Hiss, who is there?
D. Leela and D. Maya, Chennai

Recently we went to a beautiful exhibition on snakes. We had seen snakes many times before, in the Guindy Snake Park, and this exhibition had only photos, no real snakes. But it was so well done that we were fascinated. 

Snake Facts
Snakes have been around for more than 100 million years, from the Cretaceous period and have overlapped with the now extinct dinosaurs.
Spotting a snake is unnerving for many people.
An amazing fact we learned is that in some sense snakes have permanently closed eye-lids. The skin of a snake is covered with scales. If you have seen a moulted snake skin (when the snake grows a new skin and sheds its old one, almost completely intact), you will see the scales and feel the dry, smooth texture.
These scales have specialised forms. In the eye region, the eyelids of a snake are made of transparent "spectacle" scales called brille, which are permanently closed. So the snakes do have eyelids that are permanently closed but see-through! That is why if you stare at them, they stare right back at you! Without blinking!
Snakes also have specialised scales in their belly region for gripping surfaces when they move. This is how some snakes can easily climb trees.
Snakes don't have ears, but they can spot you approaching by the ground vibrations generated by your movement.
Snakes don't have great vision; they rely on their sense of smell to track their prey. Their forked tonue is used to collect air-borne particles and these particles are examined in the mouth to see if it has come from some animal that is suitable to eat. The fork in the tongue ensures that the snake is able to tell in what direction the smell is coming from so that it can find its prey (or even its predator).
Some snakes like pythons have infrared-sensitive receptors in deep grooves on the snout (near the nostrils). This allows them to detect warm-blooded animals from the heat they radiate. 
Snakes are helpful to humans in many ways and perform important roles in the natural environment. For one thing, they help control rodents and other pests, some of which could transmit diseases to humans.
The exhibition also had many details on snakes in various cultures and how historically snakes played an important role in many ancient civilisations.

Venomous or Non-venomous?
The thing that comes to most people's minds (children and adults alike) when they first see or hear the word snake is: is it venomous? (Poisons are eaten, venoms are injected so snakes are called venomous, not poisonous).
Most of the world's snakes are non-venomous. This means they do not produce a toxin that is clinically significant to people. Many non-venomous snakes kill their prey by constriction, literally squeezing the life out of them.
However, most snakes will hear you approach (from the ground vibrations) and simply go away. Unless provoked or accidentally stepped on, they will not be agressive or bite, whether venomous or not.
There are some basic features that distinguish venomous and non-venomous snakes.
Most venomous snakes have a sharp, pointed or triangular head, elliptical pupils, and upper jaws with fangs. They also have belly plates that expand all the way across their bodies to their blunt or rounded tails. Non-venomous snakes have curved heads, rounded pupils, sharp teeth and no fangs. Their belly plates are in two parts to their sharp, pointed tails.
Having read all this, we decided that it is difficult to come close enough to a snake and look at its head or pupils! Either we hope that the scientists are right and that snakes rarely attack, or we hope that there is snake anti-venom available nearby!

BOX on How to tell venomous snakes?
1 Look at the head. Most venomous snakes usually have triangular shaped heads.
2 Check out the colors. Some venomous snakes such as the coral snake have bright colors to tell you that it's dangerous.
3 Watch for warning signs. All snakes have some kind of warnings. A saw-scaled viper will make a saw-like sound while a Russel's viper will whistle like a pressure cooker. Other snakes will hiss and snap at you.
4 Look at the eyes. Some venomous snakes have vertical eye slits, however, not all venomous snakes have vertical eye slits, and not all non-venomous snakes have round pupils.
5 Check the tail. The bottom of the tail (behind the anus) of venomous snakes looks the same as the rest of the belly. If the snake has a cross pattern (like a diamond shape) it is non-venomous.
END OF BOX

Venomous snakes in India
India is host to a number of snakes, many of them venomous, but the most venomous ones called the Big Four are widely distributed and quite venomous.

The Common Cobra
These are commonly the snakes in a basket with snake charmers. They are about 1 to 2 m in length, and have a broad head. They can spread a hood behind their head, which gives them their famous, very scary appearance.
Their body color varies based on their geographical location. Generally, cobras in southern India range in color from yellow to brown. Northern India cobras are usually dark brown or black.
Cobras are shy---they'll threaten when provoked, but would prefer to back away. If they attack, they will strike quickly and sometimes repeatedly. Larger cobras may latch on and dig in, releasing maximum venom!
In the event of a cobra bite, seek medical attention immediately: the common cobra is responsible for a large number of human deaths across India.
 
Common krait
Slightly longer than the cobra, the krait ranges in length from about 1.5 to 3 m. Their head is depressed, slightly broader than the neck, with a rounded snout. Its eyes are small and entirely black.
The krait's body is black, with single or double milky-white bands. Its scales are hexagonal in shape, but those beneath the tail are undivided.
The krait is nocturnal, and during the day can be found in dark, dry places. They're docile and shy during the day, but at night will attack if provoked.

Russel's Viper
The Russel's viper is a stout snake, with a brown body mixed with reds and yellows. The body features three longitudinal rows of eye-like spots of dark brown or black, that start from the head and fade out towards the tail. The spots on either side are smaller and more rounded than the top spots.
The head is triangular, pointed at the snout, and much broader at the neck, and features two triangular-shaped spots. Its eyes have vertical pupils, and it's tongue is purplish black.
Russel's viper is venomous enough that you should seek treatment immediately. If you provoke one (and not just step on it accidentally), it will warn with a high-pitched whistle like a pressure cooker.

Saw-scaled viper
These are the second most common vipers in India, after Russel's viper, and are much smaller, just 40 to 80 cm long. Their body ranges from dark brown to red, gray, or a mix of those colors. Light colored spots of light yellow or very light brown, with dark-colored lines woven through.
The saw-scaled viper is very aggressive when provoked, and makes a saw-like sound by rubbing its side dorsal scales together. Don't stick around if you hear that sound: the saw-scaled viper is one of the fastest-striking species in the world!
If bitten, get treatment. It may dry-bite on occasion, but only a medical expert can say for sure.

The King Cobra
Sometimes this is included along with the other four to get the Big Five. An extremely aggressive snake, 3 to 5 m long, it has distinct light cross bands mainly on the forebody. It is uniformly coloured olive, brown, or green. The underside is a lighter shade of the body colour and the back has yellow bands. It is much more rare than the common or spectacled cobra, but it is much more menacing because of its great size.

Non-venomous snakes of India
Common ones are the checkered keelback, common cat snake, bronze-back tree snake, common kukri, common sand boa, common wolf snake, rock python, and rat snake. The last is often mistaken for a cobra.

Sources: Mobile Snake Exhibition, Natural History Museum, Beijing, and
the Internet for images