Barf out! What's in that pellet..?

Nope, not the feeling that you've eaten too much cake and turkey... while out feeding the birds this morning, I couldn't help but notice what I assumed were pellets from berry-feeding birds (all birds make pellets of undigested stuff, not just owls). OK, so there were bits of seed and berry skin, but also some maggots.... mmm, nice. These were clearly dead, so I had to wonder whether they were egested by the birds (i.e. having been swallowed along with berries) rather than being parasitic. So, one of the little beasts was brought indoors to scrutinse under the microscope.

Fly larva

It was about 8mm long and 1mm wide, whitish and more-or-less cylindrical with a tapering head which included dark mouthparts (fly larvae often have dark chitinous mouthparts).There were no legs, although the underside did have rough pads likely to be used to aid locomotion.

Magnified head showing dark mouthparts.

Closer still to see where the mouthparts protrude.

One of the rough locomotory pads, near the rear end.

Now, the next step is to see if the larva is identifiable. The overall form suggested 'Muscidae' (the houseflies) to me, but to be sure requires a closer look still. So, to the dissecting needles to tease out the mouthparts...

Close-up of the mouthparts while still in the head segment.

Overall shape of mouthparts - note the prongs and notches.

Again, an overall view with attached muscle fibres etc.

Handily I have a copy of Smith (1989) which covers the immature stages of British flies though not always to species level. The prongs of the pharyngeal sclerite (ps), the long, more-or-less parallel structures pointing to the right in the two photos directly above, are about the same length and there are four; these are paired and so only two are seen side-on. In front of (i.e. to the left) of these is the small hypopharyngeal sclerite (hs) and in front of this, a larger structure made of several small sclerites analagous to jaws. In the same two photos you can see the notch between ps and hs, and in the middle photo what appears as a hole is a gap between the parts of the 'jaws'.

So, what is it? Well, the mouthparts and overall body form strongly suggest that it is in the family Muscidae, though I'm can't be sure beyond that as the spiracles were difficult to see. It looks closest to Phaonia sp. though most muscid larvae are carnivorous (e.g. hunting other larvae in damp places or organic matter) and found in various substrates. Therefore, it could easily have been picked up by a foraging bird and then deposited dead but undigested with fruit remains. Any thoughts, including revised ID (e.g. could it be Dolichopodidae instead?) most welcome...

Reference


Smith, K.G.V. (1989). An Introduction to the Immature Stages of British Flies. Diptera Larvae, with Notes on Eggs, Puparia and Pupae. Handbooks for the Identification of British Insects 10(14).