Changing of the guard

Jeez you turn your back for a week or so and the whole neighborhood changes. Gone are the picture wing flies and most all of the syrphids. Even the green bottle flies aren't out basking in the thin sunlight. Instead today there are lots of these formally dressed white on black licorice-mix flies lounging on leaf tips. Do they have a faster metabolism or internal antifreeze that enables them to take advantage of feeding opportunities left untended by cold sensitive species? Where do they go in summer? Who does their tailoring?