History of Discovery
The first documented case of loiasis was published by a French surgeon name Mongin in 1770. The parasite was in Santo Domingo in a black female servant. This was by no means the first case of the parasite, however, as there were previous unpublished accounts in 1768, also in the New World. Loa loa was encountered many time subsequently before finally being described in detail by Argyll-Robertson and Manson in 1895.
Discovery of Microfilariae and Diurnal Periodicity
Stephan Mackenzie discovered microfilariae in 1890 that did not exhibit nocturnal periodicity and were different in size than others in a patient from the Congo. He asked Dr. Patrick Manson (see picture to the left, courtesy of http://www.lshtm.ac.uk/library/archives/manson.html) to see his patient, and he confirmed that the microfilaria was not W. bancrofti and discovered its periodicity to be diurnal, with a peak in the middle of the day. Eventually, after much work in differentiating the different microfilariae and their morphologies, Manson was able to identify Loa loa's microfilaria, and other physicians set to work confirming his hypothesis.
The discovery of the vector of Loa loa was accomplished by Robert Leiper in 1912. He was funded by the London School of Tropical Medicine, and his experiment consisted of feeding many different insects with blood from an infected patient. Rapid and uniform growth was witnessed with Chrysops dimidiata and Chrysops silacea. He ceased his studies when he ran out of flies, which peaked in a seasonal pattern around June in Nigeria. Further elucidation of the development of the parasite within Chrysops spp. was accomplished by Kleine, Connal and his wife, and Stevenson, among others.
The above information is from David I. Grove's History of Human Helminthology. For more in depth information about the history of this parasite and many other helminths, please consult this source, as it is very detailed and interesting.