StayiNatHome: A Cure for the Blue Fly Blues

Guest post by Steve Kerr (@steve_kerr), Honorary Curator of Entomology (Diptera), Otago Museum.

Have you ever noticed those large dark flies with bright metallic blue abdomens? Have you ever noticed how there seem to be many flies that look like this and wondered what they are? Well, so have I and I feel your pain!

In NZ there are approximately 2,500 species of fly that have been formally described and named. In addition, it is estimated there may be as many as 2,000 more awaiting description. That’s a lot of flies.

Flies (order Diptera, meaning ‘two wings’) are grouped into families, and then further grouped into genus and species. These groupings are generally based on gross and microscopic differences, and many of them can be differentiated at a glance. But many others are not so easy to distinguish and the differences can be quite subtle. Which brings us back to the topic here. Why so many blue flies and how can they be distinguished?

The three groups that contain the majority of large blue flies are the families Calliphoridae (the Blow Flies), Tachinidae (the Bristle Flies) and Muscidae (the House Flies). Others can be seen in a few other families and we’ll get to those shortly.

The purpose of this short piece is to introduce the NZ Blue Flies and to give a handy guide as to how to tell them apart with the naked eye. Photos of these species are included to facilitate comparisons, but a quick search on the iNaturalist NZ–Mātakai Taiao website will bring up many more. So, let’s get cracking.

European blue bottlefly (family Calliphoridae)

The family Calliphoridae holds the majority of large, dark, blue-tailed flies (see Dear, 1985). By far the most common is the European blue bottlefly, Calliphora vicina. It’s a cosmopolitan species (that is, found worldwide) and is readily seen on a daily basis in any urban garden here in NZ, almost year round. C. vicina is relatively large (about 1–1.3 cm) with a medium gray thorax (mid-section) with thin black stripes, and a bright metallic blue abdomen (tail section) which is said to have a ‘tessellated’ appearance. Tessellated means having a ‘tile-like’ appearance, and in shifting light or from different angles the abdomen looks as if it is covered with many small overlapping silver, black and blue tiles; this in fact is the one feature which will ID this fly at a glance.

Here's an example of the European blue bottlefly, Calliphora vicina, viewed from behind. Note the metallic blue "tessellated" abdomen. Photo CC-BY Steve Kerr

The adult fly when viewed from above is still quite distinctive (see below).

The European blue bottlefly, Calliphora vicina, viewed from above and the side. Its body length is about 14 mm. Photos CC-BY Steve Kerr.

NZ Blue Blowfly (family Calliphoridae)

Another common NZ blue fly is the magnificent New Zealand Blue Blowfly (Calliphora quadrimaculata). I say magnificent because it is large … sometimes really large (approaching 2 cm) … and its abdomen is a shiny dark metallic royal blue. It is fairly common in forests and glades, often seen resting in the sun on tree trunks and on the leaves of trailside shrubs.

The NZ Blue Blowfly, Calliphora quadrimaculata. Its body length is about 13 mm. Photos CC-BY Steve Kerr.

In addition to the distinctive solid metallic blue abdomen, C. quadrimaculata has a pair of distinctive bright orange-yellow spiracles (breathing pores) on the side of its thorax. For a better look at this stunning fly have a look at some close-up images on iNat NZ.

Xenocalliphora (family Calliphoridae)

Now things get a bit tricky. The family Calliphoridae has two other flies that look similar to both C. vicina and C. quadrimaculata. The differences are subtle and difficult to establish with the naked eye or in the field.

The first of these look-alikes is a small group of flies in the genus Xenocalliphora (see Dear, 1985). Several of these have bright blue abdomens (either glossy or tessellated with a silvery dusting) and they all have large, bright yellow-orange spiracles. In addition, the genus Xenocalliphora can be distinguished from all other New Zealand Calliphoridae by their yellow palpi (small antenna-like mouth parts or ‘feelers’) and the presence of numerous long glossy black bristles on the top of the head.

