you make your first mistake in the second sentence. you are correct, it sounds abserd to assume that 850,000 species of insects could survive on the ark. the Bible says in Gen 6, that two of every kind, not species were brought onto the ark.
as for the Ephemeroptera, it is only the adult live cycle that lives for only 1 day. also, assuming that the mayflies were not on the ark for some reason, after the rain that had to happen and did happen for forty days and nights, the salt water would not have a very strong consentration of salt.
amasingly, I have personally seen some wasps survive an alaskan winter of colder than -70 F in trees, if they were in the ground, the would be crushed by the contraction of the earth at that temperature. Here in AK, we have what is called a Black Spruce Wasp with stingers over an inch long, these rugged insects manage to survive flooding that happens in the winter, a world wide flood would be nothing for them (especially on an ark). As for the chalcids,only one of every kind was saved.
The need to have living plants to survive, does nothing to disprove the flood, the people that lived during Noah's time were very good farmers, that is how the spent their lives. So Noah brought some
plants onboard.
with the bringing on of a few live plants comes some dirt, no problem for the cicadads.insects that need mature plants to grow, had no problems after the flood, there were five months to grow plants again, and it has been postulated that the O2 level in the air was much greater in the time of the flood plants grow much better in a high O2 environment than they do in the atmosphere that we now have. also the need for vegitationfor the animals shuch as giraffs indicates that Noah brought some trees on board.
the parisitic invertabrae were also brought on the ark, but who brought them, it does not matter whether the humans, or the dogs brought them, it is even likely that they hybernated as do most types of human pestalince when they are under a harsh environment
I dont even buy the these insects survived on a floating mass of vegitation, the ark was huge, so fitting one of every kind of animal on it was not a hard thing to do.