- "Brian: Look it has blue eyes?
ALF: Naturally, it is a cockroach."
- ―Brian Tanner and Gordon "ALF" Shumway
The Melmacian Cockroach is a blue-eyed insect commonly found on planet Melmac. Although naturally tiny, they can grow to the size of a Human, or even bigger, when exposed to certain earthly chemicals, including boric acid.
After Melmac was destroyed, a Melmacian Roach, named Rodney by ALF and Brian, escaped to Earth in ALF's spaceship (in a bag containing a slimeball) and became huge and aggressive after the Tanners' attempts to kill it. Eventually, ALF discovers a substance that is lethal to the Cockroach: perfume.
Since Melmac no longer exists, it's possible that Rodney was the last member of his species, meaning the Melmacian Cockroaches are now extinct.
It is unknown what the cockroach would have done to ALF if ALF had lost the battle in the bathroom. It does seem that the Cockroach chases ALF throughout the house, as the cockroach had perfect following steps to find where ALF went.
Melmacian Cockroaches grow upon the usage of boric acid & other possible materials used in bug spray, upon the usage of such chemicals, The cockroach can grow large, ALF described that they started out with a cockroach an inch long & Kate sprayed it & a minute later, it could wear a sweater & Willie sprayed it again, The cockroach grew larger which freaked out Willie & the exterminator. Trevor then sprayed the house, causing the cockroach to become a giant monster, which nearly ate ALF, the only substance toxic to these species is perfume.