How is it that a fly always seems to be buzzing around your food moments after you sit down for an outdoor meal?
The answer is practice. Or, more specifically: evolution（进化）. Flies and other insects have been on a multimillion-year journey of evolution, improving their ability to detect food. Being able to zero in on nutritious meals is a matter of life and death.
The family of flies that I study — the blowflies — are the buzzing ones that are usually a beautiful metallic blue, with bronze and green colors. They've perfected their ability to quickly sense the smells that naturally come off picnics and garbage cans. There is a lot of competition for a resource like an overflowing garbage can because of how nutritious garbage, with the meat rotting in it, is. But the blowflies can sense these smells long before their competitors or people can, and tend to show up to the scene first.
Sensing systems differ depending on the insect and species. The blowflies' main sensing organ is their antennae, two thin projections（凸起） from the head that are covered in tiny hairs. These fine hairs are made up of special cells that contain receptors（受体） for specific smells. Think about a batch of chocolate chip cookies fresh from the oven. You can detect their delicious smells because we humans have receptors on the surfaces of the cells that line the inside of our noses. These receptors send signals to the brain: yummy food ahead. They're detecting the sweet smell of sugar-based molecules, an energy-rich food source for us.
What's a "good" or a "bad" smell can differ depending on the animal doing the smelling. The attractive rotting meat stench（恶臭） that a fly finds delightful is perceived quite differently by a person passing by an unpleasant garbage can on a hot day. But any fly that can detect the useful smell signal, which means "nutritious fly food here", will have an advantage. Over time, the insects that have the receptors for those smells will have better survival rates and produce more generations.