The structure and function of the aedeagus were investigated in 9 genera of the family Helodidae, most thoroughly in Helodes and Cyphon. On the basis of this investigation, the different types of tegmen and penis represented within the family are described. The tegmen of the Helodidae, which is either entirely devoid of a sclerotized "basal piece" or possesses only rudiments of such, may be divided into three main types; in addition there are a few quite aberrant cases. The penis (median lobe), more interesting from the standpoint of comparative morphology, is extremely diversified. A separation between two main types could however be maintained: one represented by genus Helodes alone, the other by all remaining genera. The Helodes type is more primitive, though at the same time rather specialized in certain details. The second penis type is still more specialized. Its main characteristic is a separate triangular sclerite (a "trigonium") situated ventrally in the distal part of penis. In certain genera it is cleft medially into a pair of moveable appendages ("prosthemata"). A splitting of the penis apex into two lobes ("parameroids") occurs in most species of Helodes, but also among the majority of remaining genera. The evolution of the penis from a supposed primitive initial form towards the Helodes type on one side and the highly specialized type represented by certain species of Cyphon on the other, is outlined and discussed. The importance of a gradual change of the copulatory method for this evolution is particularly stressed. The morphology of the aedeagus as basis for the systematics of the Helodidae is briefly treated. The investigation was also extended to cover representatives of other families, generally regarded as related to the Helodidae. Most of them possess an aedeagus of a rather primitive trilobed type. Deviating from this pattern are the Dascillidae, with the penis completely cleft into one dorsal and one ventral sclerite, and also the Psephenidae. In this family, the penis is supplied with a separate sclerite, apparently homologous with the "trigonium" of the Helodidae and with a similar evolution.