Hemocyanins are copper‐containing, respiratory proteins that have been thoroughly studied in various arthropod subphyla. Specific O2‐transport proteins have long been considered unnecessary in Hexapoda (including Insecta), which acquire O2 via an elaborate tracheal system. However, we recently identified a functional hemocyanin in the stonefly Perla marginata (Plecoptera) and in the firebrat Thermobia domestica (Zygentoma). We used RT‐PCR and RACE experiments to study the presence of hemocyanin in a broad range of ametabolous and hemimetabolous hexapod taxa. We obtained a total of 12 full‐length and 5 partial cDNA sequences of hemocyanins from representatives of Collembola, Archeognatha, Dermaptera, Orthoptera, Phasmatodea, Mantodea, Isoptera and Blattaria. No hemocyanin could be identified in Protura, Diplura, Ephemeroptera, Odonata, or in the Eumetabola (Holometabola + Hemiptera). It is not currently known why hemocyanin has been lost in some taxa. Hexapod hemocyanins usually consist of two distinct subunit types. Whereas type 1 subunits may represent the central building block, type 2 subunits may be absent in some species. Phylogenetic analyses support the Pancrustacea hypothesis and show that type 1 and type 2 subunits diverged before the emergence of the Hexapoda. The copperless insect storage hexamerins evolved from hemocyanin type 1 subunits, with Machilis germanica (Archeognatha) hemocyanin being a possible ‘intermediate’. The evolution of hemocyanin subunits follows the widely accepted phylogeny of the Hexapoda and provides strong evidence for the monophyly of the Polyneoptera (Plecoptera, Dermaptera, Orthoptera, Phasmatodea, Mantodea, Isoptera, Blattaria) and the Dictyoptera (Mantodea, Isoptera, Blattaria). The Blattaria are paraphyletic with respect to the termites.

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