Flying spiders: ballooning!
October 1832. It’s been almost a year now since the proud Royal Navy’s HMS Beagle sailed from the port of Plymouth, England. This two-masted Cherokee-class brig-sloop, able of carrying 10 guns, however had traded some of these guns and his military role for a third mast and scientific expeditions. This is her second journey. More precisely, we are on October 31, 1832. A young academic, aboard since the beginning of this adventure, is walking on the creaking bridge of the Beagle. Here and there, sailors are working on cleaning-up the ship, maneuvering the sails using the thick rigging cords. His gaze wanders way over the ship wall. There is nothing but this vast azure plain dotted with Ivorian foams. The nearest coastline is a hundred kilometres away. They are alone, in the middle of this huge aquatic desert. The young man’s look deviates on the rigging. And then a strange scene shows up: long threads of silk, shining in the sun and floating horizontally, are attached to the ropes. Where do all these threads come from? The young man approaches some of them and finds with a great surprise that at their end there are… spiders, tiny black spiders. Some are mature, some juveniles, there are male and female. All of a sudden he realizes the ship is covered in spiders, and infers that these spiders, which can only come from the coast probably floated in the sky, hanging on to those long silk threads that then came caught in the ropes. This almost bald scientist whose name’s Darwin will later label them as “aeronaut spiders”, and will keep being interested in this curious phenomenon nowadays known as ballooning.
Knowledge of this phenomenon does not go back to Darwin. Already in ancient Greece philosophers talked about spiders flying thanks to long silk’s thread. We know this long thread as the “gossamer threads”. Ballooning, this technique of flying thanks to long silk threads is mainly used by young spiderling to disperse, but some adults also seem to be using this technique. Some individuals will travel for only a few centimetres, some will travel for a few meters, and some will travel for hundreds or even thousands of kilometres. Indeed, sea sailors hundreds of kilometres offshore have reported experiences similar to Darwin’s! Some will rise a few centimetres in the air, while others will reach altitudes of several kilometres! For instance, spiders have already been collected by balloon probes flying at… 5 km above sea level! Many individuals risk their lives and will perish on that kind of journey, especially when they flyby inhospitable areas such as seas and oceans. But in the end for the species this technique is a winning one. This is how spiders are able to colonize isolated territories, like islands in the middle of the ocean. Perhaps while walking along some fields with a diminishing sun, you already saw long silk’s thread floating across the paths. They are ballooning spiders.
How does this work? First of all, the spider is going to look for the extremity of something, it can be the end of a tree limb, of a blade of grass, or even the parapet of a bridge. Once an adequate airstrip is reached, it will put itself on “tiptoeing position”, it will stretch its legs to gain height, and will rise its abdomen, more or less straight, pointed in direction of the sky. This posture is characteristic of individuals about to do ballooning. It will then weave a long silk’s thread (actually it is several silk threads), up to several meters long. The thread will generate an aerodynamic drag in proportion to its length. Once the latter long enough, the aerodynamic force on the thread becomes important enough to lift the spiders out of their support, and they fly. Take a long piece of ribbon. Outdoors, when it’s windy, hold it by one end and lift the ribbon up. You’ll see the ribbon floating thanks to the wind, you’ll even feel the force of the wind applied on the ribbon, and if you let go, it won’t fall on the ground, but drift off with the wind. It’s the same thing with silk thread at the difference of it is thousands of times lighter than your ribbon: the slightest breeze is enough to lift in the air the spider.
But… That’s not all. Because even in dead calm, the spider seems capable of doing ballooning! This led some scientists to conjecture that electrostatics could play a role and explain this kind of observation. The Earth’s surface is indeed a huge capacitor in which an electrostatic field reigns. Thunderstorm lightning are just “electric shock”, like the one you could have while sometimes touching metallic objects like a doorknob. In 2018, this hypothesis was finally tested. Researchers locked spiders into boxes isolated from the slightest drafts and ambient electrostatic fields. Then they generated electrostatic fields in these boxes and the spiders put themselves in a tiptoeing position, weaved gossamer threads, and flew away! Researchers even had fun changing the altitude of the flying spider by changing the inside electrostatic fields. This shows that electrostatic fields can be enough to allow ballooning to occur, and spiders appear to be able to feel the ambient electrostatic field. However, the question remains: in real conditions, what place this phenomenon really occupies in ballooning compared to aerodynamic forces?
One of the critical factors of ballooning is the mass of the spider. Most of the spiders are light enough, whether at adult or juvenile stage, to be able to perform ballooning, but some spiders, particularly some mygalomorphs, are too heavy even when they’re juste spiderling leaving their egg. Ballooning is commonly linked to spiders belonging to Linyphiidae family, notably the “money spiders”, small black linyphiids one or two millimetres long. The spiders Darwin described probably were money spiders. Here you can see a Philodromus from the rufus species group (Philodromidae family) in the tiptoeing position, and the video shows another Philodromus sp. filmed later, flying off the parapet of a bridge overlooking the Rhône near Lyon, south-east of France.
Last update: August 8, 2021