The Island of Giant Insect could work as a commercial movie for manga,
because if you want to have some questions answered, you will have to go and
read it. This movie is a somehow unique story that
links 'horror' and 'ecchi' together in a half-awful experience.
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This review is written by an amateur who did not read the manga, just saw
the movie and the OVA. The spoilers will NOT be marked,
this is NOT a spoiler free review.
As for the difference between the movie and the OVA, I don't really think
there is any, since most of the scenes from the OVA are from the first
half of the movie.
All in all - the idea of giant bugs having to eat people because of how
big they are deserves points, the animation was okay, and the voice acting
was pretty enjoyable (give a round of applause to my favourite, Makoto
Kaneko - the VA of Akira).
For everyone who wants to see this, here are the main tags:

Which perfectly sums up what you're going to get. Maybe a little detail
- the CGI is bad. But then again, I hate CGI, so I might be biased.
I understand that they were working with limited time (even though anime
movies tend to have over 2 hours sometimes), but we are missing big
pieces of informations right from the beginning.
Who are they? What relationship do they have with each other? What
connection do they have to each other? Where are they going and why? We
get no real answers, instead we're thrown into the plane crash scene
right away. We are introduced to our main character, Mutsumi Oribe, who
is along with Kazuhiko Kai the most useful character in the whole movie.
There was no time to get into the other's characters, enjoy them or even
get to know them. For various reasons. We've got a few assholes, a jock,
someone I call a "self-seeking biach" and the useless glasses girl. All
the others we meet have like 2 minutes of screen time.
I was the saddest about this poor girl:

Skipping through the time, we have erotic scenes, lots of boob scenes, a
lesbian kiss, a sex scene, an almost grape scene...
Look. I enjoy horror. And I enjoy unique stories that give us something
to think about (the whole theory of why the bugs are so big). But
this... this was a mess. Such a big mess.
I didn't see any reviews for this, and I thought it was somehow
enjoyable. The art style was deformed in places, the CGI awful, the
story was lacking in detail, the ecchi scenes connected with gore
awful... I had high hopes and I was excited, but it left me kind of
frustrated and not satisfied at all.
What made me the angriest was the few last seconds just after the
ending. (I knew it would come, I knew it!)

Are You Lost? did the same thing to me, so I wasn't as angry as I was
annoyed. Even though the story was weird and the anime itself wasn't
even half-good, those three people really made you wonder:
Who are they? What relationship do they have with each other? What
connection do they have to each other? Where did they come from? (Pretty
much the same questions we had at the beginning of the movie.)