Research On The Community Succession Of Insects On Pig Carcasses In Shenzhen Area

Research backgroundForensic entomology is a discipline of using the community succession law of Sarcosaphagous insectss on the body to infer postmortem interval (PMI).The succession law of Sarcosaphagous insectss is a important method to infer the PMI of decomposed body. Each of the forensic workers often face death time of uncertainty, and the inference of PMI is very necessary, especially in criminal cases. Knowing the time of death plays an important role in case investigation and the criminal suspect screen.Currently the methods for PMI estimation are mainly based on early body phenomenon, the change of vitreous body composition, the degradation of intracellular nucleic acid degradation degree of body tissues and biochemical tests and study of sarcosaphagous insect community succession. In these methods, for the cause of death and the surrounding environment of different bodies and individual differences, there were some deviation or even error, especially for middle-late bodies of corruption, different degrees corruption could lead to more uncertain measurement. Because the observation index of a object are outside the corpse in forensic entomology, it is stable and objective to use these objective factors out of the corpse for inference PMI. With the depth of the study, we are more familiar with Sarcosaphagous insects and their life habits. Although Sarcosaphagous insects in different regions have different community succession, people have understanded the basic succession rule of Sarcosaphagous insects beacause of much successively study: The most began to appear in the early days of the cadaver corrupt is general wings flies, often reach the bodies in a few hours after the death. The most common is Calliphora, Lucilia and Sarcophaga etc; the most important is Calliphora Chrysomyia megacephala, Lucilia sericata etc.). These flies appear most early on the cadaver corrupt, then the flies larvae breed in the body with a large number, so the early inference of PMI mainly base on the flies and larvae. In the Middle corruption of cadaver corrupt, the number of early flies decrease, the number of Piophila began to increase, and appear a lot of coleoptera insects (such as ground beetle, hister beetle etc.). These insects mainly belong to predatory insect, feeding on larvae of early flies. In the latter of corruption or drying, there can be see staphylinid and sap beetle which is predaceous insects, also, Stratiomyidae insects and the larvae of the dermestidae often can be seen. There can be see Fabricius and Scarabaeidae in the late of corruption, which are Saprophagous Scarab. After drying period, besides some scattered individual flies, there can not be see the early main flies, sometimes can be see a little flies and beetle larvae, fly pupae around the cadaver corrupt. The ants can always appear in the whole process of the cadaver corrupt.According to the experimental research of researchers in our country, in the southern of China, the main corpse flies are Chrysomyia megacephala, Achoetandrus rufifacies, Ophyra nigra and Chrysomya chan, the main corpse beetle are Creophilus maxillosus, Saprinus splendens, Necrobia ruficollis, Necrobia rufipes, Dermestes maculate etc. the main corpse tabanidae is Hermetia illucens L. and Ceratopogonidae etc. In addition, there are some predatory insects by resin insects, Such as the hornets, wasps and ants. These insects have a series of growth and development succession in cadaver corrupt environment, The method of experimental observation time point according to record these insects mainly appear in the succession process in which they appear some characteristic of the key point in time, thus describe the corruption of the cadaver corrupt time period to time period.Shenzhen is one of an important city in south China, it is also a big immigrants city, the population is mixed, and more criminal cases occurred. Implications for PMI become more important for the investigation of criminal cases. Now, There is no correlation research about Sarcosaphagous insects community succession in this area, so it is very necessary to study the succession of sarcosaphagous insects as soon as possible.ObjectiveGiven all that, here we observed and studied the Sarcosaphagous insects community succession of Shenzhen area, and the purpose can be summarized as the following:1. To obeserve the main indoor and outdoor pigs corpse in summer in Shenzhen area’s Sarcosaphagous insects species and community succession rule, and compare the difference.2. To compare the difference of Sarcosaphagous insects community succession on corpse in high rainfall environment and dryness condition.3. To compare