"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

August 24, 2018

Passeromyia longicornis

This is the third and final post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2018. This particular post was written by Lachlan Thurtell and it is about a fly that parasitise the chicks of some birds in Tasmania (you can also read a previous post about how parasitoid larvae are affected by what their caterpillar host eats here, and a post about why cuckoo eggs have thicker shells here).

Top left: Pardalote hatchling with a single maggot
Passeromyia longicornis maggot (Top Right), pupa (Bottom left),
and adult (Bottom right). Photos from Fig. 1. of the paper.
Have you ever had a nightmare where you have some strange creature crawling under your skin? Only for that creature to burst from underneath your skin, wriggling around and you can’t do anything to prevent it, or even escape after it emerges? This nightmare is very real for some native Australian wildlife.

Surviving the extreme conditions of Tasmania, competing for territory with your own species and others, avoiding predation, and facing habitat loss caused by human activities are all part of a pardalote's  daily life. However along with these trials this endangered Australian bird also faces parasitism from Passeromyia longicornis, a native Australian parasite that feeds upon weak and defenseless pardalote young.

P. longicornis is a Dipteran (an order of insects comprised of flies and mosquitos) belonging to the same family as the houseflies - the Muscidae. Houseflies are often seen as vectors of disease, carrying pathogens and eggs of other parasites. Passeromyia longicornis itself is a parasite which targets avian hosts. As an adult these flies are not thought to be parasitic but rather free living flies that feast on the decaying flesh of fruit. The larvae, however, are subcutaneous parasites that burrow their way underneath the skin of newly hatched pardalote chicks, possibly hours after the chick’s birth. Location is not very important for the larvae as they bore through the skin in a variety of different places, including the head.

Once the larvae have burrowed into the body of its vulnerable host, they begin to feed on the blood of the helpless hatchlings, a form of parasitism known as hematophagous parasitism. These vampiric creatures are known to suck the blood from their hosts for up to a week! Whilst the larvae are only known to feed upon blood, the effects on their hosts can result in death. In this study the researcher found that a whopping 85% of forty-spotted pardalote (Pardalotus quadragintus) nestlings which are parasitised by the larvae end up dying. Striated pardalotes (Pardalotus striatus) seemed to be more resistant to parasitism with only 65% of nestlings experiencing mortality. The larvae begin to pupate 3-6 days after emerging from their bed ‘n’ breakfast hosts and form cocoons where they develop over the next 17 days, however the duration of the pupal stage is shorter in warmer weather. The adults emerge from the cocoon, transitioning from a parasitic to a free living lifestyle.

Parasitism by P. longicornis is quite prevalent in both species of pardalotes. The larvae of P.longicornis were found in 87% of forty-spotted pardalotes, and 88% of striated pardalotes. Other birds were found to be parasitised by P. longicornis, such as the New Holland honeyeater, house sparrow and European Goldfinch, but showed much lower levels of parasitism than pardalotes, indicating that pardalotes are important hosts for these little blood suckers.

The pardalotes themselves may be to blame for the prevalence of P. longicornis as, unlike other birds, they pack their nests full of bark strips and grass. The nesting material is used in the pupal stage of P. longicornis as it provides a toasty environment to transition from a vampire living beneath the skin into a beautiful (if that’s your thing) fly.

Edworthy, A. B. (2016). Avian hosts, prevalence and larval life history of the ectoparasitic fly Passeromyia longicornis (Diptera: Muscidae) in south-eastern Tasmania. Australian journal of Zoology 64: 100-106.

This post was written by Lachlan Thurtell

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