The small spiders used in the film were Avondale spiders (Delena Cancerides), a harmless species from New Zealand that were provided by Landcare Research in Auckland. Despite their fierce appearance, this spider is docile member of the crab-spider family and are, in fact, harmless to humans. They were not allowed back in New Zealand for quarantine reasons. The giant "spider" used in the film was a species of a bird-eating tarantula, which attains an 8" legspan or more. Those types of tarantula are not easy to handle and can give a nasty bite. The spiders in the film were managed and handled by famed entomologist Steven R. Kutcher.
Canaima is the name of the avenging spirit of the Guyana Indians. It's also the name the area in Venezuela where the beginning of the movie was filmed and home to the world's tallest waterfall, Angel Falls.
Brian McNamara, who plays Chris Collins in Arachnophobia, was actually on an the episode of Family Ties entitled "A, My Name Is Alex", from 1987. The TV is playing an episode from Family Ties in the scene the house is covered in spiders.
The spider goes from South America to California needed to measure about one foot across. There was only one such spider in the U.S. Frank Marshall named the spider 'Big Bob' after director Robert Zemeckis.
The trivia item below may give away important plot points.
In the film the species of spider are Eusocial (Division of labour, division of reproductive labour, overlapping adult generations and cooperative brood care) but in reality most species of spider are solitary and aggressive to other spiders. The social species just live communally and don't show all the traits of Eusociality.