A NZ Xenocalliphora fly. Its body length is about 8 mm. Note the long hairs on the head and the yellow palps. Photos CC-BY Steve Kerr.

The eleven recognized species in the genus Xenocalliphora are not uncommon, but seldom noticed. They tend to be small to medium in size (7–9 mm) and are seen in a wide variety of locations including coastal forests and glades, on beaches and among stands of flax along lake margins (if you're in Dunedin, have a look around Tomahawk Lagoon).

Ptilonesia (family Calliphoridae)

The second Blue Fly look-alike in the family Calliphoridae is the genus Ptilonesia (see Dear, 1985). There is but a single species in this genus, Ptilonesia auronotata, and it is closely related to Xenocalliphora (Ptilonesia was once considered a sub-genus of Xenocalliphora). It is abundant on beaches and large numbers can be seen on the seashore intertidal zone between Brighton and Taieri Mouth south of Dunedin. Ptilonesia auronotata can be distinguished from all Xenocalliphora by the densely haired eyes, and black palpi. You may also notice that the hairs on the top of the head are short and sparse.

A male of the fly Ptilonesia auronotata. Its body length is 10 mm. Photos CC-BY Steve Kerr.

Blue tachinid flies

This brings us to a new family: Tachinidae, the Bristle Flies. The Bristle Flies, as their name suggests, sport long spiky erect bristles on their abdomens. In addition, they often have long, sometimes striking antennae. There are almost 200 described species in the family Tachinidae here in New Zealand. Observations on iNaturalist NZ–Mātakai Taiao show the extraordinary range and diversity of the tachinids.

As you may have guessed by now, some of the tachinids are blue! And as is often the case, they follow the same basic pattern seen in the calliphorids; they exhibit grey and sometimes delicately striped thoraxes and metallic blue abdomens.

A couple of good examples are shown below. They are quite similar in overall appearance to Calliphora vicina, rather uncommon, and often overlooked in the field. The best ways to distinguish them requires a close examination of the antennae and wing veins.

A species of Occisor, possibly Occisor atratus (Tachinidae: Tachininae: Occisorini). Its body length is 11 mm. Photos CC-BY Steve Kerr.

A blue tachinid likely of the genus Tachineo. Body length about 11 mm. Photos CC-BY Jon Sullivan

One of the commonest and most striking of the blue Tachinids is Pales marginata. It was first described by the eminent Otago entomologist (and former Otago Museum curator) Capt. F.W. Hutton. Pales marginata (originally Phorocera marginata) can be found throughout NZ in a range of habitats (Hutton, 1900).

A blue tachinid of the species Pales marginata (Tachinidae: Exoristinae: Goniini). Body length 9.5 mm. Photo CC-BY Steve Kerr

The thorax is often a bright blue-green with several narrow black stripes anteriorly and a blue-green ‘scutellum’ (the shield-shaped structure at the base of the thorax). The scutellum generally exhibits a brown margin (see above). Another reliable distinguishing feature for all species in the genus Pales is the presence of very long antennae which, in the field, often stick straight out (see below).

A Pales marginata observed in Wellington by @wild_wind. Photo CC-BY-NC wild_wind

Blue house flies

Not to be outdone, the family Muscidae (House flies) boasts a few Blue Flies as well. Here in NZ seventeen genera containing 145 species have been recognized in the family Muscidae. A small number are blue or at least bluish in colour and deserve mention here.

One of the most striking of these is Calliphoroides antennatis, so named because when first described in 1881, Hutton wrongly assigned it to the family Calliphoridae. By 1930 Malloch had recognized and moved the species into the family Muscidae.

Although Hutton listed it as rare, it is actually fairly common around Dunedin (they can be seen in reasonable numbers at Fraser’s Gully and along the trail from Bethune’s Gully up to Mt. Cargill). The fly can be easily distinguished, even by naked eye in the field, by its bright orange antennae and smoky dark wings. In addition, close inspection reveals consistently orange ‘knees’ and a yellow spot at the wing base (see below).

