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Spiders are terribile and fascinating creatures at the same time both for their look and techniques used to capture their preys. To somebody they provoke disgust or fear, in others curiosity or admiration so much as even to start a breeding of species of particular magnificience.
Here we will make a brief dissertation on their characteristics, about venomous bite species and some spiders of particular interest living in the world and in Italy, in order to know more deeply these creatures.
Spiders belonging to the Arthropoda phylum are principally characterized by having jointed legs (from which the scientific name) a segmented body, a bilateral symmetry and an exoskeleton. To the phylum Arthropoda belong various classes including insects, centipedes and millipedes, crustacea and arachnida.
The latter includes spiders, skorpions, mites, ticks in addition to other orders less important to man. Spiders belong then to the order Aranea ( = Araneida) and in the world we can count 40.000 species grouped in 101 families.
They are divided in two sub-orders Mygalomorpha and Araneomorpha. The former includes spiders with more primitive features, big, hairy (with irritating bristles) and with the chelicerae moving in parallel way to the body axis, while to the second one mainly belong web constructors species; their chelicerae act instead in perpendicular way.
Fossil finds related to spiders are rare and by what discoverd until now we presume they have been amongst early animals living on the earth. Spiders evolved about 400 millions years ago at the beginning of the Devonian period from acquatic ancestors which might reach considerable dimensions (up to 1,5 metres).
The first fossil spider, the Attercopus fimbriungus, dates back to 380 million years ago (over 150 million years before dinosaurs’ appearance), has a segmented body and is provided with spinnerets. The biggest is the Megarachne servinei about a half meter long which lived about 300 million years ago. Most of the very primitive fossil spiders (Mesothelae) are segmented and have spinnerets placed under the half of the abdomen. It is presumed they were predators of other arthropods and their silk served various purposes as for example the interior lining of the lair or for the eggs protection, but it was not yet woven to capture preys.
Spiders with spinnerets at the end of the abdomen (Opisthothelae) appeared instead about 250 million years ago; the earliest mygalomorpha spider appears in this period. It can be retained that the really specific web weaving began afterwards in a parallel manner with the flying insects evolution. The Tertiary period is finally the richest of fossils trapped into the amber showing spiders very similar to those living today.
Spiders are often wrongly mistaken for insects even though they have well defined and clear morphological characteristics, among which the presence of eight legs is the clearest (insects have only six of them). Moreover best observers will have noted that the body is divided in only two segments instead of three as insects’ is, for head and thorax are fused together forming the cephalo-thorax.
The other segment is the abdomen at the end of which there are the spinnerets which serve to produce the silk; this comes out fluid and hardens in contact with the air. On the cephalic region we can locate the eyes, simple and in a variable number from two to four pairs, but in some cases they can be absent too; the pedipalps, with tactile functions and hooklike curved chelicerae for the presence of a mobile claw connected by a duct to poison’s glandes producing toxins which paralyze their preys.
The biggest mygalomorphae are exotic and for their showiness are often bred in terraria by arachnophiles. Being big and hairy are so often loosely referred to as Tarantulas (also Americans use the appellative Tarantulas) which instead is named after the european species Lycosa Tarantula, araneomorpho of family Lycosides.
Among mygalomorphes prevails the Theraphosidae family, of which a deserving representative is the Avicularia. This huge arboreal arachnida of about 10-15 cm of legspan (length between the first leg on one side and the fourth on the opposite side), living in the american forest habitat (Peru, Ecuador, Venezuela, Brazil etc.). She has been described for the first time in 1705 in Suriname by a swiss naturalist who named her Avicularia, which means “Birds-eater” (Avi = birds; cularia = eater). In fact her diet, very various, in addition to arthropods includes also animals of tiny size as tree-frogs, bats, reptiles, rodens and more rarely birds (humming-birds).
This spider builds a lair made of a dense silk tubular mesh and she camouflages it with fragments and leaves, both in pieces and whole, in this case using the silk as a glue for the edges. In captivity she can live for a long time, the female even ten years, while the male normally only a year.
