The term mouse fly is kind of a contradiction and one of the types of flies that brings the word fly in fly fishing almost as far away from something flying as possible - still surpassed by shrimp flies not to mention crab flies, but anyway...
One of the reasons Britain became rich was because the Industrial Revolution (factories and such) got off the ground there sooner. Britain had a large navy, and many colonies in other parts of the world, such as India, and Rhodesia.
Widespread credit, business corporations, investments and large-scale stock markets all become common. Britain led the way in this transformation. By the 1780s, the British Industrial Revolution, which had been developing for several decades, began to further accelerate. Manufacturing, business, and the number of wage laborers skyrocketed, starting a trend that would continue into the first half of the 19th century.