Illustrations in these volumes are nature-printed, a technique where the surface of a natural object is used to produce a print. This print may be unaltered or may be enhanced with lines and color. They vary from very rudimentary prints to complex and highly detailed colored works. The emphasis is on scientific accuracy while also being aesthetically pleasing. While nature printing has been around for thousands of years, major advances in technique were made in the 18th and 19th centuries.

These prints, a midpoint between specimens and illustrations, represent an important subset of scientific illustration. They were often created on long expeditions where space constraints and damage to actual specimens was common, as well as being sent via post between scientists in circumstances where specimens could not. Abundant scientific information can be gleaned from these illustrations. For the non-scientists, they were a way for people to connect with the natural world.