Phormia regina, the black blow fly, belongs to the blow fly family Calliphoridae and was first described by Johann Wilhelm Meigen. It is the only species in the genus Phormia. The black blow fly's wings are specialized with a sharp bend. These flies also have well-developed calypters. Blow flies generally are about the size of a house fly or a little larger, and many are metallic blue or green in color. Key characteristics of this species include black gena, mostly white calypters, and anterior thoracic spiracles that appear to be orange yellow due to being surrounded by bright orange setae. Phormia regina is especially important to forensic entomology. Female P. regina, like other blow flies in the family Calliphoridae, oviposit their eggs on carrion, where they hatch. The larvae develop through three instar stages until pupation. Adult black blow flies aggregate on feces in order to mate. The success of these mating interactions has been studied and appears to be related to size and diet of the adult flies. Following successful mating encounters, the adult females are then attracted to decaying material rather quickly for oviposition, allowing forensic entomologists to use development stages of larvae as a clue in determining an approximate time of death. P. regina are extremely common in the United States and other areas in North America. Combined with their importance in forensic entomology, these flies have been relatively well studied, particularly for variables affecting their development.
Taxonomy Phormia regina was described by the German entomologist Johann Wilhelm Meigen in 1826. Its specific epithet is derived from the Latin word regina meaning 'queen'.
Description
Phormia regina adults are metallic blue or green in color and have a distinctive set of orange setae near the anterior thoracic spiracle. They are also recognizable by their black gena (the side of the head below eye level), mostly white calypteres, and the distinctive bend in their wings.
Distribution Phormia regina is commonly found across the entirety of the United States as well as across other regions in North America
Habitat Phormia regina can be found on human corpses, animal carcasses, and feces. Researchers have observed these flies on a wide variety of animal excrement, but at least in the northwestern United States, they exhibit a preference for human, mink, and swine feces. P. regina is predominantly found in the northern United States during spring and summer months, but in the winter they are localized to more southern regions. This localization is tentatively linked to the importance of temperature on their development. These flies prefer rural areas, especially near water sources, as opposed to urban spaces—at least in the San Francisco Bay Area.
Life history The life cycle and development of Phormia regina is similar to that of most other Dipteran species, in which females oviposit their eggs onto a nutrient substrate. Then, after hatching, the larvae feed throughout three instar stages until they have stored up enough calories to commence pupation and finally emerge as adult blow flies. Each transition from first, second, and third instar is marked by a molt, and eventually the third-instar larvae develop sclerotized (hardened) casings which envelop and protect them throughout metamorphosis.
Larval development Relatively few studies have been conducted on the adults of this species in comparison to those on larval development, mostly due to the importance of blow fly larvae in determining the post mortem interval (PMI) of corpses during investigations by forensic entomologists. For this reason, many researchers have conducted experiments to investigate the effect of various environmental factors on the duration time of larval development in this species (as well as many others). Because the larval life cycle of this species is dependent on a climate with temperatures ranging from 12.7°C (55°F) to 35 °C (95 °F), it tends to inhabit the northern regions of the United States during summer months and southern regions in the winter. Researchers have discovered that at 40 to 45 °C, larval development occurs normally until the prepupal stage, at which point a majority of the larvae die. The few able to pupate at these high temperatures do not emerge as adults. The lowest temperature threshold for this species was found to be 12.5 °C below which females will not oviposit. The highest rate of development (with survival into adulthood) was seen at a constant temperature of 35 °C, where the average time of adult emergence was 265 hours (about 11 days). Flies at constant temperatures between 15 and 30 °C (at 5-degree increments) developed slower, with flies at the coolest temperatures taking the longest. Cyclic temperatures ranges of 25 to 35 °C and 15 to 25 °C proved to decrease the rate of development when compared to constant temperatures. (The cyclic temperature data were collected by placing specimens in an incubator which steadily alternated between the maximum and minimum temperatures of a particular 10-degree range (e.g. 25 to 35 °C). Each 10-degree fluctuation took place over a 12-hour span.) Also, studies have been conducted to assess the effects of light exposure on developmental variability in larvae. Larvae exposed to cyclic photoperiods (shifting intermittently between 12 hours of light to 12 hours of darkness) have higher rates of development than larvae exposed to constant photoperiods (24 hours of light per day). These findings suggest that darkness may be a stimulus for larval growth. However, these variations in light photoperiods failed to influence pupal duration times.
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