I have heard and read much about how ants communicate to each other using pheromones. My question is how far away (in body lengths or inches) can an ant "smell" these signals. Also, by what other senses do ants navigate around the world.
This is an interesting question, and we definitely still have much to learn about how ants sense and interpret their surroundings.
Ants vary greatly in their ability to see things and form images. Some ants are completely blind, and many ants have small eyes that can probably only distinguish light, dark, and movement. However, some ants have enormous eyes, as you can see in this iconic picture by Alex Wild.
Some ants have been shown to use visual cues to navigate, and some, like bees, can sense the polarity of sunlight.
However, most ants primarily perceive the world through smell and touch. The majority of ant communication happens in direct proximity to another ant from the same colony. The ants will tap each others antennae, and insodoing, "smell" and feel the texture of the other ant's cuticle. I wrote "smell" in quotation marks, because when something is "smelled," we usually talk about the detection of airborn chemicals. What ants are really doing in this situation is "contact chemoreception," which, I suppose, is more similar to our sense of taste. At any rate, contact chemoreception, and the tactile interpretation of cuticle structure are the two most important parts of nestmate recognition in ants. We know this because there are some species of beetles and other insects which look nothing like ants, but are able to fool them into accepting them as sisters because of carefully matched cuticle texture and "smells."
But, as you indicated, ants also communicate by pheromones, which are airborn chemical messages. The pheromone in ants that we observe most commonly is the alarm pheromone. These compounds are often detectable by ants in very small amounts, but they diffuse to undetectable amounts in less than a minute in still air (which is good, in some cases, because the cause of the alarm may have already been taken care of--think of the neighbor's car alarm that won't stop honking.) Several studies in which researchers quantified the distance that ants are aware of alarm pheromones are reviewed in the recent book "The Superorganism," by Bert Holldobler and EO Wilson (2009). They gave 6cm (or about 2.5 inches) as the maximum distance a particular species of harvester ants would respond to an alarm pheromone in still air. Weaver ants, native to Australia and Southeast Asia, have been shown to respond to alarm pheromones from more than 10cm away (a little less than 4 inches). These distances would obviously change with wind direction.
An area of ant pheromone signaling we know much less about is mate-calling. This is obviously the most interesting use of pheromones to humans and many other animals. Male moths might fly more than a mile to find a mate, if the wind is going in the right direction. Some ants are known to use pheromones to attract mates (generally the females stay put and the males "follow their nose"), but over what distances these chemicals work is quite a mystery. However, one noteworthy trait of many male moths is their feathery, fern-shaped antennae. This shape helps to maximize the number of chemoreceptors a male moth might have, increasing his chances of smelling a pheromone molecule. The antennae of male ants are much more drab, which suggests that the distances across which they can perceive pheromones are shorter than male moths.
Further information about pheromones can be found in this excellent blog post by "Wild About Ants," and in the book "The Superorganism," mentioned above.
I hope this helps!
Jesse Czekanski-Moir & the AntAsk Team