Venom, Spitting Cobras and the Continuity Equation

For most cobras, fangs are hypodermic weapons that inject venom into intruding predators and unwary prey. But a few cobra species make a point of using their venom for purely defensive purposes, before a threat is close enough to strike. Get in the way of a spitting cobra, and you could wind up with a face full of proteins that can burn your eyes or even blind you.

The Windup and the Pitch

It doesn't really have anything to do with spitting -- the flying venom is a spray that comes from holes in the front of the cobra's fangs. The cobra doesn't just focus on one spot and fire away, though. Instead, it moves its head, directing the spray in an arc or oval to up its chance of hitting the attacker's eyes. To successfully hit a target, the venom has to travel a long way, and it has to do so quickly, before gravity pulls it to the ground.

The Fangs Have It

Spraying venom is a multistep process. Take a single fang as an example. First, muscles outside the cobra's venom gland contract, putting pressure on it. The venom naturally moves from the high-pressure area inside the gland to the nearest low-pressure area -- the open air outside the fang. To do this, it moves from the venom gland through a hole in the top of the fang. Then, it flows down the fang's venom canal and through a second hole near the fang's point.

All venomous snakes' fangs have this hole, but in non-spitting snakes, it's relatively long and narrow. This makes it easy for more venom to come into contact with more of the prey's tissue. Spitting cobras, on the other hand, have a need for speed, so their venom moves through a much smaller, teardrop-shaped hole. And the better the spitter, the smaller and rounder its fang's hole is.

The Secret's in the Plumbing

The smaller hole focuses the stream of venom -- but that's only half of the story. It also makes the stream move faster because of a principle of fluid dynamics called the continuity equation. You can express this equation with numbers and symbols, but basically, the rate of flow in a pipe is the same at every point in the pipe. Under normal circumstances, a volume of fluid moves into the pipe at the same rate that it moves out. Suppose a liter of water is moving into a pipe every second. If the pipe suddenly narrows, the fluid has to move faster to keep up that liter-per-second rate. If the pipe widens again, the fluid slows down again.

Therefore a spitting cobra's venom speeds up when it reaches that tiny opening at the end of the fang. It moves faster, flying farther before succumbing to gravity and hitting the ground. It may seem like nothing more than a tiny hole, but it's what lets a spitting cobra change its weapon of choice from a hypodermic to a ballistic one.

So if you see a cobra, don't assume it would rather conserve its venom and slither off to bite another day. It still might be able to blind you from more than 6 feet (1.8 meters) away.