The rules in the basic rules (which are also in PHB page 183) do not specify restrictions on what type of creature can take damage by falling:
A fall from a great height is one of the most common hazards facing an adventurer.
At the end of a fall, a creature receives 1d6 damage per hit for every 10 feet that falls, up to a maximum of 20d6. The creature lands prone, unless it avoids receiving damage from the fall.
The additional rules suggested in the Xanathar All Guide modify the fall speed and how the fall works with the flying creatures:
Falling from a great height is a major risk for adventurers and their enemies. The rule given in the Player's Manual is simple: at the end of a fall, you receive 1d6 damage per hit for every 10 feet you fall, up to a maximum of 20d6. It is also prone to the earth, unless it somehow avoids being damaged by the fall. Here are two optional rules that expand on that simple rule.
The rule for falling assumes that a creature immediately falls all the distance when it falls. But what happens if a creature is at a great height when it falls, perhaps in the back of a tap or aboard an aircraft? Realistically, a fall from that height can take more than a few seconds, extending beyond the end of the turn when the fall occurred. If you want high altitude falls to consume the proper time, use the following optional rule.
When you fall from a great height, you descend instantaneously up to 500 feet. If you are still falling on your next turn, you descend to 500 feet at the end of that turn. This process continues until the fall ends, either because you touch the ground or the fall stops.
A flying creature in flight falls if it is knocked down, if its velocity is reduced to 0 feet, or if it otherwise loses the ability to move, unless it can move or is maintained by magic, such as the spell of the fly.
If you want a flying creature to have a better chance of surviving a fall than a non-flying creature, use this rule: subtract the creature's current flight speed from the distance it fell before calculating the fall damage. This rule is useful for a pilot who is prone to blows but who is still conscious and has a current flight speed greater than 0 feet. The rule is designed to simulate the creature flapping its wings furiously or taking similar measures to slow down its fall.
If you use the rule for the rate of fall in the previous section, a flying creature descends 500 feet in the turn when it falls, like other creatures. But if that creature starts some of its later turns still falling and is prone, it can stop the fall on its turn by spending half its flight speed to counteract the prone condition (as if it were standing in the air).
The last rule (optional) could be relevant to its proposed scenario, if it were not for the fact that the dream leaves the insects unconscious, so it would not make sense that the creatures could avoid the damage of the fall in that way. In any case, by RAW, a 10-foot drop on solid ground would kill an unconscious creature of 1 HP.