The look of the guy to the right tells the whole exhausting story...
Here is a good description of the experimental tropical uniform worn by the man on the right...Experimental Tropical Uniform
In 1943, after extensive testing in the swamplands and jungles of Florida and Panama, the U.S. Army determined that an experimental tropical uniform made of Byrd Cloth (in Britain, Grenfell Cloth), would best protect soldiers from insects and disease while cooling the body and minimizing losses from perspiration. Byrd Cloth, as used in the Experimental Tropical Uniform, was a single-layer uniform of untreated OD long-staple Egyptian cotton, made in a tightly woven herringbone twill to prevent penetration by the proboscises of female mosquitoes. In use, the uniform was intended to cool the wearer even when continuously wetted, as might be expected in a humid, rainy jungle environment.
The uniform featured a short-tailed shirt, trousers with cuffs fitted with half-inch boot-top fastening tapes, and a flap-protected fly to keep out crawling insects such as leeches, ticks, and chiggers. Pockets were shallow and kept to a minimum to increase cooling; users carried all their gear in load-bearing belts, suspenders, or in low-mounted field packs designed to minimize body contact (jungle packs). The uniform, always in short supply because of a shortage of Byrd Cloth, was used in combat by members of the Office of Strategic Services (OSS) and the Mars Task Force (Army 5332nd Brigade-Provisional) in Burma.