Dumeril describes their respiration as arbitrary, suspended, retarded, or accelerated at will. ‘La respiration étant volontairement accélerée ou retardée, les actions chimiques et vitales qui en resultent doivent être naturellement excitées ou ralenties par cette cause.’[48] ‘The electric fluid,’ says Latreille, ‘is one of the great agents in animating living beings; and upon reptiles it operates in conjunction with warmth in rousing them from their inactivity.’ The periodical torpor and insensibility which reptiles undergo cannot, however, be always associated with extremes of cold, nor in all cases called strictly a ‘_winter_’ sleep; because it is during the hottest seasons in the tropics that they resign themselves similarly to an almost death-like repose and temporary tomb, burying themselves in the mud, which is hard-baked around and over them, almost hermetically sealed until the rainy season loosens the soil, and frees them from this literal sarcophagus. In this case the so-called ‘hibernation’ is the result of drought. It is moisture now which revivifies them, rain which restores their vital functions, and like the chrysalis bursting its shell and emerging a new and brilliant creature, the reptile lives anew, doffs his muddy coat, and reappears in all his resplendent colouring. The prairie rattlesnake (_Crotalus confluentus_) is known to undergo this species of torpor, which is, in fact, estivation. It is described as having been found in this ‘stupid condition’ in the dry cañons of the Rocky Mountains during the droughts of July and August. American naturalists who accompany the Exploring Expeditions affirm that this partial torpor is common to many species of snakes, and analogous to hibernation. They are ‘sluggish, stupid, blind, striking wildly,’ says one of the official Reports. Snakes remain torpid on an average half the year. It is a winter sleep in colder and temperate climates, and a summer sleep in hot ones. The green garter-snake of the United States hibernates eight months out of the twelve. So do some of the Australian snakes, others being underground five months in the year, Krefft tells us. The duration of insensibility varies, of course, with the climate and season.