I've been reading Rachel Carson's 'The Sea Around Us' after discovering it among my books during my recent move. My grandfather gave it to me when I first expressed interest in ocean science many years ago. The book was actually presented to him in the 50's as recognition of meteorological measurements he took as a mariner. When I first was given the book I started reading it, but didn't finish so when I found it this time I thought it was time to sit down and read the whole book.
In her chapter on island formation, this intrigued me:
So bare and desolate that not even a lichen grows on them, St Paul's Rocks would seem one of the most unpromising places in the world to look for a spider, spinning its web in arachnidan hope of snaring passing insects. Yet Darwin found spiders when he visited the Rocks in 1833, and forty years later the naturalists of H.M.S. Challenger also reported them, busy at their web-spinning.
Spiders? How did they get there? St Paul's Rocks are near the equator right in the middle of the Atlantic Ocean. Discovered by the Portuguese navy in 1511 by accident, that is by crashing into them, these islands are now part of Brazil. They jut up from the ocean floor 800 km off the coast of South America, and are made up of 15 small islands and rocks with a highest point of 17 m. This group of islands is one of the few places (Iceland is another) where the mid-ocean ridge breaks the surface. Currently, the Brazilian Navy has a science station and a lighthouse on the islands.
On the 16th of February, 1832 the H.M.S. Beagle stopped at St Paul's Rocks and Darwin had an opportunity to explore. His inventory of life included: 2 types of sea birds (boobies and noddies), a type of large crab, a fly, a parasitic tick (preying on the birds), a moth that survived by eating feathers, a beetle, a woodlouse and lots of spiders. He also observed that not a single plant or lichen could be found, since that time mosses and grasses have found their way to the island, probably helped by people.
Not including the crab, the only life form on the island that can't fly or hitch a ride is the spider. No spider can fly, even though there is an Australian spider called the flying spider. Flying spiders are tiny with pretty blue and green iridescent colouring. Their abdomens have flaps that can be extended allowing them to glide when they leap, increasing their range. But even an ability to glide wouldn't help spiders colonize remote islands.
Young spiders are forced to move away from their parents and siblings to avoid competing for food and other resources with them. To begin their journey, a spider climbs to a high point, and then point its abdomen into the air. It releases a long filament of silk that is picked up by the wind, taking the little spider up into the sky. Drifting spiders have been found thousands of meters above the remote Hawaiian Islands. Or according to Rachel Carson:
Airmen have passed through great numbers of the white, silken filaments of spiders' 'parachutes' at heights of two to three miles.
Drifting is a great way to travel as it requires no energy expenditure from the spider. I wonder how many of these drifting spiders eventually find a suitable home?