By Meredith Swett Walker
The Hippoboscidae, commonly known as “louse flies” or “keds,” are a family of rather bizarre flies that are probably more familiar to ornithologists, sheep ranchers, and equestrians than they are to most entomologists. You are unlikely to see one unless you have a bird in your hand, or are grooming a horse, because hippoboscid flies are obligate parasites. They feed on blood and only blood, and they stay very close to their lunch.
Hippoboscid flies are fairly particular about their hosts. Sheep keds are not found on birds or vice versa. There are more than 200 species of Hippoboscidae, and 75 percent of these parasitize birds of various types ranging from tiny swifts to huge albatrosses. Some louse-flies even exhibit distinct preferences for a particular species of bird. One species of hippoboscid is found exclusively on frigate birds and another species parasitizes only boobies. This specificity is seen even when the two seabirds nest in densely-packed, mixed colonies where it would be easy for a hippoboscid to fly from one bird to another.
Thankfully, hippoboscids do not parasitize humans. In 1931, G. Robert Coatney conducted an experiment to determine if pigeon louse flies, Pseudolynchia canariensis, would bite humans and survive on human blood. He must have been very persuasive because he convinced two friends to join him in playing host to the flies. The answer is yes — hippoboscids will bite humans when given no other choice of host, and their bites are definitely itchy. But the flies did not survive long or reproduce when fed only human blood. Granted, Coatney’s experiment was limited in sample size and scope, but hopefully no one feels the need to repeat it.
Hippoboscids are very mobile — most species can fly. But despite their mobility, they rarely spend any time off of their hosts. A fly dislodged from its host will quickly fly back to it or the next closest host. This ability to move easily from one host individual to another makes hippoboscids an effective vector for blood-borne pathogens like avian malaria. The only phase of their life cycle when they are not intimately associated with their host is during pupation. At this stage, hippoboscids are easy to find (just find the host), which makes them very useful for scientists studying disease ecology.
The most striking thing about a hippoboscid’s appearance is its shape. Their bodies are distinctly dorsoventrally flattened (from back to belly), as if someone had dropped a book on them. This flat body shape allows them to slide between the feathers and scuttle around in the fur of their hosts. Their shape and tough exoskeletons also make them hard to squish, both for their hosts and for the humans that study them. According to one scientist who has worked on hippoboscids, you can’t just smack them, you have to “roll them between your fingers” to kill them.
But the most bizarre aspect of hippoboscid biology is definitely their life cycle. Most flies lay eggs, which hatch into larvae or maggots. The larvae feed independently, developing through various stages called “instars” until forming puparia, undergoing complete metamorphosis, and emerging as adult flies. But along with tse-tse flies and bat flies, the hippoboscid flies belong to the group formerly known as the Pupipara or “pupa-bearers.” Rather than laying eggs, female “pupa-bearers” essentially lay a pupa or “prepuparium” — a late-stage larva enclosed in a shell that quickly hardens into a true pupa. Female hippoboscids produce one offspring at a time. A single egg hatches in the female’s uterus and the resulting larva develops ther,e feeding from “milk glands.” The larva does not leave its mother’s body until it is fully-grown and ready to pupate. In hippoboscid species that parasitize birds, females usually deposit the pupae in their host bird’s nest or roosting site, where the newly-emerged adult fly will easily find a new host.
This unusual process entails an enormous energetic investment by the female fly. According to Dr. Jessica Waite, an infectious disease biologist at Penn State University who has worked extensively on pigeon louse flies, the pupa can weigh more than the mother herself since the shell encasing it is included in the weight.
Dr. Waite never really set out to study hippoboscid flies. She started out as a bird lover who was interested in avian disease ecology. Like most people who handle birds, she had no love for the louse flies. As Miriam Rothschild and Theresa Clay wrote in Fleas, Flukes and Cuckoos: A Study of Bird Parasites, “For reasons which defy analysis, louse-flies are particularly repellent insects, and most people experience a shudder of disgust at the sight of them, and are filled with a quite unreasonable feeling of horror if they happen to dart up their sleeves or into their hair while handling the host.”
For her dissertation research, Waite studied the relationships between parasites, hosts, and vectors, and she was specifically interested in the costs to each of the organisms in this cycle. The pigeon (host), avian malaria (parasite) and pigeon louse fly (vector) system turned out to be ideal for measuring costs for the vector because of the quirks of the fly’s biology. Because both sexes of pigeon louse fly feed on blood and can become infected with the malaria parasite, Waite could make comparisons that aren’t possible in other arthropod vectors, such as mosquitoes. She could measure the reductions in survival associated with malaria infection in female louse flies, which invest heavily in reproduction compared to the males, which invest relatively little.
Working on this malaria system required keeping all three organisms in the lab, and Waite performed elegant experiments that elucidated the costs inflicted by malaria infection on the louse flies and the costs inflicted by louse flies on the pigeons. The daily grind of this work involved long hours of picking through pigeon droppings, counting dead flies, and collecting pupae, and after a while she became something of an accidental expert on hippoboscid biology.
“They are really creepy, especially in the way they move, but definitely fascinating,” Waite said.
Like Waite, Dr. Iris Levin never thought she’d end up working with hippoboscid flies.
“If you had told me at the start of my dissertation work that I’d end up working on these flies, I would have laughed hysterically,” she said.
Levin, an integrative biologist now at the University of Colorado, Boulder, was studying great frigate birds and a different species of avian malaria parasite, Haemoproteus iwa, in the Galapagos Islands. She was pretty sure that the hippoboscid fly, Olfersia spinifera, was transmitting the malaria, but to confirm the fly as the vector and understand the patterns of malaria transmission, she had to prove that individual flies bit more than one bird, and then estimate how frequently this host switching occurred. Instead of marking individual flies and trying to recapture them on different birds as had been done with insect vectors in the past, Levin found a clever work-around. She dissected the flies and extracted the bird blood from the last meals in their guts. She then compared DNA in that bird blood to DNA from the bird on which the fly was captured. If they didn’t match, it proved that the fly had fed on one frigate bird and then moved on to another.
Levin was also interested in the population genetics of the flies and their hosts. She compared DNA samples from populations of frigate birds and flies breeding on different islands. Levin found that different populations of flies were more closely related to each other than were the populations of frigate birds on which they were found. This meant that despite always staying close to their hosts, the flies were more frequently breeding with flies from other islands than the birds were. Because flies are very unlikely to fly from island to island on their own, this meant that the frigate birds were congregating and swapping flies at places and times when they weren’t breeding. So looking at the population genetics of the flies told Levin something about the birds’ movements that she wouldn’t have learned by studying the birds alone.
“In a way, hippoboscid flies are the world’s cheapest geolocator,” said Levin half seriously.
There is still much to be learned about the fascinating, yet slightly disgusting, hippobscid flies. Unlike most other biting diptera, both males and females feed on blood. What can that tell us about the evolution of blood feeding in flies? Did their unusual “pupa-bearing” reproductive strategy evolve in conjunction with their parasitic habits? Entomologists interested in studying this fly may have to learn how to catch birds or handle livestock. Or better yet, collaborate with a veterinarian or an ornithologist — you may end up converting them into accidental entomologists.
Special thanks to Jari Flinck and the community at the www.diptera.info site for assisting the author in finding images and information for this article.
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Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs http://picahudsonia.com and https://citizenbiologist.com or follow her on Twitter at @mswettwalker.