Bugs float really well, and since there would have probably been many floating mats and flotsam and jetsam, carcasses and such that bugs would float on and survive on. Noah didn't take any fish, either, he took all that had the breath of life in it's nostrils. Insects breathe through their skin.Also, there were probably numerous living throughout the ark itself, feeding off the droppings of the animals, helping keep it a bit cleaner.
As for the giraffes, these are the two the Lord brought to the ark. Of course there would be no evolution, but paleontology shows the truth that there was once a much wider variety of animals than we have today-a lot more types of, say elephants, all kinds of weird variations, but we only have two today, the African and the Indian elephants. Again, the flood probably brought that to be the case-since space was limited, only two of most animals were brought a male and female. Maybe the African and Indian are just the two variations we got from the two brought aboard the ark. Same with Giraffes. However, notice all the kinds of sheep and goats and deer-these are all clean animals, so they were brought in sevens, seven males and seven females, so maybe there was a much wider variety brought, and interbreeding would account for all the varieties we see today.
Raising goats, I know it doesn't take long to get something new. If, say, you had seven male goats and seven female goats. Say, the Lord wanting variety, brings seven variations of females, and seven variations of males. Then they breed, maybe just after the ark closes. Knowing God's plan is to repopulate the earth, and especially wanting many for sacrificial purposes, hence, clean animals, He would probably have blessed the breedings of the goats and they would all take, and all have the maximum number, which seems to be about 5. Gestation is five months, so in five months, you have up to 35 new babies, added to the 14 originals, you get 49 goats. Then, we switch, the males jumping their fences to breed females of a different 'breed'-after say two months of raising their babies. So now, about a month after the Ark has rested, you have another 35 babies, but they have different characteristics, being cross-bred. 84 goats total, from an original 14, and 35 are now slightly different from the originals, so now, you get your males and females running off in small herds, again directed by God's hand, something I don't believe He left to random chance, and now we have numerous different breeds racing off to populate the cleansed earth, and as they go, there might be more variations between them. It would not take long to populate with that sort of thing.
Now, add to them the sheep, and the cattle and the deer and antelope and the various other 'clean' animals, and you can see where the amazing variations there came from, very quickly, while things like horses and elephants and such, we see much less variation, especially animals that have a long gestation period and only bear one offspring per one or two years, and cannot bear until they are a few years old, as opposed to goats and sheep which technically, can become pregnant at about 6 months, and males can impregnate at about 4 months. Quick and efficient!
So to sum up-the larger animals had greater variation before the flood, created that way, but it was lost when the flood came, and bugs lust lived on, and from, whatever was floating in the water until the flood waters dropped away.