fer-de-lance, highly poisonous snake, Bothrops atrox, found in tropical South America and the West Indies. A pit viper, related to the bushmaster and the rattlesnake, it has heat-sensitive organs on the head for detecting its warm-blooded prey. Usually about 5 to 6 ft (150-180 cm) long, the fer-de-lance may reach a maximum length of about 9 ft (3 m). It is gray or brown with light stripes and dark diamond markings and has a yellow throat. Common throughout most of its range, it causes many human fatalities. It is classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Crotalidae.