martedì 28 aprile 2009

I ragni migratori? Una "leggenda metropolitana", dicono gli sciachimisti

L'articolo è stato aggiornato dopo la pubblicazione iniziale. Grazie a Riccardo e Fljll per le segnalazioni.

Alcuni sostenitori della teoria delle "scie chimiche" affermano che i "capelli d'angelo", i filamenti che in alcuni periodi dell'anno cadono abbondanti nei campi e nelle città, sono prodotti dalle scie stesse.

Quando si fa notare loro che l'origine più plausibile dei "capelli d'angelo" è la migrazione dei ragni che si spostano secernendo un filamento che fa da vela e li porta in quota grazie al vento e alle correnti ascensionali, arrivando ad altitudini notevolissime, i sostenitori della teoria delle "scie chimiche" spesso negano addirittura l'esistenza di questi aracnidi.

Ecco, per esempio, alcune dichiarazioni di Giorgio Pattera:

I ragni migratori non esistono...Non certamente a 5000 metri di altezza... 7000... quello che è.

(dichiarazione di Pattera durante un convegno pro-scie chimiche a Milano, 19 aprile 2009; video su Youtube)

NON SONO sicuramente la produzione (che risulterebbe a livello industriale, fra l'altro, per coprire l'estensione territoriale Vercelli-Milano/Bologna-Ferrara!) delle ghiandole serigene dei cosiddetti "ragni d'alta quota", la cui esistenza (dal punto di vista entomologico) lasciamo appannaggio delle leggende metropolitane, con buona pace del CICAP.

("Bambagia Silicea? Non Proprio!", di Giorgio Pattera; pubblicato su Scribd)

Vediamo cosa dicono invece gli esperti di entomologia e la letteratura scientifica a proposito di queste creature che sarebbero "leggende metropolitane".

John Jackman, entomologo della Texas A&M University

...dopo la nascita, un certo tipo di giovani ragni migratori secerne lunghi fili di seta che, sollevati dal vento, trasportano l'animale lontano, anche per molti chilometri. "è più facile vedere i filamenti in un giorno di sole che fa seguito a un temporale", spiega Jackman. "è un fenomeno molto comune. Quando la luce è giusta se ne possono vedere centinaia di migliaia volare in cielo".

(Scienza & Paranormale n. 40, novembre-dicembre 2001)

Encyclopaedia Britannica, edizione cartacea, Vol. 21, "spider"

Dispersal of spiderlings may be accomplished by different methods. In some species, spiderlings simply run away in all directions. But in the majority of species aerial dispersal takes place. This so-called ballooning coincides in temperate zones with the sunny days of autumn or of spring, if the eggs remain quiescent through the winter. Each spiderling climbs to the end of a blade of grass, a twig, a pole or any protruding object, raises its abdomen and spins a thin thread of silk. As this gossamer thread is caught in the breeze, the spiderling lets go of the support on which it sat, climbs to the middle of the thread and is carried by air currents to a considerable height upward and to great distances horizontally. Gossamer threads have been encountered by airplanes five miles above sea level and one observer reported that he saw spider threads in the stratosphere. Spiderlings have also been caught in the rigging of ships several hundred miles from shore.

Encyclopaedia Britannica, edizione online, 2009, "spider"

Small spiders and the young of many larger species secrete long silk strands that catch the wind and can carry the spiders great distances. This behaviour, called ballooning, occurs in many families and expedites distribution. Some species are distributed in this way around the globe within the bounds of the northern jet stream. Ballooning spiders drift through the air at heights that range from 3 metres (10 feet) or less to more than 800 metres (2,600 feet).


Journal of the Kansas Entomological Society, 1975

Glick (1939), who collected airborne spiders up to 15,000 feet above the ground...

(Factors Influencing The Aerial Dispersal of Spiders (Arachnida: Araneida), K.V. Yeargan, Department of Entomology, University of Kentucky, in Journal of the Kansas Entomological Society, Vol. 48, no. 3, July 1975)

Winter-Sleeping Wildlife, 1958

To reach new locations the spider travels by a means of transportation known as "ballooning". A spiderling or spider throws out streams of silk. These threads form a sort of "flying carpet." It rises on warm currents of ascending air, and spiders and spiderlings are borne aloft and scattered far and wide. Sometimes they go as high as 14,000 to 15,000 feet and travel hundreds or even thousands of miles.

