Their larvae are small white maggots which pupate and emerge into flies. Their life cycle under the right conditions is complete in six days with 10-12 generations per summer. There is not an over abundance of horse manure in Kansas City, or any other metropolitan city, since the advent of the automobile. There is however tons of garbage. Every restaurant has a dumpster behind the kitchen breeding flies.
Every homeowner has an excess of garbage prior to pickup.
All are prime breeding sites. The picture on the left is of a fruit fly. They are tiny flies that are attracted to and breed from rotting fruit. Their eggs are often laid in fruit and as it deteriorates the flies emerge, usually in your kitchen, restaurant, or bar. The reason I included fruit flies, fungus gnats, drain flies, and phorid flies under the same heading is that they are frequently confused.