Fauna of New Zealand, No 74: Ceratomerinae (Diptera: Empidoidea: Brachystomatidae)

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Language: English with bilingual summary in English and Maori

The Empidoidea are commonly known as dance flies and long-legged flies. There are four or five empidoid families in New Zealand depending on the classification followed. The subfamily Ceratomerinae of the family Brachystomatidae is known only from South America, Australia and New Zealand. The New Zealand ceratomerines belong to three genera: Glyphidopeza, Zealandicesa and Ceratomerus. The former two genera are endemic to New Zealand, occurring nowhere else in the world. The widespread genus Ceratomerus is highly diverse with more species found in New Zealand than in Australia and South America combined. The distribution suggests that this subfamily of dance flies is of Gondwanan origin.

Most New Zealanders have never encountered ceratomerine flies. These slender, long-legged, mostly yellowish brown flies are found along the edges and on emergent rocks of small streams, small roaring cascading rivers and in humid forests. Most species are believed to prey on small insects, except the C. dorsatus group where adults have been collected on flowers and likely feed mostly on nectar. Ceratomerines are readily and most effectively collected by setting out small yellow bowls with water and a drop of soap. The colour appears to be highly attractive to ceratomerines and many other insects. They can also be collected using sweep nets and Malaise and UV-light traps.

Glyphidopeza is known from two species in small streams in the northern region of the South Island. Zealandicesa is widespread on the North and South Islands occurring around small pools in sphagnum regions and damp cool forests. Ceratomerus is also found throughout the North and South Islands and also Stewart Island with 45 known species, of which 33 new species are described in this work based on the examination of some 1800 specimens. No ceratomerines are known from the distant offshore New Zealand islands. New Zealand Ceratomerus is divided into seven species groups defined on the basis of modified male secondary sexual characters or ornaments. There appears to be no end to the variety and form of the exaggerated male ornaments, which include modifications of the mouthparts, antennae, fore and midlegs, wings and thorax.

Major gaps in our understanding of the Ceratomerinae remain. The immature stages urgently need to be discovered and molecular techniques hold promise in achieving this goal. Mating and feeding behaviours are mostly unknown and new observations are to be encouraged

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