Kikaichū

The are a species of small, beetle-like insects that are bred and utilised exclusively by the Aburame clan, forming the basis for all of their unique techniques.

Overview
Members of the Aburame clan actually live in symbiosis with the kikaichū through the Parasitic Destruction Insect Technique: the Aburame, shortly after birth, allows the kikaichū to nest and breed inside 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. In the anime, with proper control from the Aburame user, the kikachū are also shown able to transfer their accumulated chakra to others targets, replenishing them. Other uses include simply tracking down a target, serving as 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. Aburame members can also use their bugs to pull poison out of the body of other people; the bugs themselves are seemingly immune to poison. Thanks to the kikaichū, Aburame clan members are experts in espionage and search-and-seizure missions. They can also be used in great numbers to jam chakra signals, interfering with the abilities of sensor type shinobi.

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 kikaichū, repopulating their numbers.

In the anime, the kikaichū are revealed to be unaffected by genjutsu because of their simple nervous systems. Later, in Naruto: Shippūden, kikaichū are shown to possess the ability to mutate and evolve, making 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", which basically describes the Aburame clan.