Instar determination in forensically useful beetles Necrodes littoralis (Silphidae) and Creophilus maxillosus (Staphylinidae).

In order to estimate postmortem interval from immature insects, it is necessary to accurately determine which instars are present in a corpse sample. Unfortunately, most forensically useful beetles lack morphological features specific for particular instars, and the only way to distinguish larval instars of particular species is to measure their size. The aim of this study was to test which measurements are useful for instar determination of Necrodes littoralis (Silphidae) and Creophilus maxillosus (Staphylinidae) and how to combine them to get accurate and easy to use instar classifier. Six morphological features were measured: distance between dorsal stemmata, width of the pronotum, length of the body, width of the mesonotum, width of the eighth abdominal tergite and length of the first segment of urogomphus. Linear discriminant analysis (LDA) was used to create and validate classifiers. Validation was made with fully sclerotized larvae and larvae just after ecdysis (not fully sclerotized and coloured). All the features were found to be useful for instar determination. The most useful features were the width of the mesonotum and the distance between dorsal stemmata. Complete classifiers (the ones incorporating all features) assigned larvae to instars with no misclassifications, unless larval specimens were just after ecdysis. Even in the case of larvae just after ecdysis complete classifiers were highly effective, although some third instars of C. maxillosus were misclassified as second instars. Simple classifiers (the ones incorporating only two, the best features) performed similarly well with fully sclerotized larvae, but in the case of larvae just after ecdysis they revealed higher misclassification rate than complete classifiers. These results indicate that measurement of any highly sclerotized larval structure of N. littoralis and C. maxillosus may be useful for instar determination. They also show that fully sclerotized larvae may be accurately classified according to instar from just two measurements, whereas larvae just after ecdysis will require more measurements.