Adaptations and Limitations of the Cricket Tracheal System

Navigate the knowledge tree: 🌿 Biology ➡ NCEA Level 2 Biology ➡ 2.3 Plant & Animal Adaptations

Learning Intentions

By the end of this topic, you should be able to:

Discuss specific adaptations cricket have for each of the four characteristics of an efficient gas exchange system.
Discuss specific limitations of the cricket tracheal system, and how they limit the efficiency of gas exchange in crickets.

Glossary

Key terms and definitions.

word: Definition

Overview of Tracheal System Adaptations

The cricket tracheal system needs to make sure it possesses the 4 characteristics of an efficient gas exchange system, to be able to keep up with the high metabolic demands of this flying insect.

There are adaptations for:

Maximise the large SA : V of tracheoles.
Preventing damage to the tracheae and tracheoles.
Retaining moisture and keeping tracheoles moist.
Minimise the diffusion distance across tracheoles(thin).
Maximise the concentration gradient across tracheoles.

Adaptations for a Large SA : V

A large SA : V is a requirement for efficient gas exchange because the more sites for gases to enter and exit the tracheoles, the faster the rate of diffusion.

The tracheal system has two adaptations for maximising the SA : V ratio.

1) Extensive branching of tracheoles

First is the system of highly branched tubes that permeates or directly goes into tissues. This extensive branching of tracheae and tracheoles increases the SA : V ratio available for gas exchange.

2) Small hairs lining the spiracles

Any structures that protect the airways from getting damaged also contribute to maximising the SA : V ratio. Because if the delicate tracheoles get damaged, there would be less surface area available for gas exchange.

So the second adaptation for maximising SA : V ratio are the small hairs or bristles that line the inside of spiracles. These small hairs or bristles filter air as it enters to prevent dirt and debris from entering and clogging the airways/tracheal system which would reduce the surface area available for gas exchange.

This is an important adaptation for the cricket's survival because they inhabit dusty environments. Dirt and dust particles could seriously damage the delicate respiratory surfaces.

Adaptations for Moisture

Gases must first dissolve in water before they can diffuse across the specialised respiratory surface, therefore the tracheoles must be kept moist. So it is a problem for crickets that air is dry, because inhaling dry air could risk desiccating the tracheoles. In fact, due to their large SA : V from being such small animals, insects are more susceptible to desiccation, and needs to have adaptations to create and retain moisture. The cricket tracheal system has 3 adaptations to keep the tracheoles moist.

1) Internal tracheal system

The first adaptation is the cricket’s tracheal system is internal, located inside the body to limit its exposure to environmental factors like sun and wind. The tracheal system is enclosed behind an exoskeleton which is impermeable to water, and therefore stops any water escaping or evaporating out of the cricket’s body.

2) Spiracles and hairs

The second adaptation for moisture is to do with the cricket’s spiracles. The exoskeleton contains valved openings called spiracles, which open and close to control water loss and ventilation.

Remember from Lesson 11-12 that when spiracle muscles relax, they open the valves, and when they contract, they close the valves. This is a tightly controlled process because if the spiracles are open for too long, too much moisture will evaporate out of the cricket, but if the spiracles are closed for too long, not enough new air will diffuse into the tracheoles and this will reduce the concentration gradient at the respiratory surface.

3) Tracheole fluid

Finally the third adaptation for moisture is the tracheole fluid. Tracheole cells produce a watery fluid at the very tips of the tracheoles (blue in the picture below), to allows gases to dissolve into the fluid so that it can diffuse across the specialised respiratory surface.

Adaptations for a Short Diffusion Distance (Thin)

Since diffusion is a passive process, it actually takes a relatively long time for gases to diffuse from one area to another. Diffusion of oxygen and carbon dioxide is only efficient when the diffusion distance is short.

To keep the diffusion distance short, crickets have a thin respiratory surface. The very tips of tracheoles, where gas exchange happens, are very fine tubes, made up of one tracheole cell. For O₂ to diffuse from air in tracheoles directly into the body cell, it only has to pass through one tracheole cell - the same thing applies for CO₂ to diffuse from a body cell directly into the air in the tracheoles - it only has to pass through one tracheole cell.

Adaptations for Maintaining a Steep Concentration Gradient

The concentration gradient of oxygen and carbon dioxide across the specialised respiratory surface is what drives diffusion and therefore, gas exchange. The steeper the concentration gradient across the tracheole, the more efficient gas exchange is. The cricket tracheal system has 2 adaptations for maintaining a steep concentration gradient across the tracheoles.

1) Rhythmic Body Movements

Rhythmic body movements happen naturally during flight, when flight muscles contract and relax, changing the shape of the cricket’s abdomen. Rhythmic body movements compress and expand airways and air sacs, increasing the ventilation. Increased ventilation and air flow increases the concentration gradient across the tracheole, because air in the tracheole will be oxygen rich, and cells will have low levels of oxygen due to the high rate of aerobic respiration during flying.

2) Chitin Rings

Tracheae are lined with a spiral fold of chitin rings which keeps the tracheae open allowing for ventilation to maintain a steep concentration gradient.

