Flies are familiar creatures. Australian icons even. But I’ve discovered that doesn’t make them any easier to identify than other insects. Let me share my fly ID challenge with you!

The story starts with moths

Last Spring we discovered two apparently identical caterpillars in the forest - one was grazing on leaf blades of the Grass-tree (Xanthorrhoea concava), the other on the low-growing forb Broom Spurge (Amperea xiphoclada).

As is our wont, we moved both caterpillars into alternative accommodation in a terrarium - our ‘buddy tank’ - to try to work out what type of moth they would turn into. This is a fun hobby which also has a serious scientific purpose. The link between larva and adult is known only for relatively few Australian moths.

We supplied the buddies with foliage of the same type of plant each was found on, which they happily accepted. They grew, moulted and a month later settled down in a quiet corner of the terrarium and began to spin a cocoon. This is the normal start of the pupal stage during which the larva transforms into an adult moth. But life then took quite a different turn for each of the buddies.

The Grass-tree caterpillar finished construction of its cocoon, then did nothing - at least to the outside observer. Five months later it emerged as a very handsome moth, shown below. I was able to identify it as the anthelid Pterolocera leucocera, a species I’ve encountered on numerous previous occasions in the forest.

In contrast, the Broom Spurge caterpillar stopped moving only part-way through making its cocoon and the next day was found dead on the floor of the tank. Three days later, 2 pale maggots emerged from the distinctly malodorous corpse of the caterpillar and pupated. Each formed a pupal case typical of flies - a dark reddish-brown, seed-shaped cylinder, called a puparium.

After 8 days the caterpillar has almost completely distintegrated with just its empty skin remaining. Kerri dissected this open and found another 8 pupae inside.

We were curious to know whether these were indeed pupae of flies and if so, what type of flies they were. So we transferred them to a dish and waited…

…for over 7 months. Then on 7th July this year, Kerri reported the exciting news that a fly had emerged from one of the pupal cases!

Over the next 10 days an additional 9 identical looking flies emerged. So the whole clutch of pupae had developed successfully to adulthood - at the expense of one caterpillar.

We had a bunch of questions. What were those fly larvae doing inside the caterpillar? How did they get there? Who are these flies? It seemed likely that we would have to find the answer to the last question before we could address the first two.

Thus began the fly ID challenge!

Our flies are higher

Where to start? Just eye-balling the flies gave me a clue. They looked pretty familiar. Like small blowflies or house flies. That would make them members of an evolutionary advanced group - higher flies. (Few people realise that the humble ‘blowie’ stands at the pinnacle of fly evolution).

How could I test my guess that these were higher flies? Know a fly by its face…

A very close front-on view of our flies reveals a distinctive shape just above the antennae which looks like an inverted “U” - seen in the LHS image below. This structure is called the ptilinal fissue. It doesn’t actually have a function in the adult fly. It’s just a carry-over from the pupal stage.

When it’s ready to leave the puparium and begin life as an adult fly, the insect inflates a sac - the ptilinum - from the top of its head. This pushes against a zone of weakness in the puparium, popping off a circular cap and allowing the fly to make its escape. Flies that make a circular hole to get out of the pupal case are placed in the taxonomic group Cyclorrhapha, meaning “circular seam”.

You can see the hole thus created in this photo of one of the empty pupal cases of our clutch of flies.

Once the fly is out of the puparium, the ptilinum collapses and is pulled back into the interior of the head. A split or scar is left on the adult’s head - the ptilinal fissue.

Flies that use a ptilinum to open up the puparium belong to the Schizophora (“split-bearers”) (ref. 1). This includes most of the higher flies. (Some flies make a hole in the puparium without the use of a ptilinum).

The face of our flies displays other features that mark them as being high up on the dipteran evolutionary tree.

• the antennae are relatively short (compared to the body) and end in a bristle-shaped arista

• a pair of large, incurved bristles - vibrissae - extends from the sides of the face

These structures are shown in the pair of photos above.

However, the most distinctive feature evincing the higher status of our flies is to be found, not on the head, but on the body, just below the wings - a pair of white flaps called calypters.

If you possess this structure, you belong to the Calyptratae and are related to house flies, blowflies and flesh flies. This is the most advanced group within the Schizophora (ref. 1).

Next time a house fly lands on your hand, have a close look for the calypters, ptilinal fissure, vibrissae, etc. You can see a surprising amount with the naked eye if you look closely.

Our flies belong to the family Tachinidae

Knowing that our flies belong to the Calyptratae helps to narrow down the search for their identity. However we are still left with 1067 Australian species in 13 different families (ref. 2).

How do we limit the choices further?

Well again, an obvious feature helps. The abdomen of our flies bears many long, stout hairs (LHS photo below). This is a widespread feature of flies in the Tachinidae, although it is not unique to that family.

If our guess is correct, our flies should possess a particular structure which is much less obvious, but which is exclusive to the Tachinidae - the subscutellum (ref. 1). This is a pale flap located beneath the scutellum, which is the triangular-shaped last section of the thorax, viewed from above.

