In contrast to most blood sucking flies, the structure of the spermathecae in phlebotomine sandflies exhibits extraordinary diversity. The objective of this study was to investigate the fine structure of the spermathecae in relation to mating of phlebotomine sandflies. Light microscopy and electron (scanning and transmission) microscopy were used to examine species of four representative subgenera from the Old World Sergentomyia (Parrotomyia) babu, Phlebotomus (Euphlebotomus) argentipes, P. (Phlebotomus) papatasi and P. (Larroussius) langeroni. The spermathecae of P. papatasi is composed of a pair long spermathecal ducts, a cylindrical spermatheca surrounded by a visceral muscle and a spherical gland attached by an epithelial layer. The internal histology of the spermathecae in all the four species includes: a simple epithelial layer of class 1 epidermal cells, elaborate glandular cells (class 3 epidermal cells) each having receiving and conducting ductules ("end apparatus") and a cytological apodeme, which is a new finding for the insect epidermis. The glandular cells are connected to the lumen of the spermatheca by cuticular ductules passing through a cuticular block which has a high resilin content. The spermathecal complex is enveloped in a supercontracting visceral muscular system and has a myoneural junction. Based on this study a new terminology for the spermathecal morphology is proposed. The spermathecae undergo substantial physiological changes during the female gonotrophic cycle based on studies with Phlebotomus papatasi and P. argentipes. A histochemical study revealed a mucopolysaccharide secretory-mass in the newly emerged fly. During insemination, spermatophores are deposited in the spermathecal ducts. Previous reports of mating plugs in sandflies appear to be artifacts. The histology of the spermatophore is described and the anatomical evidence for sperm competition and and II displacement presented. Blood-feeding stimulates the release of sperm from spermatophoroand then they migrate to the lumen of the spermatheca. The fine structure of spermatozoa in P. papatasi and P. argentipes during their morphological change, such as acrosomal membrane casting off described. Physical evidence suggests that P .papatasi is inseminated more than once in each gonotrophic cycle, and that further insemination is necessary for subsequent cycles. The lock-and-key hypothesis has been suggested as an important mechanism for species evolution. To test this, the length of the spermathecal duct and aedeagal filament were examined from 28 species of 13 subgenera of the Old World phelebotomine sandflies. In general, there was a positive correlation between these variables but in most taxa the aedeagal filaments were long for direct insemination of the spermathecal proper. that These findings indicate.sperm competition takes place. Given the structural diversity of the spermathecae and its significance in mating strategies, and the importance of spermathecae in phlebotomine systematics, a comparative morphological study of spermathecal variation with male genitalic variation d· was made to classify the Old World phlebotomines. Twenty nine taxa representative.all the Old World subgenera were analyzed by cladistic methods. The phylogenetic groupings based on these characters generally confirmed the presently accepted system, and that all the vectors of human leishmaniasis are in recent terminal taxa