Which tool would be most useful for observing the details of an insect's wings thermometer or balance or hand lens or graduated cylinder?

A hand lens would be the most useful tool for observing the details of an insect's wings. It allows for magnification, enabling you to see fine structures and patterns that would be difficult to discern with the naked eye. The other tools, such as a thermometer or balance, are not designed for detailed visual observation, while a graduated cylinder is meant for measuring liquid volumes.