The bird caught a bug on the fly.

那鸟在飞住了一只昆虫。

This is a new program that creates GIF images on the fly.

这是一种新的程序，能够即时生成GIF图像。

Being able to answer them on the fly gives you a real advantage over the crowd of innumerate amateurs.

能够飞速地回答这些问题，将使你获得一个位于无业余交易者人群之上的优势。

The HTML string is different from the traditional selectors in that it creates the DOM elements representing that HTML string, on the fly, to be (assumedly) inserted into the document later.

这个HTML字符串不同于用来表示那些HTML字符串、并据以查询相应DOM元素的选择符，一，随后都会把这个HTML代码串插入到文档去。