A blue Calliphoroides antennatis, a blue blowfly-like species that's really in the house fly family (Muscoidea: Muscidae). Photo CC-BY Steve Kerr

Another Calliphoroides antennatis. Its body length is 10 mm. Photos CC-BY Steve Kerr.

Another blue muscid is quite common on the beaches around Dunedin (Tomahawk Beach is ideal). It is fairly large and a bluish-grey in colour. Microscopic examination confirms it belongs in the genus Spilogona, but it has yet to be formally described and is nameless (although I like to call it the Blue Beach Muscid). The fly can be observed resting on piles of kelp on the foreshore and can be easily confused with Calliphora vicina.

A blue species of Spilogona (Muscidae). Its body length is 10 mm. Photos CC-BY Steve Kerr.

One final blue Muscid is Australophyra rostrata (the black carrion fly; formerly known as Hydrotea rostrata). The fly is found throughout Australia and first appeared in NZ at least 100 years ago. Apparently it is still rather uncommon, although Otago Museum does have several specimens, collected in the Invercargill area in 1990. Pont (1973) noted that the fly is generally glossy black with a blue abdomen, although the Otago Museum specimens are a brighter blue all over—perhaps a bit of genetic drift over the many years that it has been here.

The black carrion fly, Australophyra rostrata (Muscidae). The fly on the left was photographed by @epitree in Te Aroha (CC-BY-NC). The fly on the right was photographed by @pohanginapete in the Manawatu (CC-BY-NC-ND).

Blue flies in other families

Two last groups are shown below: one in the family Polleniidae and the other in Syrphidae.

The family Polleniidae is interesting because until just this year, all of the flies in this family (genus Pollenia, 35 species total) were considered to be part of the family Calliphoridae (Dear, 1985). After decades of debate, a recent phylogenetic analysis by Cerretti et al. (2019) resolved the issue, and a whole new family was erected.

Several of the NZ flies in the genus Pollenia have grey striped thoraxes and metallic blue abdomens, and look very much like small versions of C. vicina. The distinguishing features for Pollenia generally require microscopic examination, and I refer the interested readers here to Dear (1985) for a good treatment of the subject.

Photos of a museum specimen of Pollenia consectata (OMNZ IV106766) . Its body length is approximately 5 mm. Photos CC-BY Steve Kerr.

In contrast, the one blue fly in the family Syrphidae (Helophilus hochstetteri, shown below) is quite easy to spot and distinguish in the field, even from a fair distance. Like all flies in Syrphidae (the ‘Hover Flies’), Helophilus hochstetteri can hover over flowers and hold perfectly motionless like a humming bird. Its glaringly brilliant metallic blue abdomen shines like a jewel in the sun. It is quite common around Dunedin and can be seen visiting flowers in most any urban garden or field.

The blue syrphid fly Helophilus hochstetteri. Its body length is 10 mm (left) and 12.5 mm (right). Photos CC-BY Steve Kerr.

Concluding remarks

One has to wonder why so many flies in so many different families exhibit these seemingly constant colour patterns. The obvious assumption is that grey and black striped thoraxes and metallic blue abdomens impart some evolutionary advantage to the species. Perhaps at some time in the ancient past, a fly with these colours proved to be utterly unpalatable to other animals that might feed on it, and then over the eons other flies ‘adopted’ these same patterns as a means of protection. Such a thing has been proposed for the Viceroy butterfly in North America, which perfectly mimics the bitter tasting (or so I’ve heard) Monarch butterfly.

Of course, all of this is conjecture. It may be that we’ll never know. But there’s no denying that some of these flies are quite beautiful and special in their own ways, and the ‘wardrobe’ they’ve chosen is certainly popular!

Further reading

Cerretti, P. et al., (2019) Reclustering the cluster flies (Diptera: Oestroidea, Polleniidae). Systematic Entomology 44(4): 957–972.

