The ability to turn one's body into a living hive, and control the creatures in a symbiotic relationship.
The user has a symbiotic relationship with whatever life forms (insects, worms, snakes, etc.) they are sharing a body with. The insects get food from either the user or from whatever the person wants them to devour, and they use them for their needs.
In several cases, the user is actually a hive-minded swarm of the creatures.
Using ants they can lift things, several thousand could lift and subdue targets. With millions they could lift a vehicle.
With the poisons most insects produce as a means of defense, they can easily poison and kill people with millions of bites or stings. By directing insects down the throat, they could easily suffocate a victim. Even a few ants down the throat could cause suffocation due to the small amount of stings closing the wind pipe. By swarming thousands of bees, hornets or wasps the target can be stung to death.
With the use of thousands of locusts or caterpillars they can devour surrounding plant life.
In the event of being composed of insects, the user is effectively intangible, allowing attacks to simply pass through them.
With any insect of a small size (so that means most insects) they can scout areas. The insects will report what they find to the "hive".
The Nesk (Animorphs) are alien ants that can join together to form bodies.
In his new insect form, Clancy (Ben 10) can project swarms of insects from his mouth.
After being hit by The Shadowed One's disintegration eyebeams, Zaktan (Bionicle) was turned into a living swarm of millions of Protodites, each one containing a portion of his consciousness.
Norman Pfister (Buffy the Vampire Slayer) is a hive-minded swarm of maggots that can take on the appearance of a normal human.
Aragami like the Vajra (God Eater) are thousands of semi-sentient unicellular life forms attached to a core, all sharing a swarm intelligence.
Megaera (God of War) has numerous parasites inside her and which she can call forth through rashes on her body.
Destoroyah (Godzilla) is a sentient hive of mutated Precambrian crustaceans that can take on a variety of forms.
The Bogeyman (Martin Mystery) is able to turn his body in a maelstrom of maggots.
The Thousand (Marvel Comics) is, hence his name, a hive-minded swarm of a thousand spiders.
The Pain (Metal Gear Solid 3: Snake Eater) houses mutated "bullet bees" within his body.
D'Vorah (Mortal Kombat X) houses a variety of insects and worms within her body.
Grayroad (Nanatsu no Taizai) is a single being composed of numerous pudgy, dwarf-sized gray demons.
Shino Aburame (Naruto), like all members of his clan, houses chakra-eating kikaichū insects within his body.
In his true form, Orochimaru (Naruto) is a giant snake composed of many smaller snakes.
Oogie Boogie (The Nightmare Before Christmas/Kingdom Hearts) is a colony of bugs wrapped in a burlap sack.
Mushizō's (Ninja Scroll) hunched back is used as a hornet's nest.
Gotou (Parasyte) is an experimental Parasite created by Reiko which his body is composed of five different parasites, one in his head and four in his limbs, something he can utilize to the fullest extent.
After being assassinated by the Umbrella Corporation, James Marcus (Resident Evil) reanimated himself into a monstrous leech with the ability to control and house other leeches.
Mimicry Marcus (Resident Evil Zero) is a hive-minded swarm of T-virus infected leeches.
Million Ants (Rick and Morty) is a colony of ants.
Nrvnqsr Chaos (TYPE-MOON) is a living mass of chaotic matter analogous to a primordial soup, created from the accumulation of the lives of 666 creatures.