I have no idea how they got in, but possibly due to a small missing gromet out of the inner layer of the window. to remove them you could try GENTILY Hoovering with low suction over the gromet holes to see if they will come out the same way as they possibly went in. Then of course replace the gromets with new ones.
Julia
------------- Just love to be out amoungst Nature and Wildlife
Celebrating 37 years of Caravanning in 2019, Recently Considered Retiring, but Totally Addicted for Life!
I had the same prob with one of my vans.Seems they crawl in the missing bung holes in the inner glass.As RG said,i took of the window..its easier that way and,shook them down to a bung hole then sucked them out with a hoover,then replaced the bungs,hope this helps.