Well consider this:

The Tamron lens, which I also own (for Nikon AF) has a maximum
magnification of 1:1.  What that means is that if you shoot with
maximum extension, the size of the subject on the film will be the
exact same size it is in real life.  Were you to take a picture of a
flower, for example, and later hold the slide next to the flower, they
could perfectly overlap, size-wise.

Of course, 35mm film is always enlarged, either for printing or
projection, so what you end up with is a large shot of a small object,
revealing detail not visible to the naked eye.

If you want to shoot close-ups of a compound eye, you can do this
unaided, unless you want the image to fill the whole frame.  It also
depends on the type of insect.  Dragonfly?  Sure.  You'll see the
compound eye patterns at 1:1.  Horsefly?  No way.

For refererence, here are two quite awful hand-held 1:1 shots of
insects that I've found in my apartment:

http://betelgeuse.umeqs.maine.edu/chris/details/Centipede1.jpg
http://betelgeuse.umeqs.maine.edu/chris/details/ant.jpg

For the type of photography you seem to be after, you're going to need
extra extension at a minumum.  Depending on what type of camera system
you own, this might result in the loss of TTL metering.

For the truly hard-core close-ups (photomicography), one set-up that
I've heard of is mounting a motion picture camera lens backwards at the
end of a macro bellows.  John Shaw discusses this technique in his
book, Nature Photography Field Guide.  Or perhaps it's in Close-ups in
Nature, I forget which.

Never having done that, however, I can't offer much help other than to
point out that such a technique exists, in hopes that you'll be able to
find out more about it by searching for info on your own.

Hope this helps some.

- Chris

You can go 1:1 (lifesize) with this lens (assuming 35 mm) so about 1 x
1.5" ... to magnify further, like to get a fly's eye, you'll need a
different lens or more extension ... I second Chris Sprague's
recommendation for John Shaw's books, he covers the options very well
... I'd try another 50 mm of extension or so with the lens you have
first.  As you'll quicly see there are many issues with depth of field,
steadiness and focussing once you go higher than 1x and I find a
focussing rail works best ...

If you REALLY get into this kind of work and shoot Canon I'd recommend
their special 65 mm lens that starts at 1x and magnifies to 5x ... I
got this for my wife last year for Christmas and it can give
outstanding results but is very hard to use and we almost always use it
with a ring flash ... here's a write-up on this lens with some pics
showing you what 5x can do ...
http://www.luminous-landscape.com/reviews/lenses/mp-e-65.shtml

Bill

Thank you Chris and Bill for your very helpful comments.  I will
certainly try and locate John Shaw's book and in the meantime have a
lot of fun experimenting with my macro lens.  I have used a zoom lens
for many years for my wildlife work, so the macro world is a new
challenge to me.

I've just started using newsgroups again after a break of about 10
years and always find people in the photographic groups so helpful.
Thank you to you both again.

Jenny