The animals on the cover of Mastering the Lightning Network are wood ants (Formica rufa).
Many of the animals on O'Reilly covers are endangered; all of them are important to the world. To learn more about how you can help, go to animals.oreilly.com.
The cover illustration is by Karen Montgomery, based on a black-and-white engraving from a loose plate, origin unknown. The cover fonts are Gilroy Semibold and Guardian Sans. The text font is Adobe Minion Pro; the heading font is Adobe Myriad Condensed; and the code font is Dalton Maag's Ubuntu Mono.