Insects and the germs they carry
The hypothesis that flies and cockroaches have more germs on them is proven to be only partially correct. The cockroach and the ant had the highest amount of bacteria, followed by the fly, and finally the beetle.
When insects like mosquitoes bite and suck our blood, they inject some of their saliva into our skin to prevent our blood from clotting. This helps facilitate the feeding precess. This is how disease- causing germs enter our bodies- through the mosquitoes’ saliva. However, some insects like bees and wasps have stings that cause pain, but do not spread any disease.
The science fair project can be repeated by using other types of insects like dragon flies, bed beetles or spiders.
Try to repeat the experiment by capturing flies from various locations like a garbage dump, near animal feces, within the house, or in the garden and compare the amount of germs in them.
Also, have you considered the body sizes of the insects? Perhaps the cockroach has more germs than the ant because it has a larger body size and hence a larger surface area to carry microorganisms? A more accurate way to assess which insects are "dirtier" is to compare a certain quantity of insects that it takes to cover a certain surface area.
Insect borne diseases - http://www.traveldoctor.co.uk/insects.htm
Insects transmit germs - http://purpleslinky.com/trivia/science/insects-transmit-germs/
Insect borne-disease - http://www.watchtower.org/e/20030522/article_01.htm
Always follow laboratory safety guidelines and always practice sterile technique when handling microbes. Never have any food or drink at your workstation and always thoroughly wash your hands with disinfectant soap or alcohol before leaving your workstation. Always dispose of used material in a biohazard bag. If none are available, the bacteria should be destroyed with bleach before being disposed of. Knives should be handled under adult supervision.