There are some things that might need some thought before throwing down the money.
Are you going to shoot live in nature or inside in a controlled enviroment?
Are you going to use a stand or freehand?
Are you going to use a flash or not?
Are you ok with the short distance like 1-2 inches from the subject or do you need distance not to frighten the critters ( and flowers...
)
Personally I love to shoot fairly close without a stand but with a diffused flash or two. A big part for me is the actual hunt to get into a good position without scaring off the bug.
Here is one I shot using that method:
I used a Tamron SP 35-80 at 3.2 (i think 3.2) with a 21mm extension tube and a flash.
Heres another way that uses a Tokina 90mm f2.5 with a reversed Mamiya SX 55mm at f8.
This rig gives about 2:1 and for me that is about as large as I can go and still manage to take usable shots. Not possible to focus with the aparture open though so this needs plenty of surrounding light to be able to even see the subject.
Heres a shot of a small screw laying on a ruler using the above rig. The screw is about 2mm long.
I cropped the image and resized it to about 25% of the crop. Notice the very thin depth of field.
As you can see this isnt a great picture...... Getting a nice, smooth DOF is quite hard. Some use stacking to achieve greater DOF but I dont really have the patience for that. It also requires the use of a stand and a lot more work, both before the shot and after.
The critters will have flewn to another forest before I'm done with the setup
Heres another using the same setup.
This might have been a good shot if it had been a critter doing another critter... or something... but it at least shows the amount of detail at 2:1.
I marked it with the estimated sizes.
When you use a reversed lens on a bigger like this you have to use the aparture on the reversed lens. The 90mm in this case can be at 2.5 or maybe a little smaller but it wont do any difference to the DOF. It will however vignete if you use a to small aparture.