the differnce of Sarcosaphagous insects community succession in different experimental material (such as adult pig carcasses, young pig carcasses and rabbit corpse).4. To investigate the role of Chrysomya megacephala in the process of body corruption all the year round, and get a more reliable observation index for PMI inference in this area.Method 1. The selection of experimental animals and the experiment siteAdult pigs,which of weight close to the human body were choosed to be experimental animals, hammering the pig head to death of closed injuries with a hammer. Experiment site was selected in the suburbs of Shenzhen (about37’, north latitude22°east longitude114°09’), on the grass of the center of Shenzhen city public security bureau of forensic examination and a surrounding old floor to simulate the indoor and outdoor environment. The experiments started on May6and last for45days, then reapeated the experiments on June26in another similar environment. An adult pig carcasses and young pig carcasses were placed in similar outdoor enviroment for experiments on September21and on November6seperately. Four rabbit corpse were average placed outdoor in Shenzhen area on January5th and on March12th for experiments, respectively to simulate winter, spring outdoor environment.2. Experimental observation and recordWithin3days of the beginning place pig carcasses, observed four to five times a day, photo shooting, and recorded. Observation time of death, the temperature and humidity, Fresh stage, bloated stage, decay stage and the post-decay stage of pig carasses were observated and recorded. Insect appear in the pig carcasses and its quantity and time in some important point time of growth and development, respectively is:T1(the time of adult appeared); T2(the time of a lot of larvae appeared); T3(the time of a lot larvae from food); T4(the time of large quantitative larvae pupa); T5(the time of a lot of large quantitative pupated). Collected the first Daikin fly larvae larvae, and measured the length of the worm in the PMI.3. Insect specimens and photographCaptured all kinds of insects which appeared at the scene with nets as much as possible, in addition to capture the adult larvae to capture. The adult larvaes poisoned with ethyl acetate, and dried in the sun, then were made of insect specimen with a needle in a specinmen box. Beetle larva poisoned with ethyl acetate and inserted into specimens by the needle, the larvae of flies poisoned with XA solution (xylene ethanol=1:1) to make the body straightened and convenient to measure the length of poisoning after preserved in75%alcohol. In addition, a part of adult insects were stored in75%alcohol for DNA detection. Screened the activities of the Sarcosaphagous insects, maked a good insect specimens by stereoscopic photographs, then maked forensic entomology of Shenzhen area.4. The experimental index of recorded dataThe several experiment log data of index as the following:a. carcasses of corruption through various stages of the calendarb. the temperature and humidity of a day in the experiments of, and weather conditionsc. the main Sarcosaphagous insect arrived in the timing of the carcasses and its growth and development stages of the key point in timed. the type and the number of Sarcosaphagous insects appeared on the carcasses every day.e. in corresponding observation point in time, collected bulk gold larva and made into specimens, used a vermiercalipers measured the length of the specimens and compared the difference.Results1. The indoor and outdoor pig carcasses corruption would experience four consecutive period in the summer of Shenzhen area, the period of indoor carcasses corruption was slower than outdoors’. Outdoor carcasses corruption experienced each phase were20.5±2.5h,51.5±0.5h,100±11h and127±17h; Indoor carcasses corruption experienced each phase were32.5±5.5h、67.5±4.5h、 89.5±9.5h�'�200.5±16.5h. The four stage of outdoor carcasses corruption in spring, summer, autumn, winter in Shenzhen area respectively were45h、46h、127h、139h;21h、52h、100h、127h;25h、71h、94h、115h;96h、125h、173h、287h。