Fig. 1. (A) Close-up view of the head of a western diamondback rattlesnake Crotalus atrox Baird and Girard 1853 showing the location of the pit organ. (B) Frontal section showing the internal structure of the pit organ of a Pacific rattlesnake Crotalus o. oreganus Holbrook 1840. To aid visualization, the anterior air chamber was filled with red acrylic before the entire head was infiltrated and embedded. However, this may have displaced the membrane closer to the back of the posterior chamber. The angular aperture _i varies from 23° (included angle 45°) laterally to 10° (included angle 20°) when looking ahead and to the contralateral side.