Chitin is in a spiral arrangement to allow flexibility, and bendiness, which is needed during flight and rhythmic body movements. Chitin rings are like reinforcing wire that keeps the airways open during rhythmic body movements while allowing some flexibility.

Without the chitin, external forces like gravity and rhythmic body movements would compress the tracheal tubes and prevent ventilation.

Limitations of the Tracheal System

1) Incompatibility with water

The tracheal system cannot be used to "breathe under water". The tracheal system is incompatible with water for two reasons.

First, is that water is too dense/viscous to be ventilated by passive diffusion in and out of the spiracles, and water is too dense to be ventilated by rhythmic body movements. Without ventilation, gas exchange would stop because a concentration gradient of oxygen and carbon dioxide would not be maintained across the tracheoles - there would be nothing driving the diffusion of oxygen and carbon dioxide.

Second, is that the tracheoles would not be able to exchange gases efficiently enough with water to meet the oxygen demands of aerobic respiration, and sustain life. This is because the SA : V of tracheoles is not large enough to absorb enough oxygen from the 1% of oxygen available in water. The SA : V of the tracheal system is not compatible with the extremely low oxygen availability in water; it can only efficiently carry out gas exchange with air, which has 21% oxygen availability.

2) Limited to a small size

The cricket tracheal system is only efficient if the organism is small. This means that the cricket’s tracheal system limits the size these insects can grow to, because if they grow any larger, the tracheal system would not be efficient enough to meet their metabolic demands. This is for 2 reasons.

First, is that crickets do not have a closed circulatory system to pump oxygenated blood around the body. They rely on diffusion alone to move inhaled air through the networks of tracheae and tracheoles to reach all tissues and cells of the body. Because of this, insects are limited by the size to which they can grow. If crickets were to grow larger then the diffusion distance from spiracles to the body cells would be far greater, reducing the rate of diffusion. As a result, body cells may not get the O2 they need to meet metabolic demands fast enough.

The second reason is that the chitin rings that surround the tracheae are relatively heavy, especially when you consider the fact that a large proportion of the insect’s mass is taken up by the tracheal gas exchange system. If the cricket increases in size, the number and length of tracheae containing chitin would increase significantly, and the cricket won’t be able to move or fly due to the weight and physical restrictions of the chitin.

3) Tidal ventilation

Remember from Concept 4 that tidal ventilation describes how air enters and exits the lungs through the same way - in the cricket’s case, air enters through the spiracles, then the tracheae, then the tracheoles, and exits through the same way from the tracheoles, through the tracheae, and out through the spiracles.

Tidal ventilation is a limitation of the tracheal system because the new oxygen-rich air breathed in mixes with old air trapped in the tracheae or tracheoles. This old air is called the residual volume because it is the volume of air that is always left behind (residue) in the tracheae or tracheoles. Because it is left behind in the airways, it is oxygen poor and carbon dioxide rich. This mixing of new and old air during tidal ventilation lowers the oxygen concentration of the air that reaches the tracheoles, thus reducing the concentration gradient across the tracheoles.

4) Dead space

Remember from Video 4 that dead space refers to the air that enters the gas exchange system, but remains in the tracheae and upper parts of the tracheoles, and does not participate in gas exchange.

Dead space is a limitation because not all of the air inhaled gets to participate in gas exchange. Specifically, this is the limitation of the SA : V of the tracheal system. Not all all volume of air inhaled will have enough surface area on the tips of the tracheoles to participate in gas exchange.

More on Adaptations and Limitations of the Cricket Tracheal System

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Glossary

Key terms and definitions.

air sacs: Pockets of air that extend from the tracheae and increase the volume of air taken in, in flying insects.

breathing / ventilation: Movement of air / water in and out of the gas exchange system.

chitin rings: Rigid ring structures that prevent the tracheae from collapsing during ventilation and rhythmic body movements.

concentration gradient: A difference in the concentration of a substance between two areas. This characteristic is the sole driver of diffusion.

cricket: Example of an with a tracheal system for gas exchange.

desiccation: Drying out

exoskeleton: Hard, waterproof external covering that supports and protects the body of insects.

gas exchange: The process of obtaining oxygen from the environment and releasing carbon dioxide.

large surface area to volume ratio (SA : V): Characteristic that increases the rate of diffusion because there are more sites for gases to enter and exit the respiratory surface.

moist respiratory surface: Characteristic that increases the rate of diffusion because gases must first dissolve before they can diffuse.

rhythmic body movements: The process of compressing and expanding the air sacs to increase the rate of ventilation.

spiracles: Openings in the exoskeleton of insects that connect to the tracheal tubes.

thin respiratory surface: Characteristic that increases the rate of diffusion because the diffusion distance for O₂ / CO₂ is short.

tidal ventilation: Describes how air enters and exists the system through the same way, causing new oxygen-rich air breathed in to mix with oxygen-poor residual air trapped in the system.

trachea (insect): A network of airways that connect the spiracles to the tracheoles.

tracheoles: Many narrow, highly-branched airways that connect the tracheae and all cells of the body; their tips are the site of gas exchange in insects.

volume: The amount of three dimensional space a substance takes up.