what do paper wasps go after catpillars and spiders, bettles but not flies?

Answer 1

The true paper wasps tend to prefer large caterpillars. Perhaps flies are just too quick for them to catch, and don't provide as much of a meal.

The yellowjacket wasps, close relatives of the paper wasp, will go after flies, however. There is one type of yellowjacket, known as the aerial yellowjacket (Dolichovespula arenaria) which specializes in taking aerial prey, including many flies.

Answer 2

Predators generally hunt certain kinds of prey, as the adaptations necessary to catch some kinds may not be as useful for catching other kinds. Some predators get very specialized, and are very good at catching only a few (or even only one) kinds of prey. Many predatory wasps are pretty specialized in what they go after - there are wasps that specifically hunt spiders, even only certain kinds of spiders (Tarantula Hawks being a good example). There are wasps that may be closely related that specialize on certain kinds of caterpillars instead. Our common Yellowjackets are much less specialized - I've seen them carving up everything from fruit and caterpillars to baby birds; they'll collect almost any kind of food as long as it's not very active.

Answer 3

Because flies are too small and fast. Wasps can't lay their eggs in flies because flies are so small that the larvae would not have sufficient food.