Eye-background contrast as a quantitative marker for pupal age in a forensically important carrion beetle Necrodes littoralis L. (Silphidae)

Abstract

insect pupae sampled at a death scene may be used to estimate the post-mortem interval. the pupal age is however difficult to estimate, as there are no good quantitative markers for the age of a pupa. We present a novel method for pupal age estimation based on the quantification of contrast in intensity between the eyes of a pupa and the middle grey photography card as a standard background. The intensity is 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 is 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. To develop the model, pupae were reared in 17, 20 and 23 °C, with a total of 120 specimens. The method was validated by three raters, using in total 182 pupae reared in 15, 17, 20, 23 and 25 °C. We found a gradual increase in eye-background contrast with pupal age. Changes followed generalized logistic function, with almost perfect fit of the model. Using our method pupal age was estimated with the average error of 8.1 accumulated degree-days (ADD). The largest error was 27.8 ADD and 95% of age estimates had errors smaller than 20 ADD. While using the method, different raters attained similar accuracy. In conclusion, we have demonstrated that eye-background contrast is a good quantitative marker for the age of N. littoralis pupae. contrast measurements gave accurate estimates for pupal age. our method is thus proven to be a candidate for a reliable approach to age insect pupae in forensic entomology. Insects visiting human cadavers are used for postmortem interval (PMI) estimation 1,2. Most frequently, forensic entomologists use insect age as a basis for the minimum PMI estimation 3,4. However, estimating the age of some insect life stages may be problematic. The pupa, for example, lacks reliable and easy to use quantitative age markers. This is particularly important, as a considerable portion of premature development takes place at this stage, at least 50% in the case of flies 1,5-7 and at least 30% in the case of beetles 8-11. Pupae are frequently killed and preserved in alcohol during insect collection at a death scene. Various methods have been proposed to estimate the age for such specimens, mostly for puparia of forensically relevant blow flies. It has been suggested to analyze changes in cuticle composition 12-14 , gene expression 15-23 or morphology and anatomy 24-33. All these approaches have some advantages, which, however, are usually overshadowed by drawbacks, i.e. qualitative markers for age, changes of markers covering only a small portion of the pupal stage,

Temperature-Dependent Development of Nitidula rufipes (Linnaeus, 1767) (Coleoptera: Nitidulidae) and Its Significance in Estimating Minimum Postmortem Interval

“…The pupal stage of the beetle also accounts for a large part of the immature stage, but, compared with the obvious morphological changes in the pupa of flies [ 109 , 110 ], the morphological changes of most beetles, including N. rufipes , are very slight. The study of Novak et al [ 111 ] on the compound eyes of Necrodes littoralis (Linnaeus, 1758) helps to improve the ability of Silphidae evidence to estimate the PMI min . In this study, the color changes of the compound eyes and mouthparts can be used as two important indicators for the preliminary estimation of the pupal maturity of N. rufipes .…”

Post-Mortem Interval Estimation Based on Insect Evidence: Current Challenges

“…Intra-puparial development has been documented for many forensically important species [35,65,78,[170][171][172][173][174][175]. Although these techniques have obvious advantages (e.g., they cover most of the intra-puparial development, they are generally non-destructive, low-cost and they need a stereomicroscope only), they have also important disadvantages (e.g., they are qualitative in nature and therefore less accurate and they are also impractical due to the need to have an expert knowledge in the intra-puparial morphology) [82]. Recently, a simple-to-use technique has been developed for aging pupae of the carrion beetle Necrodes littoralis by means of the quantification of the eye-background contrast, with very encouraging results of the initial validation [82].…”

The complete mitochondrial genome of a corpse related necrophagous beetle, Necrodes littoralis (Coleoptera: Silphidae)

“…And it frequently colonizes human carcasses in the later stage of decomposition, especially in forest environments (Matuszewski et al 2011). In the recent years, N. littoralis have been studied for determining PMI, including the calculation of the developmental data of the larvae and pupae (Frątczak and Matuszewski 2014;Nov ak et al 2020), but only little genetic information can be available. Therefore, this study provided the complete mitochondrial genome (mitogenome) of N. littoralis.…”