In the Midwest termites usually swarm when the yellow Forsythia bushes bloom. This swarm can be inside or outside of your home usually around the end of March or early April. They occasionally also swarm latter in the spring or summer. The swarmers are the reproductives of a colony. Their job is to mate and start new colonies
The ones that swarm inside and do not make it back to the ground, shed their wings and die in a few hours. They swarm out of soil tubes that were built by the worker termites. If they swarm inside your home, you know that there are already termite tubes and workers present. Termite colonies do not swarm until they are three years old, but that does not mean you have had them in your home for three years. They may have started the colony with another food source and moved into your home morerecently. Termites usually swarm between noon and two pm which is the warmest part of the day. They then head for sunlight. Often they are not found until later, after they have shed their wings. This is why it is common for the homeowner to only find the wings left behind, laying on their window sill or by some other sunlight source.
Termite swarmers are often mistaken for ants. The picture below shows the differences. Both have bodies about a half inch long. Note that the ant has a narrow waist and uneven length wings, where the termite body is almost straight. Termites also have two pair of even length wings that are longer than the ant.