PHENOLOGICAL RELATIONSHIPS OF SOME ACULEATE HYMENOPTERA, THEIR DIPTERAN MIMICS, AND INSECTIVOROUS BIRDS.

Since 1961 the senior author has collected at various locations in Illinois and observed Syrphidae and other Diptera which mimic aculeate Hymenoptera. During that time it became apparent that specialized mimetic syrphids are present in this area in May and June and again in September and early October, but that they are absent during most of the summer. With few exceptions the spring and fall groups of mimics consist of different species. We offer the hypothesis that specialized mimics of aculeate Hymenoptera are absent during most of the summer because they have been selected not to fly when large numbers of young and inexperienced birds are still learning to avoid the hymenopteran models. This investigation confirms by systematic sampling the above observations, and establishes the phenological relationships between mimics, models and insectivorous birds. Rothschild (1963) was the first to point out that model and mimic could be separated in time. She suggested that the buff ermine moth, Spilosoma lutea (Huf.), is a mimic of the white ermine, Spilosoma lubricipeda (L.). The two occur in the same area, but the white ermine emerges before the buff ermine, and there is only a short period of overlap when both are present. We believe that the Diptera in question are Batesian mimics of aculeate Hymenoptera, and that insectivorous birds are the main selective agents for the phenological relationships of the mimics, as well as for the morphological and behavioral aspects of their mimesis (Rettenmeyer, 1970). The available evidence (Mostler, 1935; Rettenmeyer, 1970) suggests that the avoidance of aposematic insects by birds is generally a learned rather than an innate response. Our investigations encompassed only specialized m,imics which strongly resemble aculeate Hymenoptera, as opposed to less specialized mimics which only vaguely resemble them. The specialized mimics exhibit a variety of morphological and behavioral adaptations which mimic salient features of bees or wasps. For example, all of the wasp mimics included in this investigation (Fig. 1, Complexes I-III) have a dark area along the leading edge of the wing which imitates the dark band produced by the longitudinal folding of the vespid wing at rest. All of these mimics also have some provision for imitating the long antennae of the models. Temnostoma spp. and Spilomyia spp. have the short, three-segmented antennae typical of cyclorrhaphous Diptera, but wave about their distally darkened front legs in imitation of the long, dark antennae of the models. The remaining wasp mimics, Tentkredomyia abbreviata, Ceriana signifera and Chrysotoxum sp. have different morphological adaptations which lengthen their threesegmented antennae (Waldbauer, 1970). On the other hand, the vague or less specialized mimics have few specifically mimetic adaptations. Many species of the Tribe Syrphini, for example, bear little resemblance to wasps except for their patterns of alternating black and yellow bands. The dronefly Eristalis tenax (L.) is generally considered to be a mimic of the honeybee, Apis mellifera L. We did not include these two species in this study because both are introductions from the Old World and may, therefore, have atypical phenological relationships.