The head of third instar larvae of Hydroscapha natans was reconstructed three dimensionally on a computer. This technique allowed a detailed examination and presentation of internal features of a representative of the 'suborder' Myxophaga, which is characterized by the very small size of the immature stages and adults. Larval character states of H. natans were compared with features found in other representatives of the Coleoptera. The monophyly of the Myxophaga (excluding Lepiceridae) is supported by several autapomorphies of the larval head: a broadened, transverse head, scale-like cuticular surface structures, round and flattened labral sensilla, short antennae with only two antennomeres, a ligula with papillae, and a broadened tentorial bridge. A monophylum comprising the Hydroscaphidae and Microsporidae is characterized by a very unusual semientognathous condition of the mouthparts and an unusual shape and large relative size of the brain. The last common ancestor of the Hydroscaphidae, Torridincolidae, and Microsporidae was probably living in hygropetric habitats. Several apomorphies have evolved in correlation with this peculiar life style. The very dense arrangement of muscles and other internal structures, and the unusual shape and size of the cerebrum have resulted from miniaturization. The overall complexity of the head is not reduced in comparison to larvae of other representatives of Coleoptera. A negative allometric relationship between body size and the size of the brain, and a correlation between brain size and the size of neurons was found in several species of Coleoptera examined.
