The Pnumann: mites with hydraulic brains

You would be remiss if you thought that sentient life is constrained to a single order of magnitude when it comes to size. The "pnumann" are a sentient species of carbon-based life forms whose physical size is on the scale of house dust mites. They live in nutrient-rich liquid environments at normal human-relatable temperatures. Besides their tiny size, their main claim to exoticism is how their brains work: they "think" using hydraulics. Since their very existence is based on the flow of liquid, their technology is about one order of magnitude smaller than the technology we humans use. They have an intimate understanding of small things, including microbes, which are about the size of small ants to them.

One of the pillars of pnumann culture is personal space. Since their hydraulic brains have no real boundary separating them from the water they exist in, it is only in very personal situations that pnumann will get within one "brain length" of each other. These situations are almost always reproductive in nature. 

Pnumann do not live for very long; their meat-bodies only last about a month in human time before dissolving. However, their hydraulic brains stay somewhat coherent over the course of generations. This results in a society that is heavily based on the transfer of memories. It is not understood how this memory transfer works, as it is of secretive, almost religious importance to the pnumann. 

The pnumann are very compatible with humans. Our blood vessels, surprisingly enough, contain a cocktail of nutrients and other dissolved solids that allow the pnumann to "breathe" it. 

By the time we humans encounter the pnumann, they will have had space travel for about as long as we have. In fact, we will have been encountering their ships for centuries before noticing that their ships are actual spaceships instead of oddly-shaped space debris. The most common pnumann spaceship designs resemble syringes in both size and shape. They are filled with a "serum" in which the pnumann can "breathe." 

The pnumann, like most other sentient beings in the galaxy, have run into the Malthusian problem of population hunger. Like most other species that make contact with humans, the introduction of lots and lots of food species (the pnumann are especially fond of house dust mites) has allowed them to greatly increase their standard of living. 

The pnumann, being able to "breathe" in human blood solution, make extraordinary medical assistants. Their dexterity (able to manipulate single cells with their bare hands) and knowledge of hydraulics, chemistry, and micro-scale technology makes them perfect partners with humanity. 

In fact, it can be said that no two species have ever been made for each other like humans and pnumann. The humans provide the food and housing; the pnumann provide medical and technological advances far beyond what humans could achieve alone.