1 Department of Ecology, PO Box 84, Lincoln University, 7647, Canterbury
2 Landcare Research, Private Bag 1930 Dunedin 9054
3 Dep. of Entomology Faculty of Agricultural & Food Sciences University of Manitoba Winnipeg, MB, Cana
4 Museum of New Zealand Te Papa Tongarewa, PO Box 467, Cable Street, Wellington 6011, New Zealand
5 Department of Zoology University of Otago P.O. Box 56 Dunedin 9054 New Zealand
Menacanthus eurysternus (Burmeister, 1838), is the most widely distributed louse species being found on at least 176 host species throughout the Passeriformes. The cosmopolitan nature of M. eurysternus is all the more interesting given that mean number of host species sharing louse species for passerines is 1.9. Menacanthus species are not generally widespread with most species found on only a few host species. M. eurysternus, therefore, represents an interesting anomaly amongst lice. There are several hypotheses that may account for the pattern observed. M. eurysternus may represent:  a true multi-host species that has exploited an as yet unknown mechanism to move easily between different host species at a level that maintains high levels of gene-flow; a species that was found on a group of European passerines, e.g. blackbirds, sparrows and starlings, which have been extensively introduced around the world and from which endemic passerine species have been recently colonised;  a morphologically conservative group which forms a cryptic species complex, e.g. ‘M. eurysternus’ from some hosts in a given area may be genetically distinct from lice of other hosts or areas. We have collected M. eurysternus samples from New Zealand, Australia and England and from several common species of passerines. We have examined molecular markers (COI and EF1alpha) and they imply a complex level of genetic variation that suggests hidden diversity among the lice and some structures that match the phylogeny of host species.