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Spiders

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I have always had a special interest in spiders. I am amazed by their different techniques in catching preys, sometimes their own size or larger. Some spiders simply stalk and jump their victims, killing them with a poisonous bite. But many spiders use techniques so fascinating that they make the aliens in most science-fiction horror films seem dull. There is for instance the spitting spider Scytodes thoracica, which sneaks very slowly within firing range, and then from glands in its head shoots a jet of extremely sticky and poisonous material in a zig-zag pattern over its prey, immobilizing it in seconds. Or the bolas spider Dichrostichus magnificus which uses just one string of silk with a ball of glue at the end, holding it with one of its second front legs, and swinging it round and round by jerking its body up and down. The purpose of this is to catch one particular type of male moth, Dasygaster nephelistis, which is attracted by an irresistible scent, similar to that of a female moth, but this time it is produced by the spider, bringing death instead of love.

THE TECHNIQUE OF PHOLCUS PHALANGIOIDES

Spider webs are a well-known technique to catch preys. They come in very different shapes and sizes. Some are horizontal sheets of silk, with the spider either on top or hanging underneath. Some are beautiful orb webs, or are dome-shaped, or a combination of both. Some webs seem just like unorganized tangles, and are not as sticky as other types of web. This sort of web only seems to catch dust particles, but that is not true. A good example is the web of Pholcus phalangioides, a very common, vulnerable looking spider with a body length of about 8 mm, and very long and thin legs: front pair up to 50 mm. (See picture) Still, as far as I know one of the most effective killing machines among spiders, as it is able to capture preys much larger and stronger than itself. A prey would hardly notice that it was in a web at all, just slightly touching a string now and then when passing through. However, not all are able to complete the journey. When a string is touched, a vibration is produced that reaches Pholcus which is waiting in the shadows, informing it on the direction and type of intruder, like a blip on a radar screen. The three-dimensional and mostly empty structure of the web enables Pholcus to get very quickly to the spot. The most spectacular catch by Pholcus that I witnessed was another, much heavier and stronger spider, Tegenaria domestica, body length about 16 mm, and front legs of about 40 mm. When I saw it, it had been wrapped up and Pholcus had already started sucking out its fluids.
This is how it must have happened: Tegenaria had touched a few strings unwittingly. Pholcus had rushed to the scene, quickly turning its back towards its victim. Then, with a very quick movement of the hind legs it had first thrown a very sticky and elastic string of silk over Tegenaria, attaching the other end firmly to the web. Of course, Tegenaria had realized by then that it was being caught, and it must have tried frantically to pull free, but in vain. Pholcus had kept coming back, throwing more sticky threads, pulling them tight, anchoring them to different parts of the web, and so depriving Tegenaria little by little of its remaining foothold. Finally, Tegenaria had become completely suspended, and had then been then covered all over with ever more tightly wound strings of silk. Finally, when Tegenaria had been rendered totally immobile, Pholcus had come in for the venomous bite, which it could now safely deliver with its ridiculously small and weak mandibles. (This venom seems to be one of the strongest. Fortunately for us, Pholcus is unable to penetrate a human skin and is therefore totally harmless to man.)
Pholcus must certainly have had a good meal, but because of the size of its prey, it could never have hoped to suck out everything from the inside of Tegenaria. Most of it was wasted. Pholcus does not care, it just attacks anything that it can be encapsulated. It also has no qualms about eating its own brothers and sisters.

My study of spider behaviour from a very early age has made me question a popular notion that I grew up with: "nature is pure harmony, predators take only what they need, and it is just the old and the sick (who's time has come anyway!?) that are caught". Instead, I became convinced that it's a jungle out there, and that it's pure luck that Pholcus does not reach a body length of 30 cm, because you and I would be in very serious danger indeed when entering dark cellars!
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NEPHILA MACULATA, ARGIOPE SP.

