found here in the Basque Country. To date, they have undertaken sampling in the Leioa campus and in Bilbao and they have found that there are differences between insects that live in urban areas and those in natural zones. For example, species found at the Leioa campus but not in Bilbao and viceversa. This could be a significant piece of data to be taken into account if the crime is committed in one place and the body then transferred to another.