I, personally, would almost never feel a fly on my back, so my answer would be the face. I could test my choice by getting something as light as a fly, such as a piece of small paper or a bread crumb.
Definetly its face since face have more sensitive nerons than our back. thats why a small cut in face hurts more than back.
You would likely feel the fly on your face first because the skin there is more sensitive than on your back. To test this, you could use a lightweight piece of paper to simulate the weight of a fly and compare the sensations on your face and back. Repeating the experiment can confirm this result.
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