Here is a list of macro lenses that will work with your D3100. The longer focal lengths allow more working distance between the lens and the subject which may be nice if you're dealing with small creatures that would be frightened or be aggressive by the proximity of the lens. All these lenses are capable of excellent results.
AF-S DX Micro-NIKKOR 40mm f/2.8G http://en.nikon.ca/Nikon-Products/Pr...%252F2.8G.html
AF-S Micro Nikkor 60mm f/2.8G ED http://en.nikon.ca/Nikon-Products/Pr...2F2.8G-ED.html
AF-S DX Micro Nikkor 85mm f/3.5G ED VR http://en.nikon.ca/Nikon-Products/Pr....5G-ED-VR.html
AF-S VR Micro-Nikkor 105mm f/2.8G IF-ED http://en.nikon.ca/Nikon-Products/Pr....8G-IF-ED.html
Sigma 105mm F2.8 EX DG OS HSM Macro http://www.sigmaphoto.com/shop/105mm...g-os-hsm-sigma
Sigma 150mm F2.8 EX DG OS HSM APO Macro http://www.sigmaphoto.com/shop/150mm...po-macro-sigma
Tamron SP 60mm F/2 Macro http://www.tamron.com/en/photolens/di_II_macro/