Forensic Science International

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Chrysomya megacephala (Fab.) 12 intensity and relative humidity were also measured every night at the experimental site. The baits were put in the field duringĀ ...
Forensic Science International Further observations on the nocturnal oviposition behaviour of blow flies (Diptera: Calliphoridae) Devinder Singh"', Meenakshi Bharti

Calculating the age of immature stages of blow flies showing the longest period of association with a dead body often gives a fairly accurate estimate of the post-mortem interval (PMI). Determination of the exact time of oviposition by these flies had generally been made in the light of the conventional belief that blow flies are neither active nor do they lay eggs during night. This method of estimating the time of oviposition was modified when Greenberg [J. Med. Entomol. 27 (1990) 807] reported nocturnal oviposition by three calliphorid species that are occasionally used as forensic indicators. However, a technical problem with his experiment, having long term consequences, was placement of the bait on the ground among bushes. This could have made it possible for the flies already resting near the bait to climb over the piece of meat and lay eggs. Though Greenberg's experiment proves beyond any doubt that blow flies do lay eggs at night as well as by day, active attraction of these flies at night towards the oviposition medium had yet to be proved and the present experiment has been designed for this purpose. & 200 I Published by Elsevier Science Ireland Ltd.

It had generally been believed that blow flies do not oviposit at night [2-4] before Greenberg [I] reported that blow flies do lay eggs during the night as well. This observation has wide implications since it can alter the calculated minimum post-mortem interval (PMI) by several hours. Though Greenberg's experiment was meticulously planned, it did have one major shortcoming. In this experiment, he put the oviposition medium under the bushes on the ground. So, the flies could have crawled rather than fly to the piece of meat. Hence. one question still remains unanswered, i.e. can the flies lay eggs on a dead body after being attracted from some distance? To put it in a simple way, the point that needs investigation is whether the calliphorids fly actively during the period between sunset and sunrise. The present studies have been undertaken to find an answer to this question. Another reason for undertaking these investigations is the report by Tessmer et al. [5] that blow flies fail to lay eggs at night both in urban habitats with lighting and rural habitats without lighting.

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Frozen pieces of mutton were thawed and put in ziploc plastic bags before carrying them to the site of the experiment. The latter was an area in Patiala city without any streetlights or other sources of illumination. One such piece was put in a petri dish that was placed on a wooden platform fixed on the top of a pole 6 ft in height and fastened in the ground (Fig. I). A 9 in. broad band of sticky material (stikem special, manufactured by Seabright Laboratories, CA, USA) was made I ft above the ground to prevent the flies from climbing up the pole. The piece of meat was left on this platform from 10 p.m. to 3 a.m. daily for a week each during March and September, 1999. After exposure, each bait was placed in another ziploc bag along with wood shavings and its opening sealed. Minute perforations were made in this bag with the help of an entomological pin to allow the air to enter. These bags were brought in the laboratory and placed in empty fly cages at room temperature and observed for I month for the presence of fly maggots. The adults that emerged were counted after they died within the respective bags. Minimum and maximum temperature was measured at the site of the experiment for the duration of exposure. Light

200 I Published by Elsevier Science Ireland Ltd.

Table 1 Oviposition Serial no.

at night by three species of blow flies Date

Minimum-maximum

temperature I 2 3 4 5

14 15 17 10 12

March 1999 March 1999 March 1999 September 1999 Septem bel' 1999

16-23 18-24 18-24 23-26 24-27

CC)

Relative humidity (%)

Light intensity (Ix)

Species

75 78 80 75 85

0.8 0.8 0.7 0.6 00.7

Calliphora Chrvsomw Chrysomm Chr)'somya Chrysomya

intensity and relative humidity were also measured every night at the experimental site.

The baits were put in the field during 14-20 March and 6-12 September 1999. Oviposition OCCUlTedthrice during March and twice during September as has been summarised in Table I. On the other hand, baits put in during the day time

No. of adults reared vicina (Meigen) megacephala (Fab.) megaceplwla (Fab.) rufifacies (Macquart) megacephala (Fab.)

25 30 52 72 12

in the same period unfailingly attracted blow flies and eggs were laid in almost every trial within few hours. Calliphora vicina has been observed to visit the bait during day time in the March trial only. This experiment substantiates the report that calliphorid flies can lay eggs during the night time as well [I]. Three species of blow flies that have been used as forensic indicators by a number of workers have tendency for nocturnal oviposition. However, the probability of egg laying and the number of eggs laid was greatly reduced as compared to day time. It is a matter of common observation, as has also been seen during day trials that any exposed piece of meat will always attract blow flies within a few hours. This period can be in minutes or even in seconds [6]. In the 14 night trials undertaken during this study, oviposition occurred in five cases which incidentally matches Greenberg's [I] data of 33% success rate. Similarly, the number of eggs laid at night is remarkably reduced (Greenberg reported the emergence of a single adult of C. vicina from one of the baits). While determining the age of maggots, the possibility of night oviposition by blow flies should always be taken into consideration. Another question that emerges from this study and that needs to be investigated is the relationship between light intensity during the night and the probability of oviposition by blow flies.

The authors are thankful to Prof. B. Greenberg, University of Illinois at Chicago, for suggesting the present problem as well as for going through the manuscript. We are also grateful to Dr. K.G.Y. Smith and Dr. M. Leclercq for their scholarly review. Financial assistance provided by the Department of Science and Technology (GOI) for this research (Project No. SP/SO/C-30/97) is gratefully acknowledged.

[1J B. Greenberg, Nocturnal oviposition behaviour of blow flies (Diptera: Calliphoridae), J. Med. Entomol. 27 (1990) 807-810. [2] P. Nuorteva, Studies on the significance of flies in the

transmission of poliomyelitis. III. The composition of the blow fly fauna and the activity of the flies in relation to the weather during the epidemic season of poliomyelitis in South Finland, Ann. Entomo!. Fenn. 25 (1959) 121-136. [3] P. Nuorteva, Sarcosaprophagous insects as forensic indica-tors, in: e.G. Tedeschi, w.G. Eckert, L.G, Tedeschi (Eds.), Forensic Medicine: A Study in Trauma and Environmental Hazards, Saunders. Philadelphia, 1977, pp. 1072-1095. [4] B. Greenberg, Distribution and medical ecology of the blow

flies (Diptera: Calliphoridae) of Peru, Ann. Entomo!. Soc. Am. 78 (1985) 565-587. [5] lW. Tessmer, e.L. Meek, VL. Wright, Circadian patterns of oviposition by necrophilous flies (Diptera: Calliphoridae), Southwestern Entomo!. 24 (1995) 439-445. [6] G.D. De Jong, An annotated checklist of the Calliphoridae (Diptera) of Colorado. with notes or calTion associations and forensic importance, J. Kansas Entomo!. Soc. 67 (1995) 378-385.