Ballooning is the term used to describe the mechanism (mechanical kiting) used by a great many spiders, to disperse through the air. Many small spiders make use of gossamer (very fine silk) to lift themselves off the surface.
After hatching, spiderlings climb as high as they can then stand on raised legs, abdomens pointing upwards. They then start to release silk threads from their abdomens into the air, forming a triangular shaped parachute. This allows them to be carried away by even the most gentle of breezes.
While most only fly a short distance, others are taken up into the jet stream, some ascending into the upper atmosphere. Sailors have reported spiders being caught in their ship’s sails, over 1600 km from land. They have even been detected in atmospheric data balloons collecting air samples at slightly less than 5 km (16,000 ft) above sea level.
This is how spiders invade isolated islands and mountaintops. Apparently, they can survive flights up to 25 days or longer without food!
Spider – Wikimedia Creative Commons