Dear, J.P. (1985) Calliphoridae (Insecta: Diptera). Fauna of New Zealand 8: 1-88.

Hutton, F.W. (1900) Synopsis of the Diptera brachycera of New Zealand. Transactions and Proceedings of the Royal Society of New Zealand 1868-1961 (originally Transactions of the New Zealand Institute 1900); vol. 33, pp. 1-95.

Macfarlane, R.P., et al. (2010) Phylum Arthropoda, Subphylum Hexapoda. 2,500 speciesNew Zealand Inventory of Biodiversity; vol. 2, chapt. 9, pp. 233-467. Canterbury University Press.

Malloch, J.R. (1930) The calyptrate Diptera of New Zealand. Part I. Rec. Canterbury Mus. 3: 289-306.

Pont, A.C. (1973) Studies on Australian Muscidae (Diptera). IV. A revision of the subfamilies Muscinae and Stomoxyinae. Aust. J. Zool. Suppl. Ser. 21: 129-296.

A blue syrphid fly, Helophilus hochstetteri. Photos CC-BY Steve Kerr.

Posted on April 09, 2020 12:48 AM by steve_kerr steve_kerr


Thanks for posting @steve_kerr Very enjoyable read. Who knew there were so many blues!

Posted by jacqui-nz about 4 years ago

It's an interesting mystery why they've all converged on the same general appearance.

Posted by jon_sullivan about 4 years ago

Yes, an interesting read, I will have to start taking more detailed photos of any I see. But my brain can't cope with all the possibilities, I'll have to come back here for reference!

Posted by tony_wills about 4 years ago

@steve_kerr — excellent post, Steve. I'd never noticed this, let alone thought about it. Looking forward to more posts (hint!)

Posted by petemcgregor about 4 years ago

@jon_sullivan A post about identifying features/habitats of Coppers & Blues butterflies would be handy :-)

Posted by jacqui-nz about 4 years ago

Heh-heh ! I think Jon has opened a can of Annelids. Might make an interesting blog as well.
But seriously ... I agree with Jacqui. Lycaenidae is in need of some review.

Posted by steve_kerr about 4 years ago

@jacqui-nz There's a cart-before-the-horse problem with Lycaenidae. A Lepidoptera taxonomist needs to first secure funding from someone to pay for their time to properly review the coppers in NZ. Then we can write an iNat NZ article about it.

Brian Patrick (@butterfly4) and Hamish Patrick made a useful start in their "Butterflies of the South Pacific" book. However, a lot still needs to be done before their suggested new species can be robustly identified and described with new scientific names.

Funding that kind of basic taxonomic work is really difficult in NZ. I wonder if we could crowd source part of the funding with donations through something like Givealittle? It's embarrassing that NZ doesn't have its copper butterflies sorted and I'd love to be able to put species names on all the coppers I photograph.

Having said all that, an article now on NZ's coppers could be useful for drawing attention to current state of knowledge and the urgent (in my view) need for taxonomic work. Blues we could manage though.

Posted by jon_sullivan about 4 years ago

Thanks Jon and Steve. The crowd source funding sounds like a good idea. In the meantime, it would be nice to at least see an overview NZ Coppers article, and Blues, to kick things off. @john_early Any suggestions re the Coppers taxonomic work?

Posted by jacqui-nz about 4 years ago

Very cool blog post! I wish somebody would write a similar guide to all of our North American blue bottle look-alikes. We also have tachinids & muscids to consider as well as several calliphorid genera including a whole bunch of blue Calliphoras that are hard to tell apart in pictures.

Posted by treegrow about 4 years ago

Good effort! Thx :D
@wild_wind @robert_briggs in u haven't seen this yet.

Posted by dave_holland almost 4 years ago

Thank you @dave_holland , I have seen this already. If you'll just do a some more of these for all the rest of our flies that would be great thanks @steve_kerr :)

Posted by wild_wind almost 4 years ago

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