Grafe, T.U., H.H. Ahmad Sah, N. Ahmad, A. Borkent, I. Meuche & O. Konopik, 2018. Studying the sensory ecology of frog-biting midges (Corethrellidae: Diptera) and their frog hosts using ecological interaction networks. Journal of Zoology, vol. 307, pp. 17-27, https://doi.org/10.1111/jzo.12612
The loudspeaker broadcasting a frog call had barely been switched on before a tiny blood-sucking midge silently floated in and landed squarely on the speaker. I jumped up to catch it but it escaped. It was just after sunset, in a hot and humid peatswamp in northern Borneo in the sultanate of Brunei and I had just seen how these unusual insects locate their frog hosts acoustically.
Frog-biting midges, close relatives of mosquitoes, had been reported to attack calling frogs in Panama and Costa Rica, but little was known of their habits in Southeast Asia. Would they also be attracted to frog calls? How many species were there and did they vary in the hosts they preferred and in their sensory ecology? Over a period of ten years, I regularly returned to the same site in which I had had my first encounter and many students and fellow scientists joined me on collecting trips and to conduct experiments in Brunei and in the Malaysian states of Sabah and Sarawak.
The aim of our project was to determine how many species of blood-sucking flies there were and how specialized their relationship was to the frogs that they parasitized. Would they feed on the blood of many frog hosts or prefer just one or two frog species? What were the selective pressures on the design of frog advertisement calls and calling behavior? We knew that only calling males were parasitized and that in addition to taking the blood meal, midges are likely to transmit trypanosome blood parasites to their frog hosts. Thus, calling to attract females has the unfortunate side effect of attracting midges.
To unravel the relationships between midges and their frog hosts, we spent many hours collecting midges from calling males, concentrating on two habitats in Brunei: a peatswamp and a rainforest. We then viewed the relationships using interaction plots to reveal the degree of specialization between frogs and midges.
We found that not all midges are equal and that frogs vary in their ability to evade these eavesdropping parasites. The degree of specialization was highly variable. Some midges sucked blood from only one or two frog species, while others did not discriminate. Likewise, some frogs were better at avoiding being bitten than others. We found that most midge species did not approach frogs calling at frequencies above 4 kHz. Furthermore, the degree of specialization was higher in the rainforest than in the peatswamp, possibly because species have had more time to form such dependencies in the much older rainforest environment, estimated to be 130 million years old.
The relationships between midges and frogs, forged over millions of years, have revealed to us how intricate and multi-faceted the selective pressures on animals are and have highlighted the arms race that exists between parasites and their hosts.