The first collar insignia for members of the Medical Corps was a gold shield prescribed for wear on the sack coat in 1892 (The Encyclopedia of United States Army Insignia and Uniforms by retired Lieutenant Colonel William K. Emerson). This gave way to a Maltese Cross in 1896 before the Medical Department adopted a caduceus in 1902. The caduceus, of course, still serves as the insignia not just for the Medical Corps, but for all the branches of the Medical Department (with affixed letters indicating each the branch).
But the introduction of the caduceus was not met with a universally warm reception. As the staff of Hermes, the symbol was associated with business and commerce, not the healing arts, and its design was somewhat confusing because it bore a strong resemblance to the rod or staff of Asclepius, the son of Apollo was the god of healing.
Interestingly, the caduceus had been first introduced over fifty years earlier for the insignia of Hospital Stewards; perhaps the outcry ensued in 1902 because it would not be the insignia of an enlisted profession but of officers in the entire Army Medical Department. In any event, the topic was touched on in the June 28th edition of the Army and Navy Register, with an editorialist concocting an unusual interpretation of the insignia’s symbolism—the rod represents power, the serpents symbolize wisdom, and the wings connote activity—while simultaneously decrying that the design did not “lend itself easily to a corps emblem.”
In the case of the Surgeon General’s selection of the caduceus, however, one thing can be said with surety: nothing succeeds like success. Following the Army’s lead, numerous private physicians and medical-care providers adopted the caduceus as a symbol of their practice—with the notable exception of the United States Air Force. It instead chose the rod of Asclepius for its Medical Service, likely to distinguish itself from its parent branch of the United States Armed Forces.