Flies of Forensic Importance: Blow Flies and Flesh Flies

Calliphoridae are medium to large flies, typically 6-16 mm in body length, with a metallic body coloration produced by structural iridescence in the cuticle. The family includes approximately 1,000 described species worldwide, of which a handful account for the majority of forensic casework in any given region. All are holometabolous. Developmental data sets, particularly for the most common temperate species, are among the most rigorously validated in applied entomology.

The wing venation shared by Calliphoridae is the first character for family placement: the costa is unbroken, the anal vein reaches the wing margin, and (critically) there is a calypter (a lobe at the wing base) that is large and prominent. Compared with Sarcophagidae, blow flies tend to be shorter and more compact. Compared with Muscidae, the metallic sheen is usually diagnostic at a glance.

• Lucilia sericata (common greenbottle): metallic green, pale basicosta, cosmopolitan in temperate regions. The most studied species for PMI work in Europe and North America. Arrives on fresh remains within minutes under ideal conditions.
• Calliphora vicina and C. vomitoria (bluebottles): metallic blue or blue-grey, orange or red facial jowls (parafacials). Common in cooler, shadier conditions. C. vicina colonises indoors as well as outdoors and is often found in winter when Lucilia is less active.
• Chrysomya megacephala and C. rufifacies: dominant first colonisers in tropical Asia, Australia, sub-Saharan Africa, and expanding into southern Europe. C. rufifacies larvae are predatory in later instars, preying on other fly larvae. Their presence can distort succession patterns.
• Phormia regina (black blow fly): blue-black with white or pale squama. Common in North America. Tolerates cooler temperatures and is sometimes recorded ovipositing on cool days when Lucilia is inactive.

Flesh flies are immediately distinguishable from blow flies by appearance. They are non-metallic, typically mid-grey, with three black longitudinal stripes on the thorax and a chequered (tessellated) pattern on the abdomen. The terminal segment of the abdomen, which contains the genitalia in males, is often red or orange. Most Sarcophagidae are 8-14 mm in length and somewhat stouter than Calliphoridae of similar size.

The most forensically significant genus in many regions is Sarcophaga (now subdivided into multiple genera in modern classifications, including Blaesoxipha, Neobellieria, and Ravinia among others). Species-level identification within Sarcophagidae relies heavily on male genitalia, making identification of females from external morphology unreliable without supplementary molecular data. This is a practical difficulty in casework: many of the individuals colonising remains are gravid females, and their identification requires either rearing to adult males or DNA barcoding.

In cool temperate regions, Sarcophagidae are frequently co-colonisers with Calliphoridae on fresh remains, though blow flies tend to dominate numerically. In dry or semi-arid environments, the proportion of Sarcophagidae relative to Calliphoridae can increase. In some desert regions, Sarcophagidae are the primary colonisers.

Muscidae is a large family of generally dull-coloured, non-metallic flies. Most members are grey or brown with four longitudinal thoracic stripes. They lack the metallic sheen of Calliphoridae and the striped-with-chequered pattern of Sarcophagidae. The most familiar member is Musca domestica (the common house fly), which is cosmopolitan and found on most forensic scenes with a human remains context.

In outdoor scenes, Muscidae typically arrive after Calliphoridae as the remains progress through the bloat stage. Hydrotaea leucostoma and Hydrotaea aenescens (dark muscids, sometimes called 'secondary screwworms' in older literature) colonise remains after active decay has progressed. Hydrotaea larvae are partially predatory and can consume other fly larvae, complicating succession interpretation.

In indoor scenes, Muscidae can be the dominant or even sole Diptera present, particularly if remains were enclosed and inaccessible to outdoor blow flies initially. The indoor succession is generally less predictable and more variable than the outdoor sequence, and species from Muscidae, Phoridae (humpback flies), and Sphaeroceridae (lesser dung flies) are proportionally more significant indoors.

The postmortem interval estimate produced from insect evidence is always a minimum estimate. The entomologist can say: the oldest larvae on this body require at least X days to reach their current stage at the recorded temperatures. But the oldest larvae are only present if the first colonisers arrived promptly after death. If there was a barrier, a wrapped body, an enclosed room, a sealed vehicle, the colonisation was delayed, and the insect evidence underestimates the true PMI.

This is why the concept of the colonisation-free interval (CFI) or access delay matters. In every case, the report must address whether remains were accessible to flying insects from the moment of death, or whether the evidence points to a delayed colonisation. Blow fly eggs or first instar larvae on a body found inside a car with windows closed, for instance, are evidence that the vehicle was opened at some point rather than that death occurred at the point of discovery.

Adult Calliphoridae can be identified using standard dichotomous keys with a hand lens or dissecting microscope. The characters used most often are body colour (metallic green vs. blue), facial coloration (jowl or parafacial colour), the basicosta (a structure at the wing base: pale yellow or black), the squama colour (a lobe under the wing), and bristle counts and positions. Greenberg and Kunich (2002) provide illustrated keys for North American species; Rognes (1991) covers the Calliphoridae of Fennoscandia and Denmark in detail. Keys for South and Southeast Asian species are distributed across journal literature and regional handbooks.

For larvae, the primary characters are body size, posterior spiracle slit number and peritreme completeness (instar determination), anterior spiracle lobe count (family determination), and cephalopharyngeal skeleton morphology (genus and sometimes species determination after KOH clearing and slide mounting). Third instar larvae of the most common Calliphoridae can generally be keyed to genus; species separation often requires rearing to adulthood.

• Adult key characters (Calliphoridae): basicosta colour, parafacial coloration, number of post-sutural dorsocentral and acrostichal bristles, arista plumosity, hind coxa hair patch.
• Larval key characters (Calliphoridae 3rd instar): anterior spiracle lobe count (5-8 for Calliphoridae vs 2 for Sarcophagidae vs 4 for Muscidae), posterior spiracle slit number (3), peritreme completeness, and cephalopharyngeal skeleton shape.
• Sarcophagidae identification: male genitalia are essential for species-level identification; females require molecular confirmation. Anterior spiracles with 2 lobes (lower count than Calliphoridae) is a useful larval family character.