They are not harmful to humans
No. Mayflies are adults after they hatch. All they do is mate and die.
The nymph form of the mayfly does have a mouth. It feeds on plant material. The adult does not have a mouth because they mate right away and die within 3 days of becoming an adult. they do not eat as adults
All Microbes are not harmful. All virus are harmful. Some bacteria are harmful
some are harmful but some are not harmful
Yes. They should be able to stand up for at least one season. (300-400 miles?) As long as the soles don't show much wear or cause you any pain they are probably fine.
No they are not harmful.
more harmful, most harmful
lion are useful and harmful
If you need help I recommend you type in the specific question you need help with
it is very harmful or harmful depending on your body
It is not harmful to animals but it can be harmful/fatal to children
The word "harmful" is an adjective.