There are more than 120,000 different species of flies worldwide, but if you live in the Waukesha-Milwaukee metro area then chances are the flies you are seeing are either house flies, drain flies, fruit flies or deer flies (sometimes referred to as horse flies). Most flies live an average of 21 days and usually lay their eggs in garbage or animal carcasses. Flies multiply quickly– laying hundreds of eggs at a time. Flies can only ingest liquid and because of this when they land on food they will first spit on it. Their saliva contains a chemical that turns their food into a liquid which is one of the reasons that having flies around is so unsanitary and can spread disease.