After editing heaps of articles on terrible old SF films, I want to write The Science of Monster Movies ... in the great tradition of The Science of Star Trek and The Science of Harry Potter. Some things I have learned:
Radiation makes things big. Nuclear testing creates giant ants in Them! (the 1954 classic), cosmic rays produce giant wasps in Monster from Green Hell (1957), uranium ore causes giantish spiders in Horrors of Spider Island (1959), and a tiny smear of the flesh-eating Blob rip-off in Caltiki, the Immortal Monster (1959) not only gets big but uncontrollably reproduces whenever a ray-emitting comet approaches Earth.
Radiation also mutates things – fast! Uranium in The Cyclops (1957) takes just six months to convert a lost explorer to the title's 25-foot, one-eyed horror. In Day the World Ended (1955), World War Three is barely over before radioactive fallout spawns a three-eyed, bulbous-headed monster to menace survivors. Now imagine the script conference where someone asked, "How do we establish that this mutant is whatsername's missing fiancé?" and after much heated discussion they remembered that Hollywood werewolves revert to human form on dying. Death reverses the mutation process! I must have missed that particular biology class.
Radiation works differently on different things. Island Claws (1980), made soon after Three Mile Island, sees crabs on the Florida Keys enraged by radioactive leakage from a local reactor. They attack communities in terrifying scenes of stock footage. But thanks to an aspect of radiation known to physicists as "limited effects budget", only one crab becomes traditionally huge. Unable to beat its chest like King Kong, it instead roars and sticks out its tongue, reducing the audience to fear-crazed giggles.
Another principle of monster-movie biology is: "You Are What You Eat". Any animal-derived wonder drug will infect victims with horrid animal traits. Bat's blood spoils your social life by turning you into Batman, or rather into The Vampire (1957). Wolf blood serum: a werewolf in The Mad Monster (1957). Alligator DNA: The Alligator People (1959). I don't think wasps make royal jelly, but as a beauty treatment it has tiresome side-effects in The Wasp Woman (1959). Bee royal jelly: Invasion of the Bee Girls (1973) ... and so on to the recent District 9 (2009), where bodily fluid from alien prawns causes the hero to develop a prawnoid arm with Secret Prawn Powers.
This is such a whiskery SF cliché that P.G. Wodehouse spoofed it as long ago as 1926 in a story that opens with film addicts discussing the cliffhanger serial The Vicissitudes of Vera (a dig at The Perils of Pauline from 1914), in which a mad scientist plans to give our heroine a spinal injection of lobster-gland extract and turn her into a lobster. Because that's what mad scientists do.
Dinosaurs are our favourite monsters; practically every lost realm unknown to map-makers contains a few. For example, they turn up far underground in Jules Verne's Journey to the Centre of the Earth, whose 1959 film reveals the surprising fact that dinosaurs looked just like modern iguanas with fins stuck on. This is because ... but you're already ahead of me. Dinosaurs are even found on another planet in the singularly unconvincing King Dinosaur (1955), where we're firmly told that an ordinary lizard enlarged through the magic of rear projection is a Tyrannosaurus rex, crawling on four legs. Couldn't the studio lizard-wrangler have trained it to rear up a bit?
As for lost worlds in cinema ... should you ever find yourself in one, remember that they tend to explode. Especially if named Atlantis. An obscure tectonic condition called "fear of anticlimax" means that our explorers can rarely escape to civilization without the lost realm first being destroyed by volcanoes, earthquakes, tsunamis, or preferably all three. This is, frankly, no way to promote lost-world tourism.
David Langford just vanished under a Richter 9 lava tsunami.