Ballooning is a term used for the mechanical kiting that many spiders, especially small species, as well as certain mites and some caterpillars use to disperse through the air. Many small spiders use gossamer or especially fine silk to lift themselves off a surface or use the silk as an anchor in mid air. The very fine silk used for ballooning has been called "gossamer" since 1325 and was not originally known to be produced by spiders; by extension, the same word is used metaphorically for any exceedingly fine thread or fabric. Biologists also apply the term "balloon silk" to the threads that mechanically lift and drag systems.
A spider or spiderling after hatching will climb as high as it can. The spider then stands on raised legs with its abdomen pointed upwards. This is known as "tiptoeing". After that, it starts releasing several silk threads from its abdomen into the air, which automatically form a triangular shaped parachute. The spider can then let itself be carried away by updrafts of winds, where even the slightest of breeze will do. Most rides will end a few meters later, or a spider can be taken up into a jet stream, which depends on its mass, posture, the convection air current, drag of silk and parachute to float and travel high up into the upper atmosphere.
Many sailors have reported spiders being caught in their ship's sails, over 1600 km from land (Heimer 1988). They have even been detected in atmospheric data balloons collecting air samples at slightly less than 5 km (16000 ft) above sea level. Evidently it is the most common way for spiders to invade isolated islands and mountaintops. Spiderlings are known to survive without food travelling in air currents of jet streams for 25 days or longer.
It is generally thought that most spiders heavier than 1 mg are not likely to use ballooning (Suter 1999). Also, because many individuals die during ballooning, it is more unlikely that adults will do it than spiderlings. Adult females of several social Stegodyphus species (S. dumicola and S. mimosarum), weighing more than 100 mg and with a body size of up to 14 mm, have however been observed ballooning using rising thermals on hot days without wind. These spiders use tens to hundreds of silk strands, which form a triangular sheet with a length and width of about 1 m (Schneider 2001).