Ils se caractérisent à l’état adulte par trois paires de pattes (comme tous les insectes) et par deux paires d'ailes recouvertes d’écailles de couleurs très variées selon les espèces. Les Lépidoptères pondent des œufs qui donnent naissance à des chenilles. Ces dernières se transforment ensuite en chrysalides (s'abritant ou non dans un cocon préalablement tissé). Il en émerge enfin l'imago, ou papillon. Leur cycle biologique se trouve donc composé de quatre stades distincts, œuf, chenille, chrysalide et papillon. Ce sont des insectes à métamorphose complète.
The Monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae. It may be the best known of all North American butterflies. Since the 19th century, it has been found in New Zealand, and in Australia where it is called the wanderer. It is resident in the Caribbean, Canary Islands, the Azores, and Madeira,Portugal, Spain and is found as an occasional migrant in Western Europe and a rare migrant in the United Kingdom. Its wings feature an easily recognizable orange and black pattern, with a wingspan of 8.9–10.2 cm (3½–4 in). (The viceroy butterfly is similar in color and pattern, but is markedly smaller, and has an extra black stripe across the hind wing.) Female monarchs have darker veins on their wings, and the males have a spot called the androconium in the center of each hind wing. Males are also slightly larger than female monarchs.
The monarch is famous for its southward late summer/autumn migration from the United States and southern Canada to Mexico and coastal California, and northward return in spring, which occurs over the lifespans of three to four generations of the butterfly. The migration route was fully determined by Canadian entomologists Fred and Norah Urquhart after a 38-year search, aided by naturalists Kenneth C. Brugger and Catalina Trail who solved the final piece of the puzzle by identifying the butterflies' overwintering sites in Mexico. The discovery has been called the "entomological discovery of the 20th century". An IMAX film, Flight of the Butterflies, tells the story of the long search by the Urquharts, Brugger and Trail to unlock the secret of the butterflies' migration. There is evidence that eastern North American populations of the monarch butterfly migrate to south Florida and Cuba.