By telling the truth, even though is a predator she has a very meek nature, we can commonly meet her in the houses built close to the forests. Some american tribes are used to eat this and other species of Theraphosides.
Other species of Theraphosides are the Salmon-Pink brazilian spider (Lasiodora parahybana), the Brazilian Black spider (Grammostola pulchra) both with terricolous and crepuscolar habits; the Goliath bird-eater, whose scientific name is Theraphosa Biondi, lives in Brazil, Venezuela, Suriname, Guyana and French Guyana and she is, together with the Theraphosa opophysis, the biggest spider on the earth, the adult females in fact can reach and exceed 25-30 cm of legspan.
Italian mygalomorphes are represented by Atypidae families, with four species of the kind Atypus, the Ctenizidae with 22 species, the Therophosidae with the only sicilian endemic species Ischnocolus triangulifer, the Dipluridae with the species Brachytele icterica, living in the northern regions.
The Araneomorphae, include instead a much large number of families and species able not only to produce silk but to build spider-webs more or less complex in order to catch their preys. The orb-webs represent a high level of perfection which is revealed by the beauty and the regularity of the structure. In this kind of spider-web, radial threads are not sticky so that the spider is able to move on the whole surface without problems, while spiral ones are gluey and are just used to entrap preys.
Most of the spiders living on our planet are not dangerous to human beings, but we must keep away from some of them for their venom is very toxic and even mortal. In the United States it has been calculated there are at least 60 species responsible for having bitten human beings.
In South America lives a fearful aggressive and fast arachnida the Phoneutria fera, capable to kill a mouse with just 0,006 mg of her toxin. Dangerous as well as her fame is the Black Widow (Latrodectus mactans) particularly common in Canada and in almost all South America: it has been estimated that the female’s venom is 15 times more toxic than rattlesnake’s.
In Australia lives a spider, the Atrax robustus, whose male has a venom 5 times more powerful than the female’s and it may cause pulmonary edemas and even cardiac arrest to a man. In Africa spiders of the kind Sicarius may cause grave hemorrhages as an effect of their venom. In various parts of the world it is commonly reported the presence of the Brown recluse-spider, also named Violin-spider, belonging to the kind Loxosceles. It is a brown-colored little spider of 1-1,5 cm, which may cause edemas and even grave ulcerations. The european species of the violin-spider living in Italy too are less venomous and their meek nature greatly limits the possibility to cause damages.
Another arachnida almost cosmopolite living in Europe and in Italy too and potentially dangerous is the Sac-spider (Cheiraconthium punctorium) whose bite may cause necrotic skin lesions and neuro-toxical phenomena. In her nature she is not aggressive but probably she could become in the egg deposition time to defend her young spiderlings.
In Italy live other two species of venomous spiders, the Malmignatte or volterranean spider (Latrodectus tredecimguttatus) dark-colored and with thirteen red spots on her back, whose venom causes nervous system troubles (tremors, arrhythmia and muscular contractions) and the Tarantula (Lycosa Tarentula) whose bite, much less dangerous, causes only an intense pain and following skin redness.
Some other spiders can be considered hurtful for going around houses and other buildings, weaving spider-webs in angles ceilings and various hollows. Among these we mention the Dancer-spider (Pholcus falangioides) so named after the waving movement she performs when disturbed; the Tegenaria spp. and the Nuctenea umbratica, robust night predators; the little Scytodes which capture their victims throwing threads having sticky ends; the Amaurobius fenestralis which builds tubular lairs where she lies waiting for the prey appears to have stumbled in one of the outer threads.
Some other spiders prefer vegetation, among these we mention the common garden spider or Crossed-spider (Aroneus diadematus), the multicolored Bound epeira (Argiope bruennichi), Crab-spider with their typical lateral movements (Thomisidae), the Wolf-spiders (Pisauridae) low vegetation and terrestrial predators, the Jumping-spiders (Salicidoe) which capture the prey getting close slowly and then assailing it and, if disturbed, she do not hesitate shortly jumping running away.
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