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Adult and larval characters place the monophyletic Ectemnorhinus-group of genera firmly within the former Adelognatha and new concept of Brachycerinae. The group, previously with tribal or subfamily status, is most closely related to the genera Heterexis Broun and Oclandius Blanchard from the Auckland and Campbell Islands. A cladistic analysis of the genera based on 35 adult and 11 larval characters, resulted in the following six genera being recognised: Christensenia Brinck, Canonopsis C. O. Waterhouse, Disker Dreux & Voisin, Palirhoeus Kuschel, Bothrometopus Jeannel and Ectemnorhinus G. R. Waterhouse. Ectemnorhinus angusticollis (C. O. Waterhouse) is transferred to Bothrometopus, Mesembriorrhinus parvulus Jeannel, also transferred to Bothrometopus, is reinstated to species level. Ectemnorhinus curtus Jeannel is placed in synonymy with Ectemnorhinus fuscus Enderlein, which is removed from synonymy with Ectemnorhinus viridis G. R. Waterhouse and reinstated as valid. Ectemnorhinus hibernantium (Dreux & Voisin) is placed in synonymy with E. geniculatus Jeannel.

Relatively few instances of sympatric speciation have been reported in the literature, and both theoretical and empirical studies of this mode of speciation remain controversial. Recently, a model of sympatric speciation less restrictive than earlier ones has been proposed. This study presents a hypothetical speciation scenario, based on current observations, which seems to be consistent with the new model. Two morphologically similar, though ecologically separated, species are identified in the Dusmoecetes similis (C. O. Waterhouse) species complex on Marion Island (46° 54′S, 37° 45′E), based on a study of the morphology, biology and ecology of the group. Differences between the demands placed on angiosperm‐ and bryophyte‐feeding members of the complex, and the differing demands of bryophyte and angiosperm‐dominated communities, coupled with low vagility and a high degree of assortative mating, suggest that a sympatric model can be used to explain speciation in this genus, which probably occurred within the last 10,000 years. Differences between Dusmoecetes marioni Jeannel and D. Similis (Waterhouse) are discussed, and a lectotype designated for the latter species.

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