Skitters

Skitters are a species of apparently intelligent insect/humanoid creatures widely thought to be responsible for the Invasion of Earth. They move quickly on six legs, attack with formidable claws, and possess a tough exoskeleton. Skitters are functionally nocturnal and communicate using radio waves. They are able to use harnessed children to communicate with humans. They are also referred to as "Cooties" by Pope's Gang. The harnessed children refer to them as "guardians".

According to Rick Thompson, Skitters are morally repulsed by the concept of harming one's own species.

While performing a dissection, Anne and Lourdes find a harness beneath a Skitter’s exoskeleton, revealing skitters are the enslaved subjects of the Espheni. Skitters may have once been a different type of life form, as the harness seems to genetically modify the host.

It is revealed in the episode "Mutiny" that the harnesses will gradually transform the harnessed kids, a process that can continue even if the harnesses are removed. Examples of this can be seen by Rick's ability to stick to walls in addition to his enhanced strength, speed, and agility. In addition to this Megan was shown to have scales on the sides of her head, and possibly even more parts of her.