The spider

The jaws of a modern spider, Aranaeomorphae, are used to grab and crunch a prey. Most spiders use poison to kill their victims. At the end of the jaws are two syringe-like structures that are hollow and very sharp. These are used to puncture the body of a victim and to inject the poison. The poison is produced in special glands. These cells grow into small sacs with poison (vesicles) which, migrate to a special bladder where they burst. Around this poison bladder there is a spiral muscle that contracts to eject the poison through the syringes in to the prey.

Primitive spiders, Mygalomorphae, have forwardly pointing jaws that move forwards and backward in contrast to the modern spider. Therefore they can not crunch a prey. They wait until the prey contents are dissolved before they can suck it empty.

The poison of a spider contains protein, amines and polypeptides. Some of these molecules are capable of disrupting the communication between the nervous system and the muscles, which causes paralysis. Other molecules cause death of cells, which leads to necrosis. After the prey is killed the spider spits enzymes from its mouth into the victim. Enzymes dissolve the contents of the prey. Mammals dissolve their meal in a stomach using the enzyme pepsin. The spider digests the proteins in the prey itself and sucks it empty.

How lethal is the poison of a spider? This is difficult question to answer. A poison is given a number LD50 to express its toxicity. LD50 stands for quantity of a lethal dose needed to kill 50% of a tested population. The poison of a black widow spider has a LD50 of 0.9 mg per kg mouse. Therefore 0.013 mg poison is enough to kill one mouse. The spider needs 2 mg to kill a frog. So the lethality differs among animals. Such a test has never been performed on humans. Therefore it is difficult to calculate how poisonous a spider is to humans.

The lethality of spider's poison to humans is much exaggerated. However there are spiders that are dangerous to humans. The Latrodectus species (Black widow), the Australian Sydney funnel web spider, Atrax robustus, and some wandering spiders from South -America are dangerous. These spiders use a substance that disturbs the nerve system, which can cause heart rhythm disturbances, cramps, shaking, pain and dizziness. A spider sting can be fatal to children and persons with weak constitution.

There are also many spiders that give a nasty bite comparable to the stinging of a wasp. Most of the venom injected with these bites causes cell death and gives rise to a wound that does not heal properly and becomes easily infected. In Europe there is the water spider, Argyroneta aquatica, which has a very nasty bite. Immediate cooling of the wound is the best medicine.

Most modern spiders crunch their catch with their jaws. Other spiders wrap their prey in silk, taking care that the victim does not bite them. Orb weaving spiders make a parcel of the prey and wait until the prey is dissolved before sucking it empty. Crab spiders do not use silk but use a rapid working poison. Spiders of the family Uloboridae do not have any poison and put their trust completely on their silken thread.

The injected enzymes dissolve the prey and then it is sucked empty. The mouth of a spider is located between the palps which are connected to the stomach muscles that perform the sucking. Between mouth and stomach is a filtering device that is made up of thousands of fine hairs. Only particles smaller than 1 mm can pass through this filter. This filter is so precise that even the particles in India ink will be filtered out so that only water can pass through. With these filters the spider prevents bacteria, viruses and other harmful life forms from entering its own body. The filter is regularly cleaned with the spider's upper and lower lips.

Because the food that is taken up can be large in comparison to its own body volume, the abdomen of the spider can swell enormously. The digested proteins are stored in a special place. This makes it possible for the spider to live for several weeks on a single prey. The waste substances are chemically converted to harmless crystals and are stored in special cells. These white colored guanocytes are located in-groups and can be seen through the skin. This shows up as a very special pattern on the back of the orb weaving spiders.

Special excretory organs separate these waste substances from the blood. The spider kidneys consist of two long thin tubes, called Malpighian tubule.

Ed Nieuwenhuys, January 2006

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