I first became aquainted with Horseshoe Crabs in the mid-1960s when they appeared on the Outer Limits. The name of the episode was "The Invisibles".
"The Invisibles" is rarely mentioned as a top notch "Outer Limits" episode but it surely is. The audacity of its portrayal of the US Government rotten with corruption is breath-taking for its time. In 60 minutes, it manages to match the paranoia and uneasiness of "The Manchurian Candidate"...and throws in some godawful ugly aliens, to boot!
The parasites are second in ugliness only to "The Zanti Misfits" and being infected by one is shown as a terrifying ordeal. Yet I don't think it is the parasites themselves pulling the strings. Rather, it seems that weak and evil men are the masterminds of "The Invisibles"...and for the most part, they are the men we have entrusted our security to. That assumption packed a wallop then and still does now.
Source: SciFilm.org