As a bee-centered beekeeper, I only open a hive when I have very good reason to do so and am convinced that it is in the best interest of the bees. But, when I do, I love to see how the colony is propolising each of the six screened vent holes in the inner cover to control both the amount of ventilation traveling through the hive as well as its route.
Below is a photo of the bottom of an inner cover. This is the side that sits directly on top of the top hive body.
And below is a close-up of one of the screened vent holes. This was a photo taken in the field, of course. You can see how the bees chose to close down the vent with propolis.
Below are two photos of a single inner cover actually sitting on an occupied hive. Both photos are a look-down view from above the inner cover. The top hive body, and the bees, are below the inner cover. This first photo shows the three screened vent holes at the front of the hive:
In this case, the colony has chosen to completely seal the middle and right vents with propolis while leaving the left one partially open.
This second photo shows the three screened vent holes at the back of the hive:
Here, they have closed the middle vent hole with propolis, but left the two corner vents largely open.
Keep in mind that the insulated, ventilated base allows a slow, diffused movement of air up through the hive at all times. The colony can control both the amount, and route, of that air flow by propolising the vents.
But, there is also an inflow of air through the upper entrance hole in each of the hive bodies, which, of course, is on the front of the hive and in the center of each hive body.
It's interesting that the bees closed the center vent at the back of the inner cover but left the back two corner vents open. This causes the ventilation from the entrance holes to move across all of the combs before escaping out the vent holes.
Ventilation is critical for controlling such factors as temperature and relative humidity inside the hive (plus many more).
Honey bees are brilliant, and I love giving them this kind of control over their ventilation!
What's also interesting is that I've seen them remove propolis, and re-open the vent holes in the inner cover, when the weather has become exceptionally hot or when their population has expanded.