Brown, Rainbow and Unstable Ants are completely harmless. Red ants are not! Nor are Termites (which can easily be confused with ants). There is rumoured to be a Red Ant colony in the End, providing a convenient way back!
Be sure to collect plenty of nests and keep them in a zoo cage near your home so you can teleport anytime!
Right-click an ant to teleport to a new dimension!
Brown Ant: takes you to the Utopia Dimension. There are no mobs and it is very peaceful.
Red Ant: takes you to the Mining Dimension. There is a ton of material but it's not always safe.
Rainbow Ant: takes you to the Village Dimension. So many villages to save!
Unstable Ant: takes you to the Danger Dimension. Be careful. No, seriously.
Termite: takes you to the Crystal Dimension. Be sure not to take anything with you. Keep termites away from your house if it is wooden, unless you want it to be turned into dirt.
Your Girlfriend and other pets will automatically teleport to the ant worlds with you (unless you've told them to sit, of course).
Right click the same type of ant again to get back home! Or you may be there a while...
How to Kill:
A good pounding with your fist will do fine. If there are a lot any weapon that does splash damage, chainsaw and others, will kill many of them at once because they only have 1 health.