Revisited larval morphology of Thanatophilus rugosus (Coleoptera: Silphidae)

Abstract

Determination of insect species and their instars, occurring on human remains, is important information that allows us to use insects for estimation of postmortem interval and detect possible manipulation with the body. However, larvae of many common species can be identified only by molecular methods, which is not always possible. The instar determination is even more challenging, and qualitative characters that would allow a more precise identification are mostly unknown. Thanatophilus rugosus (Linnaeus, 1758) is a common necrophagous beetle in the whole Palaearctic region from Europe to Japan. The species is often encountered on corpses of large vertebrates including humans, and its potential to become a useful bioindicator for forensic entomology is therefore high. Adults can be easily distinguished from other species; however, larvae were never thoroughly described to allow species and instar identification. The aim of this study was to provide reliable morphological characters that would allow species and instar identification of T. rugosus larvae. The material for morphological study was obtained from rearing under controlled conditions (20 °C and 12:12 h of light/dark period), and specimens that were not studied morphologically were allowed to complete their development. Quantitative and qualitative morphological characters for instar and species identification are described and illustrated. Additionally, we report observations of biology and developmental length for all stages of the species.

Metrological framework for selecting morphological characters to identify species and estimate developmental maturity of forensically significant insect specimens

“…Our observations on Thanatophilus larvae size and shape are the most detailed about beetles to date in the forensic entomology literature. Observations of immature insects in published studies were usually superficial [ 14 , 17 , 51 , 52 ], and these changes in size and shape were sometimes not even discussed, despite being visible in figures [ 53 ]. Additionally, average measurements at a given time point or long measurement intervals can disguise changes in shape or the magnitude of these changes.…”

“…The larvae of Afrotropical Thanatophilus species can be easily separated using categorical (sternite and urogomphus shape), meristic (thoracic setae), or continuous–quantitative (ratio of urogomphus length:sternite 10 length) features [ 11 ]. Although the length of the urogomphus showed overlap between instar–species combinations ( Figure 4 ), using a ratio to assess this character reduced the effect of allometric variation to an acceptable level [ 11 , 14 , 17 ]. Characters used in species determination are usually limited to categorical or meristic features, but it is actually the discreteness of a feature that is important.…”

“…The known larvae of Thanatophilus have fewer known qualitative morphological characters [ 11 , 14 , 17 , 32 , 40 , 41 ], and those characters which have been identified are currently only sufficient for identifying the larvae of closely related Thanatophilus species ( Figure 2 , Table 2 ). This provides a potentially interesting model to explore continuous quantitative characters that might be useful for identifying species and instar, using morphometrics.…”

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

“…There is just a single identification key for the larvae of beetles that colonize cadavers [166] and a single key for the adult carrion beetles (Silphidae) that frequent cadavers [167]. Although some descriptions of larval identification features have been published for forensically important species [168,169], this group needs more attention. Otherwise, we will still have to base our identifications on the taxonomic references that may be inaccessible to forensic entomologists with no experience in beetle taxonomy.…”

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

“…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 .…”