In winter the air is cold, the ground is hard and many trees have no leaves. So bugs do what they have to do to survive.

WASHINGTON — We bet that on cold wintry days, many of you love to snuggle up in your warm home and, every now and then, pop into the kitchen for a snack.

Unfortunately, plenty of creepy-crawly critters like to do the same thing!

Winter is the time when bugs — such as camel crickets and brown marmorated stink bugs, ladybird beetles and boxelder bugs — invade your house without an invitation.

The season can be tough for such creatures, said Michael Raupp, an entomologist (an expert on insects) at the University of Maryland.

In winter the air is cold, the ground is hard and many trees have no leaves. So bugs do what they have to do to survive.

Monarch butterflies head south to warmer climates. Ants crowd in deep underground colonies and eat food they have been storing all year.

Many insects go into a deep sleep called diapause, Raupp said. There are different kinds of diapause, but all are similar to hibernation, a time when bigger animals become inactive in the cold. Insects go into an inactive period, too, but it often isn't when the temperature dips.

They rely on more dependable signals in the environment. For example, many insects can tell how much sunlight there is each day. They use that cue to tell them when to shut down.

Bugs are cold-blooded, meaning that their internal temperature is the same as the outside temperature. They can't move very much when it gets below 40 degrees Fahrenheit, Raupp said.

So they search for any place that is warm.

"They are looking for protection," Raupp said. "These guys have been doing this for 300 million years, so they don't really know they are coming into your house. The home is a recent event in terms of their evolutionary behavior."

They enter through tiny cracks or come in unnoticed on your clothes or shoes.

Raupp doesn't like stink bugs in his house, because "they are really pests." They stink.

"I annihilate them," he said.

He is kinder to ladybugs, also called ladybird beetles.

Though you may call these animals "bugs" when you spot them in your kitchen, scientists don't. All bugs are insects, but not all insects are bugs.

Bugs are insects that go through a complete metamorphosis. That means they start out as one thing and turn into something different. For example, butterflies start out as caterpillars.

Insects that aren't bugs have incomplete metamorphosis, meaning they hatch from an egg and go through different stages before becoming an adult. Those include grasshoppers.

You may not care what they are when you see them in your house. Raupp said, "Remember that they may be invading your homes for warmth and food, but they don't give a hoot about humans."