Course 32.3Lesson 2 of 3
A cockroach has no lungs and its blood does not carry oxygen. This lesson explains the open blood system, the air tubes that carry oxygen straight to the cells, the tubes that remove wastes, and the nerve cord and sense organs of the cockroach.
Try the code and the simulations beside the lesson.
In your body the blood stays inside vessels. This is a closed system. In a cockroach the blood vessels are poorly developed and open into spaces, the haemocoel. The organs lie in these spaces and are bathed in the blood. This is an open blood vascular system.
The blood is called haemolymph. It has a colourless plasma and cells called haemocytes. It has no red pigment, and it does not carry oxygen. Its work is to carry food, wastes and hormones.
The heart is a long muscular tube that lies under the middle of the back. It is made of funnel shaped chambers. Many books count thirteen of them. Each chamber has a pair of openings called ostia. Blood from the body spaces enters the heart through the ostia and is pumped forwards into the spaces again.
If the blood does not carry oxygen, how does oxygen reach the cells? The answer is a network of tubes. The tracheal system is a net of air tubes called tracheae. They open to the outside by ten pairs of spiracles. There are two pairs in the thorax and eight pairs in the abdomen. Muscles called sphincters open and close each spiracle.
Inside the body the tracheae branch into finer tubes, the tracheoles. These carry air close to every cell. Oxygen moves to the cells by diffusion, and carbon dioxide leaves by the same tubes.
Why is this so good? Diffusion is quick over a short distance and very slow over a long one. The time for a gas to diffuse a distance in one dimension is
Here is the diffusion constant. In air it is about , and in water or tissue it is about . Air is about ten thousand times better.
The air tubes cut the distance in tissue to about a micrometre. Over this distance, diffusion through tissue takes only about a quarter of a millisecond.
Waste from the haemolymph is taken in by the Malpighian tubules. There are about 100 to 150 of them. They empty into the hindgut. The main waste is uric acid, so the cockroach is uricotelic. Some waste is also stored or removed by the fat body, the nephrocytes and the urecose glands. NEET lists the systems of the cockroach as digestive, circulatory, respiratory, nervous and reproductive, so the excretory system is for the boards, A-Level and IB.
The nerve cord lies on the belly side of the body. A series of ganglia, which are groups of nerve cells, are joined by paired lengthwise connectives.
Because each segment has a ganglion of its own, many actions are controlled locally. Reports say that a cockroach can live for some time with its head removed.
The cockroach senses the world with the antennae, the compound eyes, the maxillary palps, the labial palps and the anal cerci, and with sensory structures on the body. The eyes give mosaic vision. The antennae and palps sense touch, taste and smell.
The program counts the spiracles and the ganglia from the numbers in the two regions.
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