Central response to infra-red stimulation of the pit receptors in a crotaline snake, Trimeresurus flavoviridis.

The snake family Viperidae is divided into two subfamilies by the presence or absence of pit receptors. The Viperinae, found throughout Europe, Africa and Asia, apparently lack any form of infra-red receptor (Bullock & Barrett, 1968). The Crotalinae, instead, are characterized by the presence of a highly specialized pit receptor in the loreal region of either side of the face. Five principal genera make up this subfamily: Crotalus (the rattlesnakes),* Bothrops and Lachesis, all confined to North and South America; Trimeresurus, distributed widely in Asia; and Agkistrodon, which is found both in America and throughout Asia as far west as the eastern part of Europe. There are no crotalines in Africa or Australia. The pits are found in a much more primitive form in the family Boidae; in fact, some boids possess thermoreception without any external pits at all (Bullock & Barrett, 1968). The function of the boid pit is essentially the same as that of the crotaline pit (Noble & Schmidt, 1937; Warren & Proske, 1968). However, most research on snake thermoreceptors has been confined to crotaline snakes. The structure and innervation of these pits have been described in considerable detail by the following workers: Lynn (1931), Noble & Schmidt (1937), Bullock & Fox (1957), Bleichmar & De Robertis (1962), Terashima, Goris & Katsuki (1970). That these pits are thermoreceptors responding to changes in background heat flux has been demonstrated amply by Noble & Schmidt (1937), using behavioural experimental techniques, and by Bullock & Cowles (1952), Bullock & Diecke (1956) and Goris & Nomoto (1967) using electro-physiological recording techniques. Terashima, Goris & Katsuki (1968) recorded the generator potential from the sensory membrane of crotaline receptors. Terashima et al. (1970) also determined the terminal nerve structure, and Meszler (1970) showed how changes in the densely packed mitochondria of the nerve endings indicate that these mitochondria are involved as transducers of heat energy to electrical potentials.