An unfamiliar shape clinging to a grass blade just a few centimetres above ground. Always worth a closer look. This small insect had obviously recently eclosed – the body soft and pale, unable to fly and even slow to move. And it had apparently just emerged from below ground – it was liberally sprinkled with grains of sand.

Curious, I placed the grass blade and clinging insect into a container to see what changes might take place as the cuticle hardened. And, of course, to take more images and try for a species identification.

Success! One day on, she has flown free and I have an ID.

A fly, obviously. One pair of wings plus halteres. Family Conopidae, commonly known as ‘thick-headed flies’. And I even have a species-level ID … Physocephala nigrotestacea. Thanks to the recent monograph on Australian Conopidae by Margaret Schneider (2010), identification was quite straightforward.

The full encounter

Over the course of a day, the fly gradually changed colour and took on her ultimate shape … most notably, with proboscis extended forward rather than folded beneath her body. She also groomed and removed all the sand grains. By that evening she was very active and responsive, so I chilled her briefly for some photos (below) before returning her to the dish at room temperature overnight.

The following morning she was clearly fine, fully recovered from the brief shock of the freezer (I’m pleased to report). Time to release her. And on the off chance that she would hang around for some photos in the field, I had my camera ready. What luck! She flew just a short distance to a low plant and sat there in the sun, grooming, for several minutes. Eventually she took to the air and disappeared from view, probably seeking a nectar feed, a mate, and ultimately hosts for her eggs.

So, a good story, with a happy ending. More or less. Arguably less good news for some of my favourite local, ground-nesting wasps.

A brief note on the fly’s biology

Flies in the subfamily Conopinae parasitise a variety of aculeate wasps and bees (Marshall, 2012; Schneider, 2010). Based on studies in the northern hemisphere, the female fly sits in wait, rapidly striking an unwary target and injecting a single egg into their abdomen. That impressive ‘can-opener’ shape enables her to grip the wasp, separate the sclerites (hardened cuticle plates) of the host, and gain access to the more vulnerable cuticle between. Extraordinary bit of anatomy!

The fly larva feeds on the soft tissue of the host, eventually causing its death and pupating inside the carcass … presumably underground in the host’s burrow. However, as is the case for so many of our endemic insects, almost nothing is known about the specific life histories of Australian conopids (Schneider, 2010).

But I have a theory.

It’s possible … perhaps even likely … that the life of this fly cost the death of a Cerceris wasp. I found the fly in a sandy area home to numerous nests of Cerceris antipodes. The fly is clearly a wasp-mimic, and even looks rather like Cerceris in shape, colour, size and stance. And the wasps’ nesting season is again underway, with females burrowing and delivering their beetle prey. The fly seems to have her timing just right!

When I wrote the ‘Neighbourhood Watch’ blog post in 2022 I documented a variety of marauding parasites and home-invaders. I can now add yet one more threatening actor to the cast in that ground-level drama.

Identification details

So back to the photos and the challenge of tracking down an identification. The images below illustrate the characters and interpretations upon which I’m basing my suggested ID … all with reference to the descriptions in Schneider (2010).

In conclusion, this female closely matches the physical description of Physocephala nigrotestacea. The location is also entirely consistent. Schneider’s study included a specimen collected at Cann River in eastern Victoria, less than 70km southwest from here. In contrast, P. minuta is only known from northeast Queensland.

Yet another species for our home list!

References

Marshall, S.A. 2012. Flies: The natural history and diversity of Diptera. Firefly Books, Ontario.

Schneider, M.A. 2010. A taxonomic revision of Australian Conopidae (Insecta: Diptera). Zootaxa 2581: 1-246 (open access)

The following extracts from Schneider (2010) are simply provided for ease of reference. They include the complete redescription of Physocephala nigrotestacea. For more details, including synonymies, see the original paper (link above).