Date published:

Age determination of insect pupae is important for forensics

Novák M., Frątczak-Łagiewska K., Mądra-Bielewicz A., Matuszewski S., Eye-background contrast as a quantitative marker for pupal age in a forensically important carrion beetle Necrodes littoralis L. (Silphidae), Scientific Reports 2020, 10: 14494.

Insect pupae sampled at a death scene may be used to estimate the post-mortem interval. For this purpose pupal age should be estimated, which is difficult as there are no good quantitative markers for the pupal age. In the article a novel method was proposed to estimate pupal age based on the quantification of contrast in intensity between the eyes of a pupa and the standard background. This was a transfer of methods developed to measure contrast in fingerprints to the field of forensic entomology. The intensity was measured on a standardized scale from 0 (perfect black) to 255 (perfect white) using computer graphical software and pictures of the eye and the background taken with a stereomicroscope. Eye-background contrast was calculated by subtracting the average intensity of the eye from the average intensity of the background. The method was developed and validated using pupae of Necrodes littoralis (Linnaeus, 1758) (Coleoptera: Silphidae), one of the most abundant beetle species on human cadavers in Central Europe. The authors found a gradual increase in eye-background contrast with pupal age (Figure 1). Changes almost ideally followed a generalized logistic function. Using the method pupal age was estimated with the average error of 8.1 accumulated degree-days (ADD, at base temperature for development of 9°C it corresponds to 15h in mean daily air temperature of 22°C). In 95% of cases errors were smaller than 20 ADD. While using the method, different raters attained similar accuracy. In conclusion, we have demonstrated that eye-background contrast may be used as a reliable approach to age insect pupae in forensic entomology.