(Will Barker, New York: Harper and Row, Pubs., 1958, pp. 94-96)

Microscopy UK, 1998

Ballooning allows many kinds of spiders to disperse around the world, wafted on air currents across even the widest oceans and highest mountains. When conditions of breeze and season are right, young spiders ascend to the top of any available elevation, spin a few short strands of silk, feel the tug of the wind, and let go. They arrived on Krakatau in 1883 within weeks after that volcanic island exploded and destroyed all life. Charles Darwin gathered them in mid-Atlantic during his trip on H.M.S. Beagle, and ballooning spiderlings have been collected from airplanes at very high altitude. I watched minute airborne spiders arrive on mid-Pacific islands from the nearest continent over two thousand miles away. I remember an astonishing spring day in the Eastern United States when from horizon to horizon the cloudless blue sky sparkled with pinpricks of light. It was so eerie, so beautiful, my skin rose in goose bumps—until it prickled with hundreds of tiny spiderlings alighting on me to seek their way in the world.

(The Spiders Web Part II, William H. Amos, presso Microscopy UK)

Journal of Animal Ecology, 1956

Although it has been known since the time of Aristotle that spiders can make airborne excursions, the first accurate observations were published by the pioneer British arachnologist, John Blackwall (1827).. during the last fifty years a number of arachnologists have published observations in this subject (Nielsen 1932, Braendegaard 1937, 1938, 1946, Bristowe 1929, 1939, Berland 1932, Smith 1904, Emerton 1908, 1919, McKeown 1952)... Glick was able to collect about 1500 specimens, including 4 at 5000 ft.

(Aerial Dispersal in a Known Spider Population, Eric Duffey, Journal of Animal Ecology, Vol. 25, No. 1, May, 1956, pp. 85-111, disponibile su JSTOR)

Journal of Arachnology, 1999

Bishop & Riechert (1990) used sticky traps, pitfall traps, and exclosures to demonstrate that 41-50% of the spider species found in their plots had arrived by aerial deposition, and that, in the course of the four-month growing seasons, the aerial deposition was occurring at a rate of at least 0.18 spider/day/m2. Although that estimate seems modest, it translates into an influx of 1800 spiders into a 1-ha field during each day of the growing season, and at total influx of about 2.16 x 105 spiders over the growing season.

...the occurrence of airborne spiders at high altitudes (Glick 1939; Gertsch 1979) and scores of km from the nearest land mass (Darwin 1839; Hardy & Cheng 1986)...

(An Aerial Lottery: The Physics of Ballooning in a Chaotic Atmosphere, Robert B. Suter, in Journal of Arachnology 27:281-293, 1999; disponibile presso

Charles Darwin in The Voyage of the Beagle, 1845 the morning the air was full of patches of the flocculent web [...]. The ship was sixty miles distant from the land [...]. Vast numbers of a small spider [...] were attached to the web. There must have been, I should suppose, some thousands on the ship. [...] The little aeronaut as soon as it arrived on the board was very active [...]

(The Voyage of the Beagle
, di Charles Darwin, 1845)

BBC, The Living Planet, 1984

The Sky Above, la settima puntata di questa celeberrima serie di documentari presentata da David Attenborough, mostra le immagini dei ragni che spiccano il volo. Attenborough sale poi in un aerostato alla ricerca di ragni e insetti che si spingono, portati dalle correnti aeree, fino a 6500 metri di quota.

A Falcon Flies, romanzo di Wilbur Smith (1980)

Pagina 172: "There is a tiny spider no bigger than the head of a wax Vesta which lives in the papyrus banks of the delta. It spins a gossamer web on which it launches itself into the breeze in such numbers that the gossamer fills the sky in clouds, like the smoke from a raging bushfire, rising many hundreds of feet..."

Grazie a Nodens per la segnalazione.