Memoirs: The Development of the Male Genitalia of Homoptera, with Preliminary Remarks on the Nature of these Organs in other Insects

(a) Internal Genital Ducts. In young Homoptera nymphs the male efferent genital system consists of a pair of vasa deferentia, which are anteriorly continuous with the testes, and a pair of terminal ducts of hypodermal origin which are at this stage not connected with the vasa deferentia. The terminal unpaired common ejaculatory duct arises later as a solid ingrowth of the integument; anteriorly it meets the blind ends of the paired hypodermal ducts. It soon acquires a lumen and freely communicates with the exterior. At a slightly later stage of development the paired hypodermal ducts, by a constriction in the horizontal plane along the whole of their length, become two pairs, a dorsal and a ventral, the accessory glands and the paired ejaculatory ducts respectively. The paired ejaculatory ducts communicate with the vasa deferentia at a much later stage of development. The vesiculae seminales develop from the distal ends of the vasa deferentia. The above mode of development lends great support to the view (Palmen) that primitive insects had a pair of terminal ducts, and clearly indicates that these ducts were ectodermal and not mesodermal as stated by Palmen. As shown by Nüssbaum in some other insects, of the efferent systems all organs except the vasa deferentia are of an ectodermal origin. But the way in which the common ejaculatory duct, the accessory glands, and the vesiculae seminales arise in Homoptera is quite different from that suggested by Nüssbaum. Unpaired organs are unpaired from the very beginning, they do not arise by the coalescing of paired organs, as this author believed. (b) External Genital Appendages. The male genitalia of the adult Homoptera consist of two pairs of lateral appendages, the sub-genital plates and the parameres, and a median copulatory organ, the aedeagus. They are all borne by the ninth abdominal segment. They develop from two pairs of appendages only, an outer and an inner, which appear as diverticula of the ventral region of the ninth segment. The outer pair develops into the subgenital plates, and the inner by longitudinal fission becomes two pairs; the inner one of the two pairs so obtained, by the fusion along the median line of its components, forms a single organ, the aedeagus. while the outer is transformed into the parameres. Thus the pair of appendages developing into the sub-genital plates does not belong to the eighth segment, as was believed by Kershaw and Muir, but to the ninth ; there are no appendages on the eighth in the nymphs or in the adult; nor is there any evidence in favour of these authors' view that the male gonopore in Homoptera, unlike that in most orders of insects, lies between the eighth and ninth sterna ; it is in its usual place, behind the ninth sternum. The sub-genital plates seem to be the coxites of the ninth sternum; and both the aedeagus and the parameres, derived from a primitively single pair of appendages, correspond to the endopodites. As in Homoptera, in insects in general there are, besides the median aedeagus, two pairs of appendages in connexion with the ninth (the coxites and parameres). and not one as hitherto believed. But generally one pair is developed, e. g. coxites in Ephemeroptera, Lepidoptera, Diptera, &c. ; parameres in Heteroptera, Orthoptera, Coleoptera, &c. Only rarely both pairs are present, e.g. Hymenoptera, Homoptera, Thysanura, some Diptera. The fact that usually only one pair occurs seems to be responsible for the notion that the male genitalia of insects consis of, tbesides the aedeagus, only one pair of appendages. This also explains why this one pair of appendages could not be homologized in the different orders. In male Homoptera, as in most other insects, the homologues of the anterior pair of ovipositor-lobes, believed by Kershaw and Muir to be represented by the sub-genital plates, are not present; the sub-genital plates correspond to the lateral ovipositor-lobes ; and the dorsal pair of ovipositor-lobes ia represented by both the aedeagus and the parameres, and not by aedeagus or parameres alone as hitherto believed.