The morphological features of holometabolous insect larvae are generally regarded as caenogenetic in nature, reflecting an extensive to environments. They are nonetheless significant systematically and phylogenetically, since they show the relationships among the forms possessing them (de Beer, 1940; Michener, 1953; Uchida, 1955; van Emden, 1957). Caenogenetic features appear to have evolved along two different lines, one a rather nonplastic and intrinsic, perhaps phylogenetic, trend subject to adaptation (Brown, 1958) or likely to be represented by a continuum of (Asanuma, 1950); the other a rather plastic trend, variable according to the development of adaptation (Brown, 1958) in response to varying environments. The two trends can be referred to as and special caenogenetic trends, and they exhibit complicated interaction during both ontogeny and phylogeny. The intrinsic or phylogenetic trends in larval characters may result largely from pleiotropic effects of genes selected for in other stages, while the larval characters not related to the phylogenetic trends may be adaptations to specific larval requirements. The present work was carried out with the aim of determining in what manner and to what extent these two trends are mutually interrelated, using as material the larval mouth hooks of the family Drosophilidae. The structure and metamorphosis of the buccal armatures of drosophilid larvae have been the subject of repeated investigation. In Drosophila melanogaster, for instance, Strasburger (1932) dealt with individual variation of the mouth hook dentition, while Alpatov (1929) showed on a statistical basis that there is no overlapping in number of teeth between any two successive instars. Bodenstein (1950) also reported that the number of teeth is useful as a means of distinguishing the three instars. Mayer (1935) observed in several species of Drosophila some structural differences in dentition between the left and right hooks of an individual larva. Sturtevant (1921) noted the function of the mouth hooks in mastication. Hennig (1952) believed that the development of the mouth hooks was associated with the strength of musculature and not fundamentally with the larval eating habits, e.g., saprophagy, fungus eating, and leaf mining. The present observations have revealed that there are some caenogenetic relations in the structure of the mouth hooks to the feeding habits, as well as some general caenogenetic structures characteristic of each instar. Drosophilid larvae belonging to 51 species (table 1), mostly from Japan, furnished the material for this study. They were either reared from eggs laid by flies in captivity or obtained from natural breeding sites. The buccal armature of the first and second instar larvae was rather easily dissected out from larval exuviae found ir food media, as Bodenstein (1950) noticed and that of the third from the puparia] case. The specimens were cleaned by boiling in 10% KOH solution and mounted ir euparal-alcohol. The author wishes to express his cordia] thanks to Professor Daigoro Moriwaki Dean of the Faculty of Science, Tokyc Metropolitan University, for his kind guidance in this work. He is much obliged to Professor Sajiro Makino, Hokkaido University, who was kind enough to make arrangement for publishing the present work.