If you pay attention and are lucky, you may happen to observe tiny spiders flyingsuspended in the air dragged by small triangular sails made of silk filaments. It’s about the ballooninga behavior used by various spider species (mostly their recently hatched young) as well as by mites and lepidopteran larvae capable of producing silk. The physical properties of silk make it in fact very light and at the same time very resistantsuitable for capturing small updrafts, gusts of wind or even electric fields like a sail. Since it is not a real active flight, it is more correct to speak of gliding flightwith which the animal is still able to cover considerable distances, in some cases hundreds and even thousands of kilometers traveled at the mercy of the elements. This dispersal strategy allows numerous spider species to spread their offspring over a very large area.
How ballooning works and what distances it reaches
The spider seeks an elevated position exposed to the wind, raises its hind legs, and releases numerous silk filaments from glands in the abdomen. This creates a sort of natural parachute, triangular in shape, which resists the air and drags the animal in the direction of the wind. This behavior is especially common during breeding seasonafter the eggs have hatched: masses of hundreds of thousands of spiderlings are able to cover the landing site with silk, giving it a “snow-covered” appearance. The spider is at the mercy of the wind but it can partially influence its descentreducing the length of the filaments and consequently the air resistance, as shown in an interesting video published by Science Magazine, based on research by Moonsung Cho in 2018. Some spiders are also able to use other filaments to stop their flight, attaching themselves like an anchor to branches and walls. University of Bristol studies published on Current biology they showed how wind is not even necessary, but is also enough small electric fieldstriggered by changing atmospheric conditions, to offer enough thrust to take off.
Credit: WanderingMogwai, CC BY–SA 4.0, via Wikimedia Commons
The gliding journeys of these spiders can be very long, both in height and in distance: specimens of spiders transported by the wind are often found in the oceantrapped between the sails of ships or directly in the water, helped to survive and reach land by their water-repellent legs. Some hot air balloons used for sampling revealed the presence of spiders at 200-300 meters above sea level, and monitoring carried out in the USA in the 1930s observed the phenomenon of ballooning up to 4.5 km above sea level.
It is not exclusive to light spiders
Ballooning is used almost exclusively by baby spiders or by very small species, as they are light enough to be transported by the wind – adult spiders are almost always too heavy. However, if the updrafts are strong enough, it is possible for this to occur: for example, adult spiders of the genus Stegodyphus, which can reach 100 grams in weight, have been observed using ballooning to move taking advantage of the warm currents even on windless days. Their sail, in this case, is made of thousands of filaments and can reach up to one meter in length and width.
Advantages and disadvantages of ballooning
Ballooning is risky: apart from a limited ability to descend, the spider has no control over its direction and often ends up landing on a hostile or resource-deprived areaor be preyed upon. It is therefore one evolutionary strategy which balances the reproductive costs for the animal, i.e. the loss of many young, with dispersal over a larger territory. In this sense it closely resembles the reproductive model of anemocore plants (from ánemos, wind and chōreō, to go, spread or move) which spread their seeds and fruits by exploiting the action of the wind. The best known of these is the dandelion, with its typical yellow flower which, when ripe, transforms into a soft ball of seeds with tufts of white hairs. As with baby spiders, the vast majority of these seeds are destined to die, but for the law of large numbersat least someone will find suitable soil to sprout.
In addition to spiders, also many mites of the Tetranychidae family, also capable of producing silk, ei caterpillars of some species of lepidoptera they use ballooning to disperse – an example of convergent evolution. A theory promoted by a group of English researchers hypothesizes that ballooning emerged for the first time in the Cretaceous, with the advent of deciduous plants: the animals that initially allowed themselves to be suspended from the branches with silk, and then let themselves fall, would have gradually evolved an alternative dispersal system.
