1. The coevolution of insect pollinators and their host plants is a typical example of natural selection; however, it remains unclear as to how insect pollinators avoid overdependence on one peculiar plant. As most insect pollinators exhibit a diet breadth when showing flower constancy, to determine the difference and similarity of most and less-preferred flowers by insect pollinators may be helpful to understand their trade-off between flower constancy and overdependence.

2. We addressed this question in the long-proboscid tangle-veined fly (Nemetrinus spp.). Dietary investigation indicates that the flies show constancy for the morphological characteristic of the Delphinium caeruleum, which is the most preferred plant for this Nemestrinidae fly that has blue, long-tubed flowers.

3. In a colour selection experiment, focal individuals showed obvious preference for white, which is the colour of less-preferred flowers by the fly in the natural environment. In a scent selection experiment, focal individuals showed obvious preference for D. caeruleum and Dracocephalum heterophyllum, but avoidance to Dasiphora fruticosa and Dasiphora davurica. This indicates that long-proboscid tangle-veined flies can forage on other flowers, even the existing of constancy for D. caeruleum, as long as they do not hate the scent. It seems that long-proboscid tangle-veined flies can maximize foraging efficiency by showing constancy for the morphological characteristic of the most preferred plant and for the scent and colour of less-preferred plants.

4. The tradeoff of long-proboscid tangle-veined fly in selection of nectar sources may be an adaptation to the risk of overdependence on one plant in evolution.

We have collected the dataset in the field. It has not been processed.