Mosquitoes and man in the Pacific - more than just words
Edwin Nye's recent article in Antenna (Nye, 1998) related how the word namu is used for mosquito in many Pacific languages. His primary examples, however, are all Polynesian languages and the commonality of the linguistic root perhaps is not unexpected. In writing of "the Solomons, to which Tikopia belongs", however, he may have given a misleading impression that namu is used in Melanesia. While I know little of Melanesian languages, I do know that Tikopia is inhabited by Polynesians and therein hangs a tale of mosquitoes and man.
In 1970, while engaged on malaria entomology and epidemiology work in the Solomon Islands, my late friend and colleague, Dr. Mario Maffi, and I visited the Santa Cruz group of islands. On the same trip was Dr. Roger Green, an archaeologist and anthropologist who held the Captain James Cook Fellowship from the Government of New Zealand. He was already an authority on eastern Polynesia and was extending his studies to the Polynesian peoples of the western Pacific. Together, we visited the remote Polynesian-inhabited islands of Tikopia, Anuta and Pileni (Duff Is.) and in the hours at sea I learnt something of the fascinating story of the Polynesian migration. On Anuta it turned out that the inhabitants still practised an ancient form of terraced agriculture once used by their eastern cousins, including the use of fermentation pits for breadfruit.
Some fifty years earlier, Buxton & Hopkins (1927) had written of the apparently anomalous distribution of a mosquito, "Aedes variegatus var. tongae", which they knew to be widely distributed in the Tonga Islands but, or so they thought, was also to be found on Sikaiana, an eastern outlier of the Solomon Islands chain, over 3000 km to the west of Tonga. They wrote - "The anomaly disappears when we remember that Tongans and other Polynesians were in the past great voyagers, and that the Sikaiana people are not Melanesians, like other Solomon Islanders, but pure Polynesians; it is, therefore, reasonable to suppose that the distribution of this mosquito, the eggs of which are resistant to drying, has been partly effected in canoes".
The major publication on the mosquitoes of the South Pacific by John Belkin (1962) revealed that the "species" in question was in fact a complex of species, all regarded as members of the Aedes (Stegomyia) scutellaris group. Further evidence (Belkin, 1965) showed that the Sikaiana species was Aedes varuae, which was already known from several of the islands in the Santa Cruz Group, and led Belkin to speculate that the species was originally precinctive to that Group and had been spread by islanders to Sikaiana. Our collections in 1970 included varuae from the Duff Islands, where, in 1605, another early Spanish explorer, Quiros, had encountered a man from Sikaiana. Not long after, a colleague collecting for us found varuae at Luaniua Island, in the Ontong Java Atoll. So now we had a species spread right along an arc of islands inhabited by Polynesians, although the larger Santa Cruz Group islands are inhabited by Melanesians.
The combination of the collections reported by Belkin, others listed by Huang (1972) and Huang & Hitchcock (1980), and our reports (Maffi, 1973, 1977, 1989; Maffi & Taylor, 1974, 1977; Taylor, 1973, 1989; Taylor & Maffi, 1978), has enabled me to compile the map which accompanies this article.
From this it seems that there are four major species - quasiscutellaris from the Melanesian Solomons; hebrideus from the Polynesian-Melanesian outer Solomons and Vanuatu; polynesiensis from the mainly Polynesian eastern Pacific (including the islands to the east of the map); and, marshallensis from the more northerly Micronesian islands. Within the first three of those species ranges, however, locally endemic species and imperfectly known forms add a further 16 members to the group. All told there are some 30 closely related species in the scutellaris group running eastwards from Southeast Asia. Dev & Rai (1985) examined the mating compatibility of seven species and concluded that they fell into two mating groups - of western species (hebrideus, scutellaris katherinensis, malayensis and alcasidi) and Polynesian species (polynesiensis, pseudoscutellaris and kesseli). It seems that they were able to work only with man-biting species and, I suspect, that if one looks for a "root-species" akin to the root-language in the South Pacific it may well be the non-man-biting quasiscutellaris from the Melanesian Solomons. A measure of its success there is that the normally ubiquitous colonist Aedes aegypti has been hardly able to become established. On the linguistic front, a very simplified scheme outlined to me by Roger Green looks somewhat like the "scutellaris" pattern - diminishing in numbers of speakers as one goes eastwards and with "endemicity" - viz. Eastern-Oceanic > Proto-Southeast Solomons > Proto-Hebrides-Central Pacific > Proto-Central Pacific > Proto-Fijian and Proto-Polynesian > Polynesian outliers; but with the western Polynesian languages perhaps having their origins as reverse migrations.