When I was in Australia in 1980, I saw the big golden orb weavers of the Nephila family for the first time. I saw more Nephilas in Africa, South America, an Asia. The biggest is Nephila maculata, of which the female reaches a body length of 4,5 cm, and has front legs of about 9,5 cm. This beautiful black and yellow spider is common in Thailand and other countries in South-East Asia. The mature female spider is easily recognized by the bright yellow spots at the underside of the black legs. Its web can be quite large and has a yellow-gold colour. The silk is the strongest produced by any spider. The young of N. maculata, when still small and mainly pale yellow in colour, make interesting three-dimensional webs. First, a finely woven dome is made, with small rectangular meshes. Then, partly inside this dome, the orb-web is constructed, from the apex of the dome downwards. The spokes are closer together compared to those in orb-webs of other spiders with a somewhat similar shape, like the Tetragnatha species. The first spiraling thread, which binds the spokes together, starts from the centre and is taken by Nephila to the edge of the orb. Then it starts making the adhesive spiral. Unlike many other orb weavers it leaves the first binding spiral intact. The orb is never a full circle because upwards from the centre the top sector of the orb is missing, and the dome is there instead. I am not sure what the function of the dome is. It is probably an early warning system and protection against flying animals that are too big to catch and would otherwise destroy the orb-web or even attack the young Nephila. This species seldom makes a stabilimentum: a deliberate, conspicuous pattern of white silk in the non-sticking centre of the web. I have seen one spider do this only twice, just prior to the final two moults. This seems to be evidence of the protective nature of stabilimenta, giving a warning to birds not to fly through the web when the spider is especially vulnerable. It certainly has nothing to do with attracting more insects by the reflecting UV-light from the stabilimentum, because the spider does not eat at all a few days before and after moulting.
I also think that stabilimenta that are made by Argiope spiders like the St Andrew's cross spider Argiope have this function. Argiope, like Nephila, is always in the centre of its web. It holds its legs one and two and legs three and four on each side close together. It can be no coincidence that the stabilimentum's four spokes, forming a St Andrew's cross, seem to elongate the spider's legs, making the spider look bigger and a less attractive prey to birds. Argiope, like Nephila, makes a stabilimentum before moulting, and never when it is hungry.)
When Nephila maculata has grown bigger and has acquired its conspicuous black and yellow colours, which is probably also a warning for birds not to fly through the web, the dome is reduced to a few strings from the top of the - still incomplete - orb. Because of its size and weight, and the length and thinness of its legs, N. maculata has to be very careful not to get entangled in its own web while building it. While spinning the adhesive strings of silk with its hind legs, the mature spider always keeps its head looking up, or to the left or right, but never vertically down. In a downward looking position, it would need its hind legs to carry the weight of the body to prevent it flipping over backwards and landing in an awkward position. Therefore, the hind legs cannot be used for weaving adhesive threads when facing vertically down. The similarly shaped Tetragnatha species do not have this problem, because they are much smaller and lighter than the mature Nephilas, and their webs are made at an angle of 45 degrees or more, so they can hang underneath it with less danger of getting entangled.
When a prey strays into its web Nephila gets to it, sometimes slowly, sometimes very fast especially when it is hungry and it seems an easy prey. I have always seen Nephila grab and bite its prey immediately, holding and chewing on it until it offers no more resistance. Argiope, on the other hand, which has smaller mandibles usually conquers medium size or bigger preys by first throwing broad bands of white silk over it, and then encapsulating it by rotating the victim at high speed, binding it ever more tightly, and biting it afterwards. When Argiope is still very small, it is unable to rotate its prey, and instead it runs around it, which also has the effect of tightly binding the victim.
Nephila encapsulates bigger preys very calmly, only after biting them. It uses a technique like that of Pholcus, described earlier. The spider always faces head up, hanging from its front legs, holding and manipulating the prey with the shorter legs of pair three, and applying silk all over it with the hind legs. The prey is not rotated while being encapsulated. The silk is not of the special type used by Argiope which looks like a broad white ribbon, but is more like the threads Nephila normally produces. Nephila never encapsulates smaller preys at the spot where they are caught. Instead, the spider bites them, pulls them loose, and returns to the centre, moving carefully backwards, pulling itself up along the string of silk that it made when going down. Again, this is obviously to prevent becoming entangled itself and causing unnecessary damage to its web.
In his book "Australian spiders in colour" (reprint 1979, ISBN 0 589 07065 7) Ramon Mascord stated that eggsacs and males of this species were unknown to him at the time. By now at lot more must be known about this spider. I saw a male in the Botanical Gardens near Kandy, Sri Lanka (see picture). This specimen had a body length of about 0,5 cm and front legs of about 1,5 cm. The legs and cephalothorax were orange coloured, the tips of the palps were black. The abdomen had lighter yellow-orange colour, with a slightly darker striping on top, from the front to the back. It was in the cupola part of an immature female, which had a few more moults to go, so this could mean that Nephila females already mate at this stage. I have also seen N. maculata laying eggs (see picture). The female lowers itself to the ground below the web, and starts digging a shallow pit using its powerful mandibles and its legs. The pit is then covered by a pinkish-yellow woolly layer of silk on top of which the eggs are laid. These are covered by another woolly layer of silk, after which the spider camouflages the eggsacs with dirt and pieces of fallen leaves. The whole process takes about four hours.
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ARGYRODES ANTIPODIANUS

In the tropics and sub-tropics, the webs of big orb-weavers are often inhabited by some smaller spiders. Sometimes a number of very small (3 mm) silvery droplets are seen near or in the web, moving about very carefully. These are dewdrop spiders, Argyrodes antipodianus, that like Pholcus belong to the Theridiidae family of spiders. Argyrodes prefers to eat preys that a big orb weaver has caught and encapsulated in silk, and has left hanging in the web waiting to be eaten later. Argyrodes stealthily moves towards the encapsulated prey, always ready for a sudden departure by means of a rescue line, fastened outside the web. When it has reached the prey, often much bigger than itself, it starts to attach the rescue line to it, and then very carefully begins to cut the prey loose from the web, not arousing the attention of the web's owner. When the strings connecting the prey to the web are bitten through and replaced by longer threads, prey and Argyrodes gradually move safely out of reach of the big spider.
I have observed these tiny spiders catching preys by themselves as well. In this case these were the young of Argiope keyserlingi, in who's web the silver droplets were living. Small preys that have become entangled in the web of a big spider but are not worth its attention are also attacked and conquered by Argyrodes on its own.
Although these little spiders prefer living with much larger spiders, they seem to be not totally dependant on their hosts but are quite capable of making a web by themselves. The result is a small cobweb typical of Theridiidae. The males and the females are about the same size, the male being a bit slimmer than the female and having longer legs. They stay together for their mature lives (a few months), copulating rather often. Needless to say, the males are not eaten by the female. The eggsacs are balls of about 4 mms in size, at first white and then light brown, suspended from one thick thread which is attached close to the web of the big spider, sometimes protected by a small cobweb around it.
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