When humans sniff in order to smell something, we draw a quick puff of air into our nostrils and over chemoreceptors in our nasal cavity. But octopuses, butterflies, and other animals don't have noses like ours. Instead, they've evolved other, sometimes bizarre ways of sensing the world around them.
For instance, if you were to look closely at an Oregon shore crab (Hemigrapsus oregonensis), you wouldn't see anything resembling a nose. But that doesn't mean the creatures have no sense of smell.
"Smelling is really important to most animals, and crabs are no different," said Lindsay Waldrop, a postdoctoral researcher in biology at the University of North Carolina at Chapel Hill.
Toothbrush for a Nose
"We sniff with our sinuses, and crabs actually do the same thing," said Waldrop, "only they use an external hair array that looks like a really dense toothbrush."
These toothbrushes are located on antennae near the animal's mouth. When the crab wants to take a sniff, it waves these arms through the water.
Quick downstrokes open the bristles, allowing water and odor molecules to swish between them. Slower upward strokes close the bristles and trap scents against chemosensory cells in the hairs to give the crab a whiff of what's nearby.
In a paper published this week in the Royal Society's journal Interface, Waldrop explained that crabs use their bristly sense organs to find food in murky environments, track down mates, and avoid becoming someone else's lunch.
Though snakes have nostrils, they actually receive a lot more sensory information via their tongues.
A snake's tongue flicks outside its mouth because, like the crab, it's trying to capture scent molecules. Once the tongue draws back inside, the fork fits neatly into two pits in the roof of the mouth, thereby transferring those molecules to the snake's sensory center, called the vomeronasal or Jacobson's organ.
Their forked tongue can even provide the snake with a bit of spatial information—as in, "the juicy squirrel is to the left." (See National Geographic's photos of snakes.)
Tasting With Your Toes
Given the way most of our feet smell, being able to sense the world through them doesn't sound all that appealing. But imagine if you spent each day strolling across flowers and ripened fruit.
Flies have chemosensory hairs both on their labellum (think lips for insects) and their tarsi (the equivalent of feet). So when one lands on your sandwich, it's not simply taking a rest, but is actively sampling your lunch. If the feet like what they taste, then out come the mouthparts! (Learn more about fly dining in "Flies Eating Donuts.")
Butterflies can also taste the world through their feet, but do so for a different reason. Females lay their eggs on the undersides of plants so that the caterpillars have something to eat when they hatch. Mom uses the foot taste test to avoid poisonous plants—a choice that means the difference between dinner and death.
Insects aren't the only ones that can taste with their extremities. Octopuses can have as many as 1,800 suckers on their eight legs, and each one is packed with chemical receptors. (See "Sensitive Octopus Suckers.")
Taste All Over
Perhaps no animal is as weirdly and thoroughly equipped to taste the world around it as the yellow bullhead catfish (Ictalurus natalis). Its whole body might as well be one long, slimy tongue.
This fish has over 175,000 taste buds stretching from head to tail, with a high concentration in the "whiskers" or barbels near the mouth. By comparison, human tongues usually have between 2,000 and 8,000 taste buds.
Like the crabs Waldrop studies, these catfish typically live in conditions with poor visibility; much of what they eat must be scavenged out of the mud. This hypersensitivity even helps the fish hunt live prey at night. (See "How Catfish Stalk Prey in the Dark.")