The spider

Also in spiders blood is circulating in their body. The colorless blood, the hemolymphe, transports nutrients, hormones, oxygen, and cells. The blood also serves an other purpose. It is used to raise locally the blood pressure to be used for moulting (change of skin) and stretching the legs.

Oxygen is transported by another protein than with mammals. In mammals the protein hemoglobin is used to bind oxygen en release it at the right spot. The spider uses a more complicated protein, hemocyanin. Hemocyanin is not stored in a cell but flows free in the blood, this in contrast to mammals where millions of hemoglobin molecules are stored in red blood cells. Hemocyanin is a protein that is made up of 24 sub-units with a molecular mass of 1.704.000. For comparison, the molecular mass of oxygen is only 32. Human hemoglobin is made up of four sub-units with a molecular mass on 64.500. Hemoglobin molecular disc made of nitrogen, carbon and hydrogen, with in the center an iron atom, with a high affinity for oxygen. The oxidized iron gives the molecule its characteristic red color. Hemocyanin also contains nitrogen, carbon and hydrogen but has a copper atom instead of an iron atom. The oxidized copper gives the molecule a blue color. Hemocyanin binds oxygen but only releases it after it receives the right chemical signal. For every of the 24 sub-units there is special chemical signal. Depending on the need for oxygen a cell can give many or less signals. Beside these signals the release of oxygen is also controlled by temperature. Every sub-unit has a specific temperature optimum.

The blood does not flow through blood vessels. Spiders have an open blood circulation system. Blood vessels do transport the blood to a specific place but thereafter the blood flows freely in the open spaces between the organs. On the back of the abdomen the heart is located. This is an open tube with valves that is hung in a cavity (pericardial-sinus). Elastic muscles around this cavity contract, enlarging the tube. Because of the valves in the tube the blood can flow in only one direction. If the tube is filled with fluid the muscles weakens and the blood is pressed out of the tube. The heart has it own nerve center that can let the heart beat independence from the brains. There are connections with the brain that can raise the heart frequency. This can be registered if the spider become excited and its heart frequency rise. 

In mammals the uptake of oxygen in the blood and the release of carbon dioxide from the blood takes place in the lungs. Spider uses other organs. Above the spinners there is a slit that can be opened and closed. From these slit long small tubes run deep into the body. These tubes are called trachea. By diffusion the gasses are exchanged with the blood.

Beside this system a lot of spiders also have book lungs. These are hollow leaves through which the blood is flowing. These leaves hang in an open space that is connected to a tube. The other side of the tube is in open contact with the air. The entrance is located on the underside of the abdomen.

There are spiders with only book lungs, spiders with only trachea and, most of the spiders, with both.

Primitive spiders only have two pairs of book lungs. Modern spider developed trachea. Most of them also possess one pair of book lungs. Trachea supply the spider faster with oxygen than book lungs does. There is also a larger reserve of oxygen available in the body making that these spiders can react quicker and longer than the primitive spiders with only book lungs. Fast running and jumping spiders have a good developed trachea system. The small book lung less spiders of the family Symphyltognathidae are exceptional, they do not posses book lungs but has developed and very well performing trachea system.

The size of the heart depends on the size of the developed trachea system. Spiders with a good developed system do not need a large heart because the pumping capacity can be smaller. Less blood is needed to supply the organ with oxygen.

Because of the external skeleton (exoskeleton) of a spider they have to change it regularly en (ecdysis) to grow. The hard skin made up of chitin the spider can not grow larger. The spider changes its skin 5 - 7 times in her life. Spider than can grow very old, like the Tarantula that may live for 25 years, the skin is changed every year. This is not because she keeps growing but, like our clothes, the skin becomes worn out.

Before the spider changes its skin the color of the spider becomes. Enzymes dissolve the connection between the skin and the rest of the body. The new skin is forming below the old one and is extremely folded because it has to be larger than the old one. The nerve system stays connected to the sensory organs on her old skin because she would be deprived of essential signals from the sensory organs on her legs. Just before changing the spider hangs itself, head downwards, to a thread. After several minutes the abdomen contracts to around 70% of it original volume. The blood is pumped to her head rising the pressure to approximately 40 kPa (0.4 atm). The skin starts tearing at her jaws and the crack enlarges to the abdomen. If the skin is loose from the head-breast part (prosoma) the blood pressure is raised in the abdomen. If the skin has become completely loose the spider falls out of her old skin. These old skins are the "dead" spiders you can see hanging to wires and small branches.

Regularly spiders can be observed with one or more legs missing. After a moulting these lost legs grow again. After the first change of skin new formed legs are smaller than the original one but after the second moult the differences in length are hardly observable. 

Ed Nieuwenhuys, March 1999

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