The RHS photo in the pair below shows that the subscutellum is indeed present in our flies.

Tachinidae are parasitoids

Knowing that our flies are tachinids immediately provides an answer to our first question - what are their larvae doing inside a caterpillar?

All tachinid flies are parasites. They raise their young inside the body of another insect.

Depending on the tachinid species, this could be the larva of a moth/butterfly (Lepidoptera), beetle (Coleoptera), bug (Hemiptera), wasp (Hymenoptera) or the nymph/adult of a stick insect (Phasmatodea) or mantid (Mantodea) (ref. 4).

The tachinid larvae feed on the internal tissues of the host, developing at its expense. The host is invariably killed in the process - witness our anthelid caterpillar - but its death is delayed until the fly larvae are ready to pupate. The parasite thereby gains the maximum benefit from the association. Strictly speaking, tachinids and other parasites that ultimately kill their host are termed parasitoids. The term parasite is reserved for animals that gain benefit at the expense of the host while at the same time allowing it to keep living.

So to the question of how the tachinid larvae gain entry to their host. There are various options here.

Some tachinids simply lay eggs on the surface of the host and rely on the larvae to bore inside once they have hatched out. In other species, the eggs are held and incubated inside the female fly’s body, thus reducing the amount of time the larvae must spend in the hazardous environment on the host’s body surface. Other tachinids inject their eggs into the interior of the host’s body, using a sharp ovipositor (egg-laying tube). A final strategy is to lay eggs on the vegetation likely to be consumed by the host. When the tachinid larvae hatch from those eggs, with luck they’ll automatically find themselves in the desired internal body space of a host (ref. 1).

I am unable to decide which of those strategies was used by the fly that attacked the Broom Spurge caterpillar. It is clear however that only a single female was involved as there was a high degree of synchrony in development of the offspring - all of the larvae pupated within a few days of one another and all adults emerged from the puparia within 20 days.

Sorting out the Tachinidae

So we’re making progress - we know our flies belong to the family Tachinidae. But unfortunately this is the largest calyptrate family with 4 subfamilies, 145 genera and 520 species (ref. 3). Still some work to do…

Taking it from the top, can we decide which subfamily our flies belong to?

We have a good head start here because we know that the host of our flies is a moth (order Lepidoptera). This automatically rules out two subfamilies - Dexiinae (which use only beetles as hosts) and Phasiinae (which use bugs exclusively) (ref. 4).

This leaves the subfamilies Tachininae and Goniinae, which both employ Lepidoptera (amongst other insects) as hosts.

Nothing left for it now but to break out an identification key. Fortunately, there is one available. Crosskey (ref. 4) provides keys for all of the tachinid subfamilies, tribes and genera known at that time (1973).

I have spent a good deal of time working through the keys for Tachininae and Goniinae and the associated tribes (the taxonomic level between genus and subfamily). This is a rather frustrating exercise as there are exceptions at many of the points in those keys, which means you often can’t be confident you’ve made the right choice.

As you can see in my iNaturalist activity record for this fly, I went down the wrong path in choosing sub-family Goniinae, tribe Sturmiini, which led me to an incorrect genus.

A very helpful contributor, who is doing their PhD on systematics of Tachinidae (so can probably be trusted!) pointed out that I had misinterpreted my own observations. For example, what I took for pretty much hairless eyes are in fact densely haired. The hazards of using inexperienced eyes!

They have suggested a possible candidate for our flies - Chlorotachina, a genus in the tribe Ernestiini in the subfamily Tachininae. However they acknowledge that this doesn’t quite fit. So I may have an undescribed species or it might be something completely different.

If our fly is a Chlorotachina species, then we can add a new species - the anthelid moth Pterolocera leucocera - to the list of hosts of this tachinid genus. Currently the only known host is the skipper butterfly genus Hesperilla (ref. 4).

While I still don’t know for sure who our fly is, I don’t feel my efforts have been wasted. I’ve learnt a lot about the anatomy of tachinid flies and the key characters necessary to identify them.

To give you some feeling for the nature of this activity, I’ve provided a gallery of images of our fly which shows some of those characters. The accompanying text details the questions addressed by these images which you are called upon to answer when working through a key.

I’ve enjoyed examining a fly up close and personal. It’s an exquisitely detailed piece of machinery with its own natural beauty.

References

1. S.A. Marshall. (2012) Flies: The Natural History and Diversity of Diptera. Firefly Books Ltd. NY

2. Australian Faunal Directory. https://biodiversity.org.au/afd/taxa/CALYPTRATAE/statistics

3. Australian Faunal Directory. https://biodiversity.org.au/afd/taxa/TACHINIDAE/statistics

4. Crosskey, R.W. (1973) A conspectus of the Tachinidae (Diptera) of Australia, including keys to the supraspecific taxa and taxonomic and host catalogues. Bulletin British Museum (Natural History) Entomology Suppl. 1: 1-221