Solonian The Solonians were a race with several different forms . Biology . As their planet , Solos , went through its seasons , the Solonians gradually mutated from one form to the next form . This naturally occurred with radiation and a special green crystal from the planet . These changes could also be artificially caused by pollution . Humanoid Solonians . The humanoid Solonians were the form which existed during the spring of Solos . They appear to be dark - haired human s . The main difference from human s was their ability to breathe the poisonous mists of Solos . They were at a low level of technology when they were discovered , with a simple farming and hunting culture . ( TV : " The Mutants " ) Insectoid Solonians . The Insectoid Solonians ( pejoratively known as Mutts to the human overlord s ) were the intermediate form from the start of summer on Solos ( something like a mobile pupa ) . These Mutts were a bit shorter than human s and covered in a thick black exoskeleton . They had a head , thorax and abdomen , as well as clawed arms and legs . They had bulging yellow eyes , a wide opening for a mouth and long pincers beside their mouths . Insectoids seemed to have decreased intelligence and could not speak , though this was likely just a side effect of the transformation . Near the beginning of summer , the humanoid Solonians would be instinctively compelled to seek out the necessary radiation and crystals for the transformation . Though the mutation would occur naturally from the radiation and crystals from certain caves , the changes could be brought on early with pollution . Unnatural changes could leave the victims confused and uncertain about the entire process . ( TV : " The Mutants " ) Advanced Solonians . The Advanced Solonians were the form which existed during the summer of Solos . The insectoid Solonians were transformed into advanced Solonians by more radiation . Though they had the basic shape of a human , advanced Solonians were almost angelic , with a robed body and scintillating colours . They communicated telepathically and had a wide variety of powers , including the ability to pass through solid matter and to shoot energy beams . ( TV : " The Mutants " ) Culture . The Solonians used engraved images in their writing . The Third Doctor was able to decipher engravings depicting the four seasons of Solos . Each of these seasons lasted 500 years as Solos only circled its sun once every 2000 years . These seasons triggered the mutations , but the Solonians were unaware of this due to their own cryptic writings and long periods under mutation . Solonians were often segregated from humans . They were forced to use different facilities such as a separate transmat . ( TV : " The Mutants " ) History . Sometime in the 25th century , the Earth Empire discovered Solos and started to exploit it . The Solonians were taken advantage of and their planet was stripped of its resources . By the 30th century , tensions were rising and the mutations from the modified atmosphere were starting to increase . Solos was to have independence but the Marshal had the administrator assassinated and placed " Skybase One " , and hence Solos , under martial law . Ky killed the marshal and a number of humans stayed behind to help transition the planet back to its former functionality . ( TV : " The Mutants " ) Behind the scenes . A Mutant costume was cosmetically altered and reused to represent a generic insectoid alien for the opening scenes of " The Brain of Morbius " . The novelisation of the story states that the creature belongs to an entirely different species . Despite this , the Doctor does call the creature in " The Brain of Morbius " a Mutt , like the insectoid Solonians were called , and acts like he recognises the creature . Ohica also mentions that the last ship to arrive on Karn before the Doctor was a " cruiser of the Mutts " . In the DVD commentary , Christopher Barry refers to the species as a Mutt .