Succession And Development Of Sarcosarprophagous Insects And Their Significances In Estimating PMI In Yangtze River Delta Region

Objective:It is important to infer the time of death or postmortem interval（PMI）through the succession and development of insects and the exact PMI should be based on the growth and development data of sarcosaphagous insects in the local region.It is one of the core works for forensic entomologists to study the succession of the regional insects on the corpse as much as possible.In this study,we observed the succession and development of sarcosaphagous insects in the corpses in Yangtze River Delta region in summer and autumn.Meanwhile,we investigated the growth and development law of Chrysomya megacephala（Fabricius）and Musca domestica L under different temperature conditions and built the PMI inference model,which provides the basis for forensic examination.Method:（1）In this study,the degree of corruption in the corpse was observed,recorded,photographed,and we performed the linear regression analysis according to the independent variable（the total body score,TBS）and dependent variable（accumulated degree days,ADD）.To collect eggs,larvae,pupae,adults,pupa shell and focus on monitoring the critical time points of arrival,spawning,hatching,prepuparial stage,pupation and feathering,then count the occurrence and succession of insects by the matrix per day.Jaccard similarity index was used to detect the species similarity between the corpes.（2）In this study,the larvae of Chrysomya megacephala（Fabricius）and Musca domestica L were cultured under seven constant humidity conditions,and the larvae length changed with the growth and development of insects and was measured under different constant temperature conditions.Non-linear regression analysis was used to simulate the formula for PMI inference and the isometric model（Isomegalen）from the larvae hatch to the feeding peak of insects.The time interval for different developmental markers（hatching,first molting,second molting,pupation and feathering）under different temperature conditions was measured.The isometric model（Isomorphen）is established by the data of the development milestones under different temperatures.The accumulated temperature and developmental thresholds were summarized by linear regression analysis according to the different developmental stages（egg,larval larvae,second instar larvae,third instar larvae and pupal stage）.Result:（1）The new model predicting accumulated degree days（ADD）from total body score（TBS）is a superior fit.In the succession survey,33 species of arthropods,belonging to 4 orders and 15 families,were found and most of them were found in three succession surveys.We found that the composition of arthropod in the corpses was basically the same in the same season.While the composition of phytophagous arthropods is different in summer and autumn,but the difference is not obvious.Chrysomya megacephala（Fabricius）,Lucilia sericata（Meigen）and Chrysomya rufifacies（Macquart）are the first wave to reach the body and the development process was a good indicator to infer the minimum postmortem interval,PMI_min.Ophyra leucostoma（Wiedemann）was later than above three kinds,and the beetles were followed,as the dominant species in the late stages of corruption and skeletonized remains.（2）The development time of Chrysomya megacephala（Fabricius）and Musca domestica L were shorten with the increase of temperature,suggesting that the developmental stages of the insects were significantly affected by the temperature.The larvae development rate increased with the increase of temperature,but the difference of developmental rate at each temperature was decreased when the temperature increased to28-34℃.According to these results,we generated an isomorphen diagram,thermal summation model and isomegalen diagram for Chrysomya megacephala（Fabricius）and Musca domestica L.Conclusions:For the first time,a more appropriate regression model to predict accumulated degree days（ADD）from total body score（TBS）is presented in the Yangtze River Delta region.It was also found that the seasonal distribution of phytophagous insect species in the Yangtze River Delta region and more reliable developmental data of Chrysomya megacephala（Fabricius）and Musca domestica L.The Isomorphe model,the Isomegalen model and the accumulated temperature model were established to estimation of the minimum postmortem interval(PMI_min)in forensic entomology.