Kikaichū

are a species of small, beetle-like insects that are bred and utilized exclusively by the Aburame clan, forming the basis for all of their unique techniques. All members of the Aburame clan actually live in symbiosis with the kikaichū through the Destruction Bug Host Technique: The Aburame clan, shortly after birth, allows the kikaichū to nest and breed inside the their bodies and feed off of their chakra, and, in return, the host ninja receives the life-long ability to control and command the bugs, using them as their primary fighting technique.

The most common use of the kikachū in combat is to silently ambush an opponent and drain them of their chakra before they notice the bugs at work. This process can leave the victim with so little chakra that they are unable to move or fight back. Other uses include simply tracking down a target, serving as a a protective shield and as a prison to hold and detain a target in place. A female bug can be left on a target and then they can be tracked by smell, or scout bugs can be sent out and return to tell the host information about the area. Thanks to the kikachū, Aburame clan members are experts in espionage and search-and-seizure missions.

Since the kikaichū only have a lifespan of a few hours, they are expendable in combat. To maintain their numbers they are constantly reproducing within their host. If large numbers of the kikaichū should be wiped out, then Secret Technique: Insect Cocoon can be used to rapidly breed a whole new generation of the kikachū, repopulating their numbers.

In the original Naruto anime, the kikachū are revealed to be unaffected by genjutsu. Later, in Naruto: Shippuden, kikachū are shown to sometimes possess mutations that make them better equipped for a battle. The host can then cultivate these specific bugs to increase their numbers, improving the swarms' overall survival rate should the same opponent be encountered again.

Trivia

 * Kikai, when written as 奇怪, means "strange", or "mysterious".