Joseph Leidy, one of the great American scientists of the 19th century, possessed a vast knowledge of the natural world. Best known as the Father of American vertebrate paleontology, he also pioneered the fields of parasitology and protozoology, and was the preeminent anatomist of his time. He was also an influential teacher of natural history and an expert in areas as diverse as entomology, geology and pathology.

Leidy was a passionate advocate of employing the microscope in scientific research. He was first introduced to the tool by his medical school mentor, Dr. Paul Goddard and he emphasized its usefulness to his own students. In his research Leidy used the microscope to examine a wide range of subjects, including fossils, gems, parasites, forensic medicine and human tissues. Notable among his microscopic studies are his 1848 study on the microstructure of the liver, his 1853 parasitology monograph ("Flora and Fauna within Living Animals"), his 1879 protozoology monograph ("Fresh-Water Rhizopods of North America") and his study of endosymbionts in termites ("The Parasites of the Termites").

Joseph Leidy's contribution to American parasitology is perhaps second only to his contribution to vertebrate paleontology. Although several European scientists were active during the 19th century, Leidy was essentially the only American researcher in the field. His work was highly regarded in Europe, but it wasn't until the early part of the 20th century that it was broadly appreciated by his American peers.

Leidy's parasitological studies were concentrated into two separate periods. The first period (1846-1858) began early in Leidy's scientific career, while the second (1874-1891) occupied the latter part. In between, Leidy's efforts were concentrated on Vertebrate Paleontology. Early studies included many short articles and notices covering parasites from a wide variety of animals, such as dogs and pigs. In 1874, after nearly 16 years during which his efforts were devoted to paleontology, Leidy resumed his studies in parasitology, and published longer articles on the tapeworms in birds, parasites of shad and herring, leeches and the symbionts of termites. Curious about the diet of a common termite, Joseph Leidy collected some of them and examined the brownish material from their gut under the microscope. He wrote of his astonishment of finding it full to be made up predominately of parasites.

Leidy published his findings, "The Parasites of the Termites", as a scholarly paper in the Journal of The Academy of Natural Sciences of Philadelphia, 1881. The only other references to termite parasites Leidy could find were some short passages in a monograph on termites by the French scientist Lespes in 1856. Leidy's report was the first detailed examination and description of termite endosymbionts. He recorded a total of eight species, several of which were new to science.

"Fresh-Water Rhizopods of North America" was Joseph Leidy's last monograph (he continued to write shorter publications, primarily on parasites.) It contained 324 pages and 48 lithographic plates. Leidy limited his monograph to free-living, non-parasitic freshwater forms. Most of his rhizopods were collected from marshes, ponds and streams near Philadelphia, but others were collected from elsewhere in Pennsylvania and New Jersey, as well as from Connecticut, Rhode Island, the Bridger Basin (southwestern Wyoming) and in the vicinity of the Bay of Fundy in Canada.

Leidy expressed considerable frustration with the illustrations that graced his book. He made the original drawings (later copied to make the lithographic plates), but regarded them as inadequate for representing the minature marvels he saw in the microscope. He was captivated by their complex movements, variable colors and ever-changing shapes.

Although Leidy was unsatisfied, the illustrations were cherished by others. Some of the drawings were preserved in bound volumes by Leidy's colleague and friend Joseph Willcox. Herbert S. Jennings, a leading American zoologist of the early 20th century, regarded Leidy as a scientific artist. He also regarded "Fresh-water Rhizopods of North America" as a "masterpiece" and " a section of nature permanently presented to us."