Gadfly of Horse
A gadfly, or Bot-fly, that lays its eggs on horses. After these eggs hatch on the skin, the larvae bore into the skin and create tumors (’warbles’) beneath it. When fully fed, they leave their host and drop to the ground, bury themselves, and, after pupation, emerge as flies. This illustration is enlarged, and also shows the larvae.
John H. Finley ed. Nelson's Perpetual Loose-Leaf Encyclopaedia (vol. 5) (New York, NY: Thomas Nelson and Sons, 1917)