You may wish to know that the more common name for the "gadfly" in farming and veterinary circles is the "warble fly" - Hypoderma bovis or Hypoderma lineata.
This insect causes much distress to lumbering bovines and is of considerable economic importance as it bores through the hide of cattle to lay its eggs, where they subsequently hatch. In so doing, they ruin the hide.
Once the larvae hatch, they do a great deal of damage to the carcase as they migrate through the flesh to the gullet, causing great irritation and sometimes damaging the spinal cord in the process, causing death to the hapless victim.
In other words, gadflies get under your skin, cause a lot of irritation and do a great deal of damage, sometimes killing the victim. Howard may have been more accurate than he imagined.