Which macro lens to buy, is something that many beginning macro photographers think about.
Traditionally, macro lenses were available in mostly three focal lengths, 60mm, 105mm and 180mm or 200mm. These sizes presuppose a 35mm (full frame) sensor. Camera manufacturers have adapted to the APS-C size sensors, and on Nikon at least, a 80mm lens is available for DX-type cameras. Similarly, micro four thirds cameras have the 60mm Olympus that is equivalent to a 105mm, and there is also a Leica lens of, if I remember correctly, 25mm that is equivalent to the 60mm lens on 35mm cameras.
The shorter lenses, the 60mm of DSLR, or 25mm for micro four thirds, is useful for studio work, product photography, and in my experience the short lens works particularly well with flower photos. For insects, however, I found the longer lenses more useful, just because they have a longer working distance. Below are three images, one taken with a 60mm lens, one with a 105mm lens, and one with a 180mm lens, all on a Nikon D200. Notice with the wide lens, the background is more pronounced than with the telephoto lenses. In cases where you wish to show the insect in its environment, the 60mm lens will thus bring a sense of environment. The 180mm lens gives a smooth background, and the 105mm lens something in between.
With the 60mm lens I tried to include the red kniphofia flowers on which the bees were feeding. Although this was intentional, it can quickly become an issue if you are not watching your background.
The 105mm lens gave a good balance between cost, working distance and other factors. Usually the 105mm lens on 35mm sensors, or the 60mm on micro four thirds is the general work-horse flocal length.
The 180mm lens is rather difficult to work with, giving such tunnel vision that a moving subject like a bee is easily lost. However it renders a beautifully smooth background, and the working distance is great for photographing insects that are easily scared off.