Why was it necessary for Ross to examine the stomachs of mosquitoes that had not fed on blood from malaria parents?

It was necessary for Ross to examine the stomachs of mosquitoes that had not fed on blood from malaria parents because he was trying to determine if the malaria parasite was transmitted through the mosquito's saliva. If he had only examined the stomachs of mosquitoes that had fed on blood from malaria parents, he would not have known if the parasites found in the mosquitoes' stomachs had come from the bloodmeal or from the mosquito's saliva. By examining the stomachs of mosquitoes that had not fed on blood from malaria parents, he was able to determine that the parasites were not present in the mosquitoes' stomachs until after they had fed on blood from a malaria patient. This led him to conclude that the malaria parasite was transmitted through the mosquito's saliva.