2. There were4orders,16species of10families of mainly Sarcosaphagous insects in the summer of Shenzhen area.The most representative species were eight species:Chrysomya megacephala, Achoetand rusrufifacies, Chrysomya chain, Hydrotaea spinigera, Creophilus maxillosus, Saprinus splendens, Necrobia ruficollis, Dermestes maculate (Refer to appendix)3. The changes of the length of Chrysomyia megacephala on interior pig carcasses with the time of death has certain rules, and could be modeled as a binomial equation:y=0.0009x2+0.3676x0.3676(R2=0.9808) and y=0.0002x2+0.1288x0.1288(R2=0.965). The changes of the length of Chrysomyia megacephala on outdoor pig carcasses with the time of death has certain rules, and could be modeled as a binomial equation:y=-0.0014x2+0.3166x-4.4998(R2=0.9701) and y=-0.0009x2+0.3055x-10.373(R2=0.9921);The changes of the length of Chrysomyia megacephala on interior pig carcasses with the time of death in each season of this area could be modeled as a binomial equation: y=-0.0002x2+0.1475x-0.925(R2=0.9893); y=-0.0014x2+0.3166x-4.4998(R2=0.9701); y=-0.0004x2+0.1878x-1.1079(R2=0.9878); y=0.0004x2-0.0803x+4.3670(R2=0.9949).4. The key time points(T1, T2, T3, T4, T5)of Indoor pig corpse Chrysomya megacephala were52.5±13.4h、77.5±12.0h、146±7.1h、182.5±9.2h、258.5±2.1h, respectively. Among them,the key time points(T1, T2, T3, T4, T5)of Achaean rusrufifacies were55.5±14.9h、83.5±14.9h、192.5±27.6h、220.5±24.8h、312±28.3h and the key time points(T1, T2, T3, T4, T5)of Chrysomya chain were 83.5±14.9h、240.5±21.9h、398.5±16.3h、404.5±16.3h、475±9.9h, respectively. The key time points(T1, T2, T3, T4, T5)of outdoor pig corpse Chrysomya megacephala were18.5±2.1h、40±2.8h、112.5±3.5h、136±1.4h、241.5±4.9h, respectively. Among them,the key time points(T1, T2, T3, T4, T5)of Achaean rusrufifacies were19.5±0.7h、44±2.8h、174±11.3h.201.5±7.8h、277.5±12.0h and the key time points(T1, T2, T3, T4, T5)of Chrysomya chain were56.5±6.5h、227±15.6h、330±28.3h、337±29.7h.388.5±14.9h, respectively. The key time points(T1, T2, T3, T4, T5)of growth and development of Chrysomya megacephala outdoor in each season of this area were4h、45h、141h、170h、309h;2h、69h、134h、162h、278h;2h、50h、124h、172h、311h;144h、240h、312h、343h、437h, respectively.5. The indoor and outdoor insect community succession on pig carcasses were different. Beacause the outdoor pig carcasses affected by the rain, the number of flies and maggots were much more than indoor’s, and it was affected by the rain. There were fewer beetles appeared on the pig carcasses,and beetles were difficult to breed around the pig carcasses. On the contrary, the indoor pig carcasses less affected by the rain, there were more flies and maggots in the early of corruption, and there were mainly beetles and their larvae in the late.6. Because of much rain in the summer of Shenzhen area, the Chrysomya megacephala, Achoetand rusrufifacies and Chrysomya chain on outdoor pig carcasses have produced the second generation; But other insects didn’t produce the second genration except the chrysomya chain producing less on indoor pig carcasses. Other seasons had less rain, the flies on the pig carcasses didn’t produce the second generation.7. There were significant differences in the community succession corruption duration of sarcosaphagous insects in adult pig carcasses, young pig carcasses and rabbit carcasses. The number of eosinophils corpse insect in adult pig carcasses was significantly more than in the young pig carcasses, while the young pig carcasses had more beetles than rabbit corpses, the rabbit corpses corruption duration was short,and had less beetle in the late.8. There was no reported about the gold soldier fly individual insects which was observered in this experiment before.ConclusionBased on the observational study of Sarcosaphagous insects community succession regularity on pig carcasses between indoor and outdoor in summer, outdoor in each season in Shenzhen area, we could draw the following conclusion:1. The results of the main species of Sarcosaphagous insects in the summer of Shenzhen area were similar with other research results in southern China, but there was some difference about some key points between this stuy with previous studies, which may consider to the difference of area and experimental environment.2. The length of the larva of the Chrysomya megacephala show regular change with time, the change could be used as a region index to infer PMI of Shenzhen.3. Sarcosaphagous insects activities as a whole had a certain regularity: flies and their larvae were the main insects in the early of corruption, beetles, black soldier fly and their larvae accounted for the major in the late.4. Because of much rain in the summer of Shenzhen area, the Chrysomya megacephala, Achoetand rusrufifacies and Chrysomya chain could produced the second generation on outdoor pig carcasses; but they could not hardly produced the second genreation in the indoor enviroment and autumn outdoor enviroment; Theoutdoor environment with much rain is not conducive to sarcosaprophagous beetlesactivities and reproduction.5. There were significant difference of corruption duration and insect community succession in the adult pig carcasses, youth pig carcasses and rabbit corpses. Rabbit corpses had a shorter corruption stage than pig carcasses, the Sarcosaphagous insects community succession was correspondingly simpler and fewer than the adult pig carcasses.