On June 23,1997, approximately 0805 central daylight time, a Callair A-9B agricultural airplane, N7684V, was substantially damaged during a forced landing following a total power loss near El Indio, Texas. The commercial pilot, sole occupant in the airplane, was not injured. The airplane was owned/operated by a private individual under Title 14 CFR Part 137. Visual meteorological conditions prevailed for the aerial application flight which had originated from a private airstrip approximately 5 minutes before the accident. No flight plan had been filed for the flight. According to the pilot, he was working on a boll weevil eradication contract. He stated that he had been servicing the airplane (with fuel, oil, and applicant) at the end of each day. He reported that the day before the accident he was distracted from his servicing process by the arrival of a customer and the news of a family problem. The pilot reported that the next morning, while doing the preflight of the airplane, "I checked the sumps but did not remove the fuel caps to visually check the fuel [quantity]." The pilot reported that he departed for the first flight of the day with the fuel selector on the right fuel tank; this was the fuel tank which was not serviced the day before. The pilot further reported that the right fuel gage indicated "full." While making his first application swath, the engine "quit." The pilot "saw the fuel pressure light come on and he turned on the boost pump"; he "delayed switching fuel tanks because he thought he had full tanks on both sides and that there was some other problem." The pilot performed a forced landing to a cotton field. He stated that "I got down as smooth and slow as I could but the wheels got down into the mud and it flipped over." The airplane came to rest tail first in the inverted position.