All the scutellaris-group species are container-breeders, and probably, like the much better known Ae. aegypti, all have eggs which tolerate desiccation, being laid just above the water level and hatching when wetted. This would have given them the capacity to "hitch a lift" with human voyagers. The picture becomes more fascinating when it is realised that many do not bite man but those which do can be serious pests. In a few instances, moreover, the indication is that they are vectors of human diseases. For instance, in 1975, an epidemic symptomatic of dengue fever occurred in the Banks and Torres Islands, the distribution and behaviour of aobae suggested that this had been the vector (Maffi & Taylor, 1977). Ae. polynesiensis had long been suspected as a vector of dengue in French Polynesia and the Cook Islands, also, rotumae as the vector on Rotuma (Suzuki & Hirshman, 1977), and pseudoscutellaris in Fiji (Takagi et al., 1990). A further disease of which polynesiensis is a suspected vector is Ross River Virus, epidemic polyarthritis, recorded in the Cook Islands (Rosen et al., 1981). Rosen et al. also mention earlier serological evidence of this virus in humans from New Guinea, Melanesian Solomons and Vanuatu. Firm proof of an interesting occurrence of disease is the presence of subperiodic bancroftian filariasis in the eastern Pacific. The "normal" filariasis of south Asia and eastwards into the Solomons and New Hebrides, is transmitted by nocturnal vectors (such as Anopheles farauti and the pantropical Culex quinquefasciatus in the latter island chains). In Polynesia there are few or no naturally occurring nocturnal vectors and the parasite, which presumably also arrived with its human hosts, has relaxed the periodicity of its appearance in the subcutaneous blood-vessels to coincide with diurnally-feeding mosquitoes - again polynesiensis and Aedes (Finlaya) samoanus (Samarawickrema et al., 1987). Ae. samoanus does show a pattern of early night biting in houses and perhaps this is a vector to which the filaria has come more recently. A final disease of concern utilising these two vectors in eastern Polynesia is Dirofilaria immitis, a dog parasite which will develop partially in humans, enough to cause internal lesions (Samarawickrema et al., 1992).
Without going into details, the members of the kochi group of Aedes (Finlaya) species breed almost exclusively in leaf axils, such as those of aroid root crops, including taro, and screwpines, Pandanaceae. Whereas the Melanesians, at least in the Solomons, use leaves of the swamp-dwelling sago palm for thatch roof and wall panels, the Polynesians utilise broad-leaf forms of the low-moisture tolerant Pandanus, not withstanding the fact that the latter have saw-edges to the leaves. Thus, the Polynesians took Pandanus with them on their migrations. Ae. samoanus is one example but in the Polynesian outliers of the Solomons, I discovered Ae. maffii on Rennell and later reappraised "hollingsheadi" specimens from Sikaiana as also being maffii (Taylor & Maffi, 1978). Ae. hollingsheadi itself is widely distributed through the Melanesian Solomons.
To deviate on pest species, after our visit to the Duff Islands, Roger Green and I discussed the possible reasons why most of the people there live on an artificial island, Tahua, set some 100 m out in the lagoon within inside the fringing reef to the west of the main island. An already reported explanation given by the inhabitants was that this was to avoid being bitten by mosquitoes, with the prevailing argument being that these were of the sole malaria vector, Anopheles farauti. My feeling was that the more likely truth was that on-shore two particularly voracious species, Armigeres breinli and Aedes lineatus, were abundant. These are day-active but do not venture out of fairly deep shade. Thus, they do not venture out across the lagoon, unlike the night-biting Anopheles farauti, which in any case has an innocuous bite and causes no obvious discomfort to its prey. To close, perhaps the most exciting moment of our 1970 trip was the discovery by Roger Green, at Graciosa Bay, Santa Cruz, of the long-lost camp of the Spanish expedition led by Mendana in 1595 (Allen & Green, 1972) - "Long-live multidisciplinary studies".
©1998 - Brian Taylor CBiol FSBiol FRES|
11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K.
Comments to firstname.lastname@example.org