"Apart from rubbing our legs together like some sort of grasshopper, I doubt if we can get on speaking terms with them."Continuing the series' general policy at this point of alternating history and science fiction-themed stories, we head to the planet Vortis, populated by the butterfly-like Menoptra and ant-like Zarbi. The latter have fallen under the influence of the Animus, a parasitic creature which is slowly enveloping the whole planet in webs.While the camera lens gets a bit foggy from the sheer weirdness of this planet, the TARDIS crew are split up. The Doctor and Vicki try to keep themselves alive while dealing with the Animus, while Ian ends up in the underground home of the Optera. Barbara spends the serial in the company of a scouting party of Menoptra. Ultimately everyone converges on the lair of the Animus and (barely) manage to defeat it thanks to an untested weapon built by the Menoptra.Novelized under the title Doctor Who and the Zarbi.
—The Doctor ponders the difficulties of communication.
- Alien Sky: Vortis has multiple moons, drawn there by the Animus.
- All There in the Manual: No date is given on-screen for the story, although the novelisation sets it in 20,000 AD. To a degree it doesn't matter, as this is one of the show's few stories which unambiguously has no human involvement at all.
- All Webbed Up: Well, only their heads and upper bodies, but the Doctor and Vicky get this treatment.
- Badass Adorable: The larva guns are deadly, but also very cute.
- Big Creepy-Crawlies: The Zarbi and the larva guns.
- Blinded by the Light: The Animus gives off quite a bright glow.
- Bug War: In this case, bugs versus bugs rather than bugs versus humans.
- Continuity Nod: Barbara finally gets on the same page with Vicki about her adventures in Rome. The gold bracelet that Nero gave her plays a small part in the first two episodes.
- Conveniently Timed Distraction: Barbara and the Menopteras confronts the Animus to save the Doctor and Vicki and kill it, but the Animus overpowers them with its light and blocks their attack. The Animus then gets briefly distracted when Ian and the Vrestins arrives from under the ground and Barbara quickly uses this opportunity to kill the Animus.
- Does This Remind You of Anything?: Vortis' natural ecosystem is compared to a single living organism. The Animus is constantly growing and sucking the life out of it. Its home base is called the Carcinome.
- Eldritch Abomination: The Animus qualifies both in terms of its powers and in terms of its appearance. In the Expanded Universe, it's actually named as one of H.P. Lovecraft's Old Ones, Lloigor...
- Handicapped Badass: Hrostar and Hlynia's wings were cut off when they were taken to the Crater of Needles. They still can kick Zarbi ovipositor.
- In-Camera Effects: The unique look of this serial was achieved by attaching a distorting lens to the camera.
- Insectoid Aliens: The four regular characters are the only humans (well, three humans, one humanoid) in this story. For six episodes they spend time on a planet where every other living thing is an Insectoid Alien.
- Light Is Good / Light Is Not Good: The Menoptra believe in gods of light, and convince the Optera that light is not bad. But when they see the light of the Animus, they have to remind themselves this creature is not good simply because it is light.
- Mind-Control Device: The Zarbi have a gold collar that they put around people's necks to control them, since the Animus can use gold to exert mental control over someone. Barbara is forced to leave the TARDIS in a trance-like state in episode one because she's wearing the gold bracelet that Nero gave her.
- People in Rubber Suits: Really obvious for this one.
- Puppeteer Parasite: The Animus.
- Starfish Language: The Zarbi.