Sounds like an explorer mission. You actually have to kill 40 "hermit" spiders, not any ole spider. They missions are specific to the type of spider. The mission data should be on your data pad. After each kill of the "hermit" spider you should see a message on your screen telling you how many you have left to kill. Once completed the money will be deposited to your account. If you get tired of it just go to your data pad and click details on the mission and destroy the mission.