Rachni Workers are tiny green rachni (similar to aphids) which usually act as a first wave for Rachni Soldiers. Acting in swarms, they rush up to the squad and explode in suicide attacks, causing heavy toxic damage which ignores shields. Rachni Workers are encounted in a few places in the galaxy. Given their name, they likely fill the niche occupied by workers in other hive species, tending to the needs of the queen.
One shot or a tech proximity mine will cause them to explode at a distance. Also since Workers are easy to kill, just shooting a few bullets into them will usually kill them. If the workers get too close they will explode causing toxic damage to those around it. Subsequently killing them too close will have the same effect. Best to kill them at a distance.
Mass Effect 2
If the Rachni Queen was allowed to survive Noveria, then Shepard encounters an asari on Illium who encountered the rachni and gave her a purpose. The Workers worked tirelessly to repair the asari's ship, and apparently it runs even better than before.
Mass Effect 3
If the Rachni Queen survived the events on Noveria, Shepard has the opportunity to rescue her in 2186. If Shepard chooses to rescue the queen, sacrificing the krogan Aralakh Company in the process, she will use her workers to assist in the construction of the Crucible.