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http://en.wikipedia.org/wiki/L...s_in_Marvel_Comics#P

A few alien races have had considerable "air time" in various Marvel Comics publications over the years, having a near-constant presence and/or major crossovers and storylines involving them. This includes:

• The Badoon (first seen in Silver Surfer #2) - A race of reptilian aliens that are notable for living under strict gender segregation, resulting in two separate societies: the Brotherhood of Badoon (ruled by a "Brother Royal") and the Sisterhood of Badoon (ruled by a Queen).
• The Brood (first seen in Uncanny X-Men #155) - The Brood are a race of insectoid, parasitic, extraterrestrial beings.
• The Celestials (first seen in The Eternals #1) - The Celestials have existed near the birth of the universe.
• The Dire Wraiths (first seen in Rom the Spaceknight #1) - The Dire Wraiths are an evolutionary offshoot of the Skrulls from the Andromeda Galaxy. Like the Skrulls, the Wraiths are shapeshifters where they are able to take the forms of other creatures.
• The Kree (first mentioned in Fantastic Four #64; first seen in Fantastic Four #65 (Blue) and Marvel Super Heroes #12 (Pink)) - The Kree (also known as the Ruul) are a scientifically and technologically advanced militaristic alien race that are native to the planet Hala in the Large Magellanic Cloud. The Kree are the rivals of the Skrull.
• The Phalanx in Uncanny X-Men #312 - The Phalanx are a cybernetic species that form a hive-mind linking each member by a telepathy-like system.
• The Shi'ar (first seen in X-Men #97) - The Shi'ar Empire (or Imperium), is a vast collection of alien species, cultures, and worlds situated close to the Skrull and Kree Empires. The Shi'ar are also called the Aerie.
• The Skrulls (first seen in Fantastic Four #2) - The Skrulls are a race of shapeshifting aliens that originated from the planet Skrullos. The Skrulls are the rivals of the Kree.
• The Symbiotes (first seen in Secret Wars crossover #8) - The Symbiotes are a fictional race of amorphous extraterrestrial parasites that envelop their hosts like costumes, creating a symbiotic bond through which the host's mind can be influenced.
• The Watchers (first seen, in the form of Uatu, in Fantastic Four #13) - The Watchers are one of the oldest species in the universe and are committed to observing and compiling knowledge on all aspects of the universe. After an incident with the Prosilicans, the Watchers then took a vow never to interfere with other civilizations.