How do scientists classify mammals and insects?

                        http://www.sciencekids.co.nz/images/pictures/animals/dragonfly.jpg

                       Insects have 6 legs, antennae, and wings.

                               http://www.sciencekids.co.nz/images/pictures/animals/whitetiger.jpg
             
                 Mammals are born live just like me. They are warm blooded                                            and are vertebrates.
                             Standard 2.L.5A.1   How can you classify animals?