A true macro will do 1:1  That is it will focus close enough that the
subject that it will include is exactly the same size as the 35 mm
negative-slide.  The image on the film is the same size as the original
subject.

   Many lenses call them selves macro but can only do 1:4 (subject is four
times larger than a 35 mm negative) or 1:2 (subject twice as large as a 35
mm negative.   I would guess your zoom fit into that class.

   Unless you need something less than about 1.5" x 1"   1:1 will be fine.
1:4 will do for a subject about 6"x4"

   Focusing distance can be figured, but it usually is not an issue.  It is
also a problem since the distance is so small you have to figure where on
the camera/lens you are measuring.  I seem to recall it is stated as the
distance from the optical center of the lens, which can be in front of the
lens.

    A longer focal length lens will be further away from the subject for
the same reproduction size.  Further is usually better so that 100 mm lens
should be good.

   If that lens does 1:1 you should be fine.   It also is likely to give
you better results.  True macro lenses are designed to focus close and do a
very good job, other lenses designed for general use are optimized for
distance work and don't do as well close up. (actually it is a little more
complex than that, but it is not all that important at this time.)

Macro is one of the most confusing things about photography for many
people. Most folks are used to prints and think in reference to their
prints, which is where the confusion lies. Anything can be made "life
size" in a print by enlarging it enough (it may be an unrecognizable
blob, but it will be a life size blob) but in macro lenses, "life size"
means something else. With respect to magnification ratios (1:4, 1:2,
1:1, and yes you can get 2:1 or more with the right lens) the ratio is
the relationship of the object to the size of that object ON THE PIECE
OF FILM. So, for your 24x36mm piece of film, a 20x20mm square object,
when photographed at 1:4, will take up a 5x5mm square on the piece of
film. At 1:2 it will be a 10x10mm square. At 1:1 it will be a 20x20mm
square and you should be able to hold the piece of film up next to the
object and they'll be the same size, therefore "life sized."

Sometimes the manufacturer gives the magnification ratio as a percentage
or fraction, i.e. your 70-210 does .23x magnification (according to
http://www.canon.com/camera-museum/camera/lens/f_lens.html) so the
20x20mm square is 4.6x4.6mm, or just shy of 1:4, which is not bad for an
older zoom, but not in the range of a good dedicated macro lens.

How big is the insect? If it's 36mm or shorter in the long dimension,
you'll definitely want a real macro lens that can go to 1:1. For really
large insects, you don't need 1:1, but you might as well get it since so
much of your intended subject will be needing 1:1 to 1:4 or so.

Canon says 0.31m, from the same web site as I mentioned above. That lens
does 1:1, so it should be good for what you want. If you want greater
than 1:1 magnification, you'll probably need a bellows and/or extension
tubes.

If you want to get really serious about macro, in addition to a good
tripod, you may want a macro sliding rail which lets you do very fine
controls of the cameras position. Velbon makes one that generally goes
for about $100 that I have and it works very well. Here's B&H's link:
<http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&
Q=&sku=193311&is=REG>

Really Right Stuff makes on for use with Arca-style quick releases.
<http://www.reallyrightstuff.com/specialty/index.html>

Drew

Other have explained reproduction ratios very well so I won't get into that.
I found the best way for me to get into macro was by starting with extension
tubes. Extension tubes are devices that move a lens further from the film
plane allowing for closer focus.

A set of three tubes for the 300 will run about $130 from B&H
http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=7
1515&is=REG

I linked to the Kenko set because they are significantly cheaper than the
Canon tubes and work great. Because tube have no glass you don't have to
worry about additional optics messing up your images. I would also consider
purchasing the 50mm F1.8 from B&H as well. The lens is less than $100 and
used with the extension tubes will give magnification from appox 1:4 to
1.3:1 for about the same price as the Canon 50mm Macro. the additional
advantage of extension tubes is that they can be used on all of your lenses
I often use the 12mm on my 28-70 F2.8 .

As others have recommended a sturdy tripod is a must. You could add a Very
good Bogen/Manfrotto Head/Leg combo to the above list and still be less than
the cost of the 100mm macro. This would give you a great opportunity to
experiment with macro and have versatile equipment that can be used for any
other type of photography as well.

The only really specialized piece of equipment I own that I find invaluable
is a macro focusing rail. I have the Bogen/Manfotto 3419 and it has served
me very well.

http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=1
62665&is=REG

I won't add to the technical detail from all other respondents, but just
want to assure you that the Canon 100/2.8 macro
is a great lens.  Because it can focus all the way from a few inches to
infinity, it is useful for anything suited to the 100mm
focal length - like portraits, for instance

Good luck

A 1:1 magnification will give you pretty big insect pictures - 2:1 fills the
frame with many insects and 4:1 is 'head and shoulders portrait' sized.  So
don't get hung up on needing massive magnification.

Also don't get hung up on a macro lens.  Yes it is the most convenient way
to get to 1:1, and will be optimised for its close focus performance.  It
will also have a flatter field than most lenses - but that matters to a
stamp collecter photographing his - flat - treasures a lot more than it does
to you photographing insects.  I'm not saying don't get a macro lens if it
is in your budget, just that you will also need a good tripod and probably a
focusing rail, and it would be a lot better to get these and a cheaper lens,
than get the macro and have nothing left over for the other essentials.

A 50mm lens reversed on a 200mm will give you very good results for insects,
and with a bit more working distance than the 100mm gives you.  Extension
tubes or a bellows are other options, and you could get a supplementary
close-up lens (sometimes foolishly described as a 'close-up filter', which
obviously it isn't) to experiment with.  The supplementary lens can give
quite good results - especially the two element ones - and is a very good
lower cost way to see how you like the world of macro.  If you later get a
dedicated macro lens you can sell the supplementary, or keep it as a handy
lightweight way of getting macro results when you are 'travelling light' and
don't have that macro lens with you.

There are lots of good books on all this stuff - you'll probably find John
Shaw's introductory one in the library.

Peter

Check out John Shaw's book 'Closeups in Nature'.

A quick online link to gander at (especially for us olympus freaks)
http://www.alanwood.net/photography/

JD

I'ld like to thank everybody for the very helpful answers :-)

Cheers !
Kostas