Effect of Temperature on Larvae Growth - Lab Report Example

Practical 3: Effect of temperature on larvae growth Abstract This experiment aimed at investigating the relationship between larvae growth and temperature. The various facts about forensic entomology have been discussed as a build up to this experiment. The experiment involved placing a constant number of larvae in beakers which were then placed in different temperature conditions. From the experiment it was found that the beakers that were under a mean temperature of 18 deg C recorded the shortest time of 1.5 day and the adults emerged. On the other hand at the average temperature of 6 deg C the longest time of 5 days was taken for the adult to merge. Introduction Forensic entomology also known as medico-legal is the study which is linked to human corpse in order to establish elapsed time since the time of occurrence of death (Goff, 2000). From the evidence gathered from insects it can be established whether a body has been moved to a second site after death or if there has been any form of disturbance of the body in due cause, either by wild animals or by the killer who returned to the scene of crime (Catts, 1990). However, forensic entomologists are usually interested in establishing the elapsed time since the occurrence of death. A forensic entomologist will be involved in cases that have occurred at least 72 hours or more. This is due to the fact that there are other forensic methods which are more accurate that obtained from the insect. For a time period of about three days, the evidence obtained from the insects that are associated with body, is found to be very accurate and in some cases it may be the only that can be used in the determination elapsed time since the occurrence of death or when the body was put into conducive conditions of decomposition (Arnoldos,2005). After a period of time, the eggs which are laid in batches on the corpse hatches into a first instar larva which feeds on the corpse and moults into second instar larva. With continued feeding of the larva there will be development of the third instar larva which is identified from the size and the number of spiracles (Goff, 2000; Blackith, 1990). In all this process the environmental temperature plays a very big role in determining the rate at which these processes take place. If the surrounding temperature goes lower than a specific point the growth activity may seize or even death may occur. The third instar will continue feeding for sometime then it stops and starts wandering away from the corpse and settle at the ideal place to pupate. From the pupae stage there is metamorphosis to the adult stage (Goff, 2000; Blackith, 1990). In investigations involving death, having an accurate estimate of the time of death or the time that has lapsed since the occurrence of death (PMI) is vital. The estimation of PMI is of great importance in investigating suspicious death as it will assist in the reconstruction of events and the circumstance under which the death occurred (Arnoldos,2005; Blackith, 1990 ). This may help in linking the suspect to the victim and also may help to establish how credible are the statements given by the witnesses. In addition to its importance in criminal cases it also applies for civil cases. The estimates always have implications in judiciary in matters of insurance and inheritance whether the death is due to natural cause, by accident of by suicide. There different methods which are put into use in order to give an estimate of PMI. Postmortem changes like settling of blood, the cooling of the body and the decomposition of the body are used by the pathologist when as a way to determine the time elapsed since (Goff, 2000; Blackith, 1990). It is possible to work out the specific base temperature for a particular species from a laboratory set up. This is done by monitoring the insect’s growth rate at some selected experimental temperatures. The calculations are then made bearing in mind that lower temperatures of growth will result in slow growth in the insects. By having a plot of temperature against 1/total number of days it is possible to establish the base temperatures. The total number of days is the time between when the larvae emerge from the egg and the point at which the adult emerges for the different range of temperatures. By extending the line graph to the x-axis this makes it possible read-off the x-axis. This point is the base temperature for the particular species and this method where the base temperature is established graphically is known as linear approximation estimation method. Method The base of the tank was covered with vermiculite and was moistened with precaution being taken to avoid flooding. 25ml of pig’s liver was added into the 100ml beaker with fluon being applied inside of the top 1 cm of the beaker. Into the beaker 10 first instar fly larvae were placed then the beaker was placed into the tank. The temperature reading-button was then placed into the tank and the waist of a pair of ladies tights were stretched over the top of the tank. 50% of each of the tights legs were cut off and tied into knots and then the tank was placed into a constant temperature site. These procedures were repeated for four other tanks with the tanks being placed in different temperature sites. The time taken for each of the population of flies to reach adulthood was recorded. Results The results from the experiment indicated that the growth of larvae to adulthood varied with the temperature of the chamber. The temperatures from the chambers were averaged to get the values in the table below. From the table it can be seen that the longest time it took for the larvae to develop to adulthood was 5 days and this was when the average temperature in the chamber was lowest at an average of 6 deg C while the shortest time was 1 day and a half and the average temperature recorded was 12 deg C. At the average temperature of 15 deg C it took 3 days for the adult to emerge while at 16 it took 2 days for the adult to emerge. Temperature 6 11 15 16 18 Number of days 5 4 3 2 1.5 1/Number of days 0.2 0.25 0.3 0.5 0.7 Discussion From the graph it is clear that that low temperatures will will make the period required for the larvea to grow to adulthood longer. It can also be observed that the base temperature is approximately 2.5 deg C. The base temperature is very important as it is used in the calculation of post moterm interval (PMI). The base temperature is a theoretical point where the temperature will cause lack of growth in the larvea. This point will vary from species to species. It will also vary even when dealing with the same species as a result of experimental errors. The growth rate at different temperatures can be very important when establishing the time a victim died when investigating crime. This can also help in establishing if the a body could have been moved from a different place to its present site. For example if a body is found in a freezing condition and yet it contains some dead larvea it could be a clear indication that it could have been transferred to its present location when it had started decomposing. References Goff, M. L. (2000). A fly for the prosecution. Haverd University Press. USBN 0-674-002200-2 Catts, E.P. and Haskell, N.H. (1990). Entomology & Death: A Procedure Guide. Joyces Print Shop, Inc. Clemon, South Carolina. USBN 0-9628696-0-0 Blackith, R.E. and Blackith, R.M. (1990). Insect Infestations in small corpses. Journal od Natural History 24:699-709 Goff, M.L. and Flynn, M.M (1991). Determination of Post Mortem Interval by arthropod succession: A case from the Hawaiian Islands. Journal of Forensic Sciences 36607-614 Arnoldos, M. I., Garcia, M. D., Romera, E., Presa, J.J and Luna, A. (2005). Estimation of post mortem interval in real cases based on experimentally obtained entomological evidence. Forensic Science International 149(1):57-65 Benecke, M. (2001). A brief history of forensic entomology. Forensic Science International. 120(1-2):2-14 Campobasso, C.P., Di Vella. G. and Introna, F. (2001). Factors affecting decomposition and Dipteran colonization. Forensic Science International 120:18-27 Read More