Rupendra Brahambhatt in Ars Technica:
There are some insects that fly synchronously, meaning their wings on both sides flap together and in a coordinated manner. Others demonstrate asynchronous flight, in which each wing operates independently. A big difference between these two modes is that in synchronous flight, the nervous system of an insect has complete control over the wings’ motion.
The insects can command their muscles to beat on each wingstroke with their brains, just like you or I do when we command our leg muscles to move with each step. That is what the very first flying insects likely did, as it’s common in many groups of insects today, including moths, cockroaches, and others.
In asynchronous flight, the wings flap much faster than the insect’s brain can control.
More here.