Antennae are the olfactory sense organs of insects, and the fan-like structure greatly increases the surface area available for the chemo sensors. Insects usually need to sniff out two things: food and mates. The additional ‘leaf’ carried by the male assists in detecting the pheromone used as reproductive enticement by the female. It enables males to find females even in the dark!
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As you can see from the quality of this and the previous photograph, I have reached here the technical limits of my Nikon 105 mm macro lens.
To take these close-ups, I have used a trick: I cut off the tip of a branch and placed a cockchafer (which I shook from an oak tree shortly before) on the remaining part, its head facing the cut end. In most cases the beetle crawled along the branch until it reached its end. Here, cockchafers need a bit of time to decide what to do: to fly off or turn around and crawl in the other direction. I used this short moment of stalemate to take the portrait shots.