Blow flies biology (Chrysomya megacephala)

Among all the insects that attracted by the corpse to feed, to copulate or to

oviposit, primary flies like blow flies (Calliphoridae) and flesh flies (Sarcophaga)

are always the major members, and theirs larvae can be used to estimate the

minimum PMI (Catts and Goff, 1992). Blow flies have the potential to discover

the corpse in a very short period of time after the corpse been exposed. For

instance, it has been shown that blow flies can arrive the corpse few hours after

death in the earlier research (Catts and Goff, 1992). Another research showed

that blow flies can arrive the corpse within minutes (Byrd and Castner, 2009). In

contrast, there is a problem when using flesh flies to estimate minimum PMI.

Flesh flies usually have precocious development (Villet et al., 2011). Usually when the

oviposition is about to happen, the well-developed eggs will slide through the oviduct and

fertilized when it go through the opening of spermathecal duct, then the fertilized,

well-developed eggs will be laid on the media (Smith, 1986). However, sometimes the female

adults cannot find a suitable media to oviposit right after the eggs have been well

developed. In this scenario, eggs been pushed through the oviduct and fertilized, but not

yet be laid on the media, cause the eggs started the embryonic development inside the

females abdomen. Therefore, it may result in the eggs hatched right after eggs been lay.

These may cause the overestimate of PMI (Goff, 2010). Fortunately, the precocious

development is not common in Calliphoridae species (Erzinclioglu, 1990).

The experimental subject C. megacephala (Fabricius) belongs to Diptera,

Calliphoridae, and Chrysomyinae. It originally distributed in Oriental and Australia

region (Smith, 1986), but now could be found throughout the South America, South

Africa and southern United State. They play as an important role in the forensic

entomology and used for minimum PMI estimation (Byrd and Castner, 2009). Chrysomya

megacephala was also known as the oriental latrine fly, body size are from 8.0–10.0 mm,

colors are blue or green with metallic luster (Byrd and Castner, 2009). Chrysomya

megacephala have red compound eyes, holoptic eye in male which have narrow frons.

Female have dichoptic eyes which their frons is much wider. Ommatidia arrange pattern

is different from the top to the bottom of the compound eye, form an obvious boundary

in the middle of the compound eye, which can easily be recognized (Byrd and Castner,

2009). Gena is yellow and covered with yellow hair. Chrysomya megacephala life cycle

is about one month in the wild, however, it usually can survive 2 months in the laboratory.

Larva stage have three instars. When eggs was laid, it usually would hatch within a day,

the complete larva stage is 5.4 days, and 5.3 days in pupa stage when it in the condition

of 27℃ and 60-70% R.H. (Gabre et al., 2005). Before pupation, it will go through the

post-feeding stage. In the post-feeding stage, maggots cease feeding behavior, and

disperse from the origin food resources for pupation. Egg batch produced by C.

megacephala is averaging 223.7 eggs each time. After eclosion from the pupa, C.

megacephala will take 6.6 days to reach sexual maturation and to be ready to oviposit

(Gabre et al., 2005). Mating behavior will happen after ovarian maturation (Norris, 1965).

In our anatomical observation, female blow flies ovaries maturation are synchronized,

and usually lay whole batch mature eggs when oviposit, this observation also meets

Browne’s description (Browne, 1993).

Chrysomya megacephala is a dominant necrophagous species in Taiwan, it can be

attracted to the corpse soon after exposure. Chrysomya megacephala use their third

segment of aristate antennae to receive the odor (Erzinclioglu, 1996), and it will use their

olfactory sensory to track and search the target in long distant. When C. megacephala

getting closer, visual ability replaced to ensure the location of the oviposition media

(Gomes et al., 2007). Mouth, nostril and anal, these natural orifices are the preliminary

targets for gravid female to oviposit. Besides the wounds, those natural orifices are the

only way to invade inside the corpse, and usually are soft and moisture enough for first

instar larvae to feed. When suitable location has been find, female will stretch their

ovipositor to reach inside the hole, or folds around the body to oviposit, and start the clock

of the succession.

In the recent studies, blow flies C. megacephala was found to have

gregarious oviposition behavior which have tendency to lay eggs together and

form an egg bunch, and gregarious effects do not need to be triggered by all

females individuals or by conspecific individual (Yang and Shiao, 2014). They

found female adults seldom oviposit when they are the only individual that

expose to the oviposition media. However, when they used 1 female with 9 males

in the same cage, and the single female could laid eggs.

Many factors will affect blow fly oviposition, including solar exposure, wind, heavy

rainfall, temperature, etc. (Amendt et al., 2004). Many study showed that blow fly would

not oviposit at night (Payne, 1965), however, nocturnal oviposition was still a

controversial issue for many years (Greenberg, 1990; Singh and Bharti, 2001). Besides

those environmental factors which will influence blow fly activities and oviposition

behavior, oviposition media condition will also affect blow fly oviposition. Yang and

Shiao (2012) provide different condition of pork livers as an oviposition media,

to test the blow fly of C. megacephala and C. rufifacies preference, their results

indicate that blow fly oviposition behavior will affected by media size,

conspecific or heterospecific larvae presence on the media. This study also

indicated that visual ability plays an important role in locate media location and

search for the potential competitors.

There were several researches which focused on the media status affect blow

flies oviposition behavior. Blow fly Calliphora vomitoria have shown preference

on the larger carcass, and oviposition behavior will be inhibited when oviposition

media is dehydrate (Erzinclioglu, 1996). The moisture condition of the media is

also important in maggots’ survivorship. When blow fly oviposit on a dehydrated

media, it may cause eggs to dehydrate, change and damage the shape of the

chorions, cause them difficult to hatch (Davies, 1950). Dehydration of the media will

also cause maggots dehydrated, in addition, if the water content of the media were too

high, maggots may drown in the media (Erzinclioglu, 1996). However, most of the blow

fly species can recognize the dehydrated layer on the oviposition media

(Campobasso et al., 2001).

Previous study showed that when facing nutrient depleted media or very

aged (8-day-old) media, blow fly and flesh fly have lower colonization rate

compared to that on the fresh media (George et al., 2012), however, there was no

significant difference between the specimen number on the aged (4-day-old) and

the flesh media. In their study, only aged (4-day-old) and very aged (8-day-old)

media had been tested, the tendency of colonization along the different

decay-aged media had not been tested. The effect of media older than 4-day-old on the

oviposition behavior of C. megacephala remains unknown. In order to know the

preference of C. megacephala in different decay-aged media, the sequential data

of different decay-aged media is needed. In this study, we are going to use C.

megacephala to test the tendency of oviposition preference along with different

decay-aged media, and also to known the possible duration of time differences in

oviposition happened on different decay-aged media (acceptance phase). The

changes of time interval in acceptance phase which may break the assumption of

“blow flies can reach the corpse and oviposit soon after victim’s death” and could

influence the minimum PMI estimation.