Identifying Blow Flies - Part 1

Olga Sivell currently studies at the Jill Dando Institute of Security and Crime Science, University College London carrying out research in Forensic Entomology. Her current project is on spatial and temporal distribution of Calliphoridae in Britain. She works at the Natural History Museum, London as a Research Assistant on the Darwin Tree of Life Project.

Why Study Blow Flies?

During the last year I have been putting the finishing touches to a new identification guide to blow flies in Britain. Finally, it was published in March 2021 in the Royal Entomological Society Handbooks series. But why blow flies? And why now?

Despite initial disgust, many people recognise the importance of blow flies:

• Indeed, they have a long and interesting cultural history, for example as a symbol of the devil and death in paintings, literature and Christian iconography.
• In forensic entomology, a number of species help with crime investigations.
• Their larvae feed on carcasses and can be used to estimate the post-mortem interval.
• Some blow flies cause myiasis –when larvae feed on live hosts, such as people, sheep (‘sheep-strike’) and other vertebrates.
• Others parasitise earthworms or snails.
• The larvae of some species are used for cleaning wounds.
• They are also important decomposers.
• Adult flies pollinate plants while feeding on their flowers.
• And of course they can spread disease.

Since 2015 I have been running the Calliphoridae, Polleniidae and Rhiniidae Recording Scheme, with the aim of mapping species distributions and investigating their seasonality. Over that time I have seen a growing interest in flies among the general public. Blow fly records have been gathered from both enthusiasts and professionals, including from iRecord, social media posts and museum collections. Despite all this, many species are still under-recorded in Britain. A major reason for this has been the lack of an up-to-date identification tool.

The Scope of the Blow Flies Handbook

The new book covers British species of calyptrate flies from three families: the Calliphoridae, Polleniidae and Rhiniidae, commonly known as the blow flies. The most common and easily recognisable are bluebottles (Calliphora spp.) and greenbottles (Lucilia spp.). The metallic appearance of the majority of blow flies means they are readily distinguished from other flies. However, because there are a few other metallic calyptrate flies, especially tachinids and muscids, I have included a guide to common look-alikes. This section also covers blow flies that are not metallic and might be confused with flies from different families. Collectively the blow flies are a small and manageable group.

Only 30 species of Calliphoridae have been recorded in Britain and Ireland, 8 species of Polleniidae and 1 species of Rhiniidae. There have been significant taxonomic changes in the last decade. As a result, the existing identification guides that cover this group (such as the blowflies Naturalists’ Handbook by Zakaria Erzinçlioğlu, 1996) are out of date. Biological recorders and enthusiasts should find a good list of blow flies within their local area. Since many species are seasonal and some are active in winter, there is the potential to study these flies all year round. Although specimen preparation such as genitalia extraction is sometimes necessary, I have avoided more complicated characters where possible in the key. In fact, some species can be identified and recorded from a good photograph (see below). But unavoidably for other species, some of the characters that need to be examined are too small to see on a photograph, such as the coxopleural streak and some bristles. I check all submitted records meticulously to maintain the high quality of data collected by the Recording Scheme.

Blow fly species that can be verified through photographs 

Calliphora vicina. A common and widespread bluebottle, especially in urban sites, found all year round (but not ‘on the wing’ when it is very cold). The anterior thoracic spiracle is pale yellow to orange, while the basicosta is pale.

Calliphora vomitoria. Another common and widespread bluebottle, especially in non-urban habitats like woodland, found all year round (except when it is very cold!). Look for the ‘ginger beard’: orange hairs on the gena and postgena.

Cynomya mortuorum. A widely distributed fly, but more common in Scotland and northern England (from Leeds northwards). Look for the strikingly golden yellow ‘face’: golden yellow frons, prafacialia and gena.

Pollenia amentaria. Common, in meadows, marshes and uplands. Look for crinkly yellow hairs on the thorax and the black abdomen without a dusting pattern.

Lucilia richardsi. A greenbottle, common in England and Wales as far north as County Durham. Attracted to carrion, breeding on dead birds and small mammals. Look for the pale basicosta, which varies from creamy-white to orange and 2-3 anterodorsal bristles on second tibia.

Stomorhina lunata. Superficially similar to a hoverfly, and common in southern, eastern and central Britain. Abdomen with a yellow pattern, and a thorax with 3 dark stripes. Look for the lower part of the face, which is both shiny and strongly protruding. Live specimens have stripy eyes, rather like some of the horseflies.

Records of blow flies can be submitted to iRecord.

The guide is available to order from the FSC website: https://www.field-studies-council.org/shop/publications/blow-flies/