The first time in Graph Theory a graph was characterized as “Bowtie” was in the seminal paper by Broder et. al. Though no textbook had ever mentioned this type of graph before, no less an important network than the Web Graph itself is supposed to resemble this shape. In two large collections of crawled Web pages and in numerous smaller collections, researchers discovered Bowties and Bowtie-looking graphs by studying millions of web pages. But why do collections of Web pages resemble a Bowtie? The short answer is “because, given the way the Web is created, that’s the only shape it could have”. This paper shows why this is the case and presents an algorithm and software to visualize Bowtie graphs.
Metaxas, Panagiotis. "Why Is the Shape of the Web a Bowtie?" World Wide Web (WWW) Conference, WebScience Track, Lyon, France, April 2012.