ABSTRACT While there exist extensive memoirs on the comparative anatomy of the male genitalia of several insect orders, works dealing with their ontogenetic development are very few, with the consequence that up to now there is practically no unanimous agreement as to the fundamental or basic plan of these organs in the insect kingdom. Zander (119, 120, and 121), in a series of short papers, described the post-embryonic history of the external genital appendages of Hymenoptera, Trichoptera, and Lepidoptera, and came to the conclusion that the ‘penis’ an unpaired median organ in the adult, originates in the young as a pair of appendages which sub sequently fuse with each other along the median line and develop into the single tubular organ found in the imago. Zander, however, did not attempt the homologies of the various genital appendages, and, moreover, as will be shown later, his account of the mode of origin of the above-mentioned pair of appendages is also open to question. Previous to Zander, Verson and Bisson (108) in the case of Bombyx mori, and Michaelis (73) in that of the honey bee, had also shown the origin of the penis from a pair of appendages. Verhoeff (106), Esch erich (34 and 35), &c. pointed out the paired-appendage nature of the penis even in the adult condition in some Thysanura. Christophers (23) and Christophers and Cragg (24) studied the development of the genitalia in Diptera and Cimex (Rhynchota-Heteroptera) respectively, and confirmed the main conclusions of Zander. The account of Cimex alone cannot be said to be representative of the whole order Rhynchota, as this form, apart from being highly specialized, is devoid of—as are all other Heteroptera—the subgenital plates (see below), whose nature is of immense morphological importance, and is still sub judice. Besides, my observations on Idiocerus, belonging to the allied sub-order Homoptera, while in accord with Christophers and Gragg’s findings with regard to the mode of origin of the penis, lend little support to most of their conclusions, and in some points, e. g. history and nature of the associated internal ducts, are at variance with their results.