Adult fleas are complex creatures. They are so tiny that it is hard to pick them out of your cat's fur - especially if your pet has dark fur - yet when magnified, fleas look like something out a sci-fi movie.
How Big is a Flea?
If you were to measure a single flea, it would be around 1 to 2.5 millimeters long, and either yellow or dark red in color. And yes, the dark red color comes from blood, either yours or your cat's, which means that a dark red flea is one that has just eaten.
For more scientific information on fleas, visit the wikipedia flea page!
Like almost all insects, an adult flea has six legs, three sets of two. Each of these sets is slightly longer than the others, with the front legs being the shortest, the middle ones being, well, the mid-length ones, and the rear ones the longest. Because of these long rear legs, fleas can jump pretty far. On top of this, their rear legs are covered with tiny hairs that help them cling to whatever they have landed on. Although once an adult flea has landed on one of your pets, the flea will stay there for the rest of its life, or until you manage to remove it.
What Does a Flea Look Like?
The body of an adult flea is comprised of various sections, including the head, thorax and abdomen. No matter the species of flea, it has two eyes, two antennae, and a mouth designed to siphon blood from your pet's skin. Like mosquitoes, fleas secrete saliva as they drink, and some pets, as well as people, are more sensitive to the saliva than others. This is why some people and pets itch like crazy and end up with small red bumps caused by flea bites, and others do not seem to be as affected. No matter what, fleas need to be killed, as your pets can become anemic if the infestation gets to be too bad.
This document from the CDC has some excellent diagrams of an adult flea.
An adult flea's legs emerge from the thorax, or center section of the insect, while the abdomen is where the flea digests its meals and, if female, forms eggs. In order for the eggs to be fertilized, a female flea has to mate with a male one. Once this happens, the eggs incubate for a bit, before the female flea starts to lay them. The average adult flea can lay around 40 eggs a day for ten days before exhausting her supply - all the more reason to make sure that you vacuum up any flea eggs after killing the live ones.