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Amphizoa is a genus of adephagan aquatic beetles, placed in its own monogeneric family, Amphizoidae. There are five known species of Amphizoa, three in western North America and two in eastern palearctic. They are sometimes referred to by the common name "troutstream beetles".
Troutstream beetles have a characteristic appearance. They are relatively large, oval, slightly convex, dull black to piceus. Body length ranges between 11–16 mm (0.43-0.63 in). The head is broad with a quadrate shape and small round eyes. The antenna is filiform, rather short with 11 segments. Pronotum significantly narrower than the elytra and with lateral margins slightly crenulated; prosternal processes broad and flat, rounded to truncate at apex; elytra vaguely striate and ventroapically with a series of short spines of unknown function. The legs are not well adapted for swimming, and lack long swimming setae; hind coxae extending to lateral margin of abdomen; tarsal formula is 5-5-5. 
Known species of Amphizoa
Amphizoa davidis – bodylength between 10.5-12 mm the elytron lack a carina on fifth interval. Pronotum have a lateral margin without lateral bead. Only known from the province Sichuan in China. 
Amphizoa striata – a larger species, bodylength around 13-15 mm, front tarsi with well developed groove on posterior surface, grooves bearing a fringe of long setae. Known from British Columbia, Oregon and Washington. 
Troutstream beetles can be found in streams and rivers in mountain regions of China, North Korea and western North America. Streams are often coldwater, medium to fast flowing, and the beetles can be found clinging to rocks, woody debris or at margins. Both adults and larvae are predators, especially on stonefly larvae but occasionally on other aquatic insects. Larvae may also scavenge dead insects. When disturbed, adults exude a yellowish fluid from the anus, with an odor described as that of cantaloupe or decaying wood, probably as a defense mechanism against predators like frogs and toads.
Phylogeny and evolution
Amphizoidae share some plesiomorphic features with Carabidae (e.g. Slender ambulatory legs) and other characteristics with Dytiscidae (e.g. large sensorial lobes of epipharynx). In an analysis based on the genes 18S rRNA, 16S rRNA and Cytochrome Oxidase I, Amphizoidae was placed as a sistergroup of a clade comprising the newly described family Aspidytidae, Paelobiidae(=Hygrobiidae) and Dysticidae. An analysis based on a morphological character matrix also came to this conclusion. However, two other studies with more genes have placed Amphizoidae as sistergroup to Aspidytidae. With Aspidytidae and Amphizoa share the same morphology of the apical part of the pro-sternal process and the mesocoxal cavities. The phylogeny within Amphizoa has been analysed in two studies based on morphological characters and they suggested that A. davidis is an isolated species and sister to the remaining four species as follows (A davidis, (A. insolens, (A. striata, ( A. sinica + A. lecontei)))). 
- Amphizoa LeConte 1853. Illinois Natural History Survey.
- Nilsson, Anders N., and Bernhard J. van Vondel (2005) , World Catalogue of Insects. Volume 7: Amphizoidae, Aspidytidae, Haliplidae, Noteridae and Paelobiidae (Coleoptera, Adephaga)
- Kavanaugh, David H. (1986) A systematic review of amphizoid beetles (Amphizoidae: Coleoptera) and their phylogenetic relationships to other Adephaga, Proceedings of the California Academy of Sciences, vol. 44, no. 6
- Dettner, K. 2005: 7.4. Amphizoidae LeConte, 1853. Pp. 81-85 in: Beutel, R.G.; Leschen, R.A.B. (volume eds.) Coleoptera, beetles. Volume 1: Morphology and systematics (Archostemata, Adephaga, Myxophaga, Polyphaga partim). In: Kristensen, N.P. & Beutel, R.G. (eds.) Handbook of zoology. A natural history of the phyla of the animal kingdom. Volume IV. Arthropoda: Insecta. Part 38. Berlin, New York: Walter de Gruyter.
-  Illinois Natural History Survey
- Ji, L.; Jäch, M.A. 2003: Amphizoidae: taxonomic notes and new distributional records (Coleoptera). Pp. 49-52 in Jäch, M.A.; Ji, L. (eds.) Water beetles of China. Volume 3. Zoologisch-Botanische Gesellschaft and Wiener Coleopterologenverein, Vienna.
- ITIS: The Integrated Taxonomic Information System. Orrell T. (custodian), 2011-04-26
- Ribera, I., Hogan, J. H. & Vogler, A. P. 2002 Phylogeny of Hydradephagan water beetles inferred from 18S rDNA sequences. Mol. Phyl. Evol. 23, 43–62.
- Rolf G. Beutel, Michael Balke, Warren E. Steiner Jr3 The systematic position of Meruidae (Coleoptera, Adephaga) and the phylogeny of the smaller aquatic adephagan beetle families: Volume 22, Issue 2, pages 102–131, April 2006
- Balke, M., Ribera, I., Beutel, R.G., Viloria, A., García, M. & Vogler, A. (2008) Systematic placement of the recently discovered beetle family Meruidae (Coleoptera: Dytiscoidea) based on molecular data. Zoologica Scripta, 37, 647–650.
- Balke, M., Ribera, I. & Beutel, R. G. 2005. The systematic position of Aspidytidae, the diversification of Dytiscoidea (Coleoptera, Adephaga) and the phylogenetic signal of third codon positions. Journal of Zoological Systematics and Evolutionary Research 43 (3): 223-242
- Edwards, J. Gordon (1951) Amphizoidae (Coleoptera) of the World, The Wasmann Journal of Biology, vol. 8, no. 3
- Peiyu, Yu, and Nigel E. Stork (1991) New evidence on the phylogeny and biogeography of the Amphizoidae: discovery of a new species from China (Coleoptera), Systematic Entomology, vol. 16, no. 2
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