The pantropical subtribe Megacephalina represented by more than 100 species is the most diverse of the basal cicindelid groups. Today, most taxonomists recognize eight genera within the subtribe. This is in contrast to Horn who, back in 1910, conceded only two genus-level taxa: the monospecific Aniara and Megacephala sensu Horn (a genus which united the seven other taxa). In the present study we provide a molecular phylogeny of Megacephalina based on the nuclear 18S and the mitochondrial 16S and cytochrome oxidase III genes. The dataset includes 60 specimens of more than 40 mostly South American and Australian taxa. Three cicindelid species of derived lineages were used as outgroups. The resulting phylogenetic trees are basically in agreement with the current classification system. Megacephala and Grammognatha are placed basal in the dendrogram. Pseudotetracha and Australicapitona form a monophyletic Australian clade. Phaeoxantha, Tetracha and Aniara also form a monophyletic group. The position of Metriocheila remains uncertain. The most striking deviation from the traditional classification is the well-supported placement of Aniara within Tetracha, rendering the latter a paraphyletic taxon. Several monophyletic subgeneric species groups are observed in Pseudotetracha, Phaeoxantha and Tetracha/Aniara. Within the latter the monophyletic sobrina, carolina and brasiliensis clades together represent a monophyletic group. Additionally, habitat types were assigned to the taxa and mapped on the phylogenetic tree. The basal African species inhabit non-flooded uplands. The Australian species moved to inland and/or coastal salt plains. The American groups were most likely first confined to river margins and then colonized secondarily and independently non-flooded uplands and/or coastal habitats

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The tiger beetle Phaeoxantha klugii inhabits Central Amazonian floodplains, where it survives the annual inundation period in the third-instar larval stage submerged in the soil at approximately 29 C for up to 3.5 months. Because flooded soils quickly become anoxic, these larvae should be highly resistant to anoxia. The survival of adult and larval P. klugii was therefore tested during exposure to a pure nitrogen atmosphere in the laboratory at 29 C. Adult beetles were not resistant (< 6 h). Survival of larvae decreased over time, maximum survival was 15 days, whereas time to 50% mortality was 5.7 days (95% confidence interval 3.8-7.9). Anoxia resistance was additionally tested in third-instar larvae submerged within sediment for 40 days before anoxia exposure in the laboratory. Anoxia resistance was greatly enhanced in these larvae, showing a survival rate of 50% after 26 days of anoxia exposure. It appears that the gradual flooding process and/or the submersion phase induced a physiological alteration, most probably a strong depression in metabolic rate, which requires some days for induction. The degree of anoxia resistance in larval P. klugii is remarkable among terrestrial arthropods worldwide, even more so considering the high ambient temperatures. The species is well-suited to serve as a model organism for studying the physiological mechanisms of anoxia and submersion resistance in terrestrial arthropods inhabiting tropical floodplains.

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