By the shape of the head: that of the venomous ones is triangular. That, at least, is what the saying goes – but that is a dangerous generalization. Differentiating between them is far from being that simple. Other anatomical details – such as the tail, scales and pupil – also help with the distinction, but there are so many exceptions to the rule that they, by themselves, are insufficient. The safest method is to observe the presence of a small hole between the eyes and the nostrils: the so-called loreal pit. “All venomous snakes, with the exception of the coral snake, have this orifice, which does not appear in non-venomous snakes”, says biologist Otávio Marques, from the Butantan Institute, in São Paulo. This little hole is, in fact, a thermoreceptor organ – that is, a heat detector – of very high sensitivity, capable of perceiving temperature variations of the order of 0.003 degrees centigrade. In addition to this revealing trait, there are also significant behavioral differences between the two types of ophidians.
The main one is that the poisonous ones have nocturnal habits – although they can also be seen during the day. Another thing: harmless snakes flee when threatened. Not the venomous ones: they curl up and arm the boat. But there are also exceptions in both cases. There are around 2,500 species of snakes, of which around 260 are found in Brazil. Of these, less than 30% are venomous and the most dangerous belong to the group of crotalines, grouped into three genera: Bothops, also known as pit vipers; Crotalus, popularly called rattlesnakes; and Lachesis, the surucucus. Together, they account for 99% of attacks on humans.
The characteristics that distinguish venomous snakes are subtle – and, to top it off, there are exceptions to the rules.
The scales of venomous snakes are like keels, elongated and pointed, giving a rough feel to the touch.
Venomous snakes have a triangular head, well detached from the body and with small scales. But this rule does not apply to the very poisonous coral.
The eyes of venomous snakes have a vertical slit pupil.
The snake on the left – a Dipsas albifrons species – does not have venom. The problem is that it is easily confused with the extremely dangerous jararaca (Bothops jararaca, above). Both live in the Brazilian Atlantic Forest and have very similar scale designs. In addition, even though it is harmless, the opium has typical traits of venomous snakes, such as a triangular head and pupils in vertical slits.
The most accurate identification of a venomous snake is in this small hole between the eyes and nostrils, called the loreal fossa. But there is an exception to the rule: the poisonous coral does not have this trait.