The Volus are a Citadel species that hail from Irune. Their home world has an ammonia-based atmosphere and a gravitational field 1 and a half times that of Earth, which explains their small stature. If the Volus are exposed to the common Oxygen/Nitrogen mixture it can cause their flesh to split open, that's why they must wear protective suit when away from their home planet.
Due to the physical appearance of the Volus, they are not adept for war and combat. For this reason they have never managed to gain a seat on the Citadel Council despite being one of the longest serving species on the Citadel. Instead, the Volus specialize in Finance and trade. They offer other species financial support in return for protection.
 Discovering the Citadel
The Volus were the third species to discover the Citadel around 300 years after the Asari and the Salarians. Unlike the first two species, they were not offered a place on the Citadel Council and became the first species to gain an embassy. In order to join the council a species must provide the Citadel with a military fleet, resources and economic aid, the Volus aren't capable of doing that.
 Turian Hierarchy
During the Krogan Rebellions, the Volus made first contact with the Turians. Shortly after, the two species agreed that the Volus would become a client race to the Turians. This meant that the Turians would offer military protection to the Volus in any war they pursue. In return the Volus offer their finance and trade skills to help the Turian economy.