Mecopterans are harmless creatures which are hunted for the bones from their hands which are powdered and added to soups to cure impotence. Geralt refuses to kill them, while Dandelion can confirm that the tales are rubbish, from personal experience with such soup.
“
Another asks me to kill a mecopteran and bring him a bone from its hand because, crushed and poured into a soup, it cures impotence—
That's rubbish, interrupted Dandilion, I've tried it. It doesn't strengthen anything and it makes the soup taste of old socks. But if people believe it and are willing to pay—
I'm not going to kill mecopterans. Nor any other harmless creatures.”
— pg(s). 160, The Last Wish (UK edition)