The spider

The spiders have circulating blood in their bodies. The colorless blood called hemolymphe transports nutrients, hormones, oxygen and cells. The blood also serves another purpose. It is used locally to raise the blood pressure during moulting (shedding of old skin) and stretching the legs.

In mammals hemoglobin molecules, present in the red blood cells, transport oxygen. The spider uses a more complicated protein called hemocyanin. Unlike hemoglobin, hemocyanin is not stored in a cell but flows freely in the blood of a spider. Hemocyanin is a protein that is made up of 24 sub-units with a molecular mass of 1.704.000 compared to the molecular mass of oxygen, which is only 32. Human hemoglobin is made up of 4 sub-units with a molecular mass on 64.500. Hemoglobin is a molecular disc, made of nitrogen, carbon, hydrogen and an iron atom at the center. The oxidized iron gives the molecule its characteristic red color. Hemocyanin also contains nitrogen, carbon and hydrogen but has a copper atom at the center 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 more or less signals. Beside these signals the release of oxygen is also controlled by temperature. Every sub-unit has a specific temperature optimum.

Book-lung and trachea entrances (13)

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.

The heart is located on the back of the abdomen. This is an open tube called pericardial-sinus with valves, which is hung in a cavity. 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 relax and the blood is pressed out of the tube. The heart has it own nerve center that can let the heart beat independently from the brains. There are connections with the brain that can raise the heart frequency. This can be registered if the spider becomes excited and its heart frequency rises.

In mammals the uptake of oxygen in the blood and the release of carbon dioxide from the blood take place in the lungs. Spider use other organs. Above the spinners there is a slit that can be opened and closed. Long small tubes run from this slit into the body. These tubes are called trachea. The gasses are exchanged with the blood by diffusion.

Besides trachea many spiders also have book lungs. These are hollow leaf-like structures through which the blood flows. These book lungs 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 below the abdomen.

There are spiders with either book lungs or trachea but most spiders have both.

Primitive spiders have only two pairs of book lungs. Modern spiders have developed trachea systems. Most of them still possess one pair of book lungs.
The trachea supply oxygen faster than book lungs. The modern spider uses these two systems together. This allows for the utilization of more oxygen and gives the advantage of quicker and longer reactions than the primitive spiders which have only book lungs.
Fast running and jumping spiders have a good developed trachea system. The small spiders of the family Symphyltognathidae are exceptional. They do not posses book lungs but have a well 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.

Skin of a spider

Because of the hardened skin made of chitin, the spider can not grow larger. Therefore the spider needs to shed external skeleton (exoskeleton) regularly (ecdysis) so that it can grow. The spider changes its skin 5 - 7 times in her life.
Spiders that can grow very old, like Tarantula's that may live for 25 years, change skin every year. This is not because she keeps growing but, like our clothes, the skin becomes worn out.

The color of the spider becomes darker before the changing of its skin. Enzymes dissolve the layer between the skin and the rest of the body. The new skin begins to form below the old one. It is extremely folded because it has to be larger than the old coat. The nerves stay connected to the sensory organs on her old skin so that she is not deprived of essential signals from the sensory organs on her legs. Just before shedding the old skin the spider hangs itself upside down with a thread. After several minutes the abdomen contracts to around 70% of it original size. The blood is pumped to her head raising the pressure from 20 kPa 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. When 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.

Spiders can often be seen with one or more missing legs. After moulting these lost legs regenerate. After the first change of skin, newly formed legs are smaller than the original legs. After the second moulting these differences in length are hardly observable.

Ed Nieuwenhuys, January 2006

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