Cricket Tracheal System

Concept 7: Overview of the Cricket Tracheal System

Click this drop-down menu to see the Success Criteria.

  • I can briefly describe the ecological niche of crickets.

  • You need to be able to identify that crickets are terrestrial animals living on land for key requirements 2 in the rubric to get Achieved.

  • You need to be able to identify that air is the source of oxygen for crickets, and that in air, the oxygen availability is 21% for key requirement 3 in the rubric to get Achieved.

  • I can describe ventilation and gas exchange in the cricket tracheal system.

      • A brief description of ventilation may apply to key requirement 6 in the rubric to get Achieved if you link ventilation to an adaptation.

      • May apply to key requirement 7 to get Merit if you explain in-depth how ventilation helps maintain a high concentration gradient across the respiratory surface.

      • May apply to key requirement 8 to get Excellence if you compare ventilation across the three taxonomic groups.

Click this drop-down menu to see the list of Vocabulary.

Do Now:

Do Now in your books:

Compare adaptations for SA : V between humans and snapper. What are their similarities and differences?

Ms. AdviConcept 5: Overview of the Snapper Gill System

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Ecological Niche of Crickets (Insect)

CRICKETS belong to the insect taxonomic group. They are TERRESTRIAL animals which means they live on land and their source of oxygen is air, which is 21% oxygen. Air is dry so they are susceptible to desiccation or drying out. As a matter of fact, because insects are so small compared to mammals, they have a higher surface area to volume ratio, they’re susceptible to drying out faster than mammals. Air also contains debris, which may block or clog the airways.

Crickets have high metabolic demands, which means they need a lot of energy to carry out necessary activities to survive - like flying away from predators. Flying is a vigorous activity that needs a lot of energy. Also, to make those distinctive mating calls that crickets are notorious for, male crickets stroke their wings against each other to attract females. This wing stroke behaviour requires a lot of energy.

What are the parts of the cricket tracheal system?

The tracheal system consists of the following structures: spiracles, tracheal tubes, tracheoles, and air sacs.

Insects take in air through spiracles along the sides of their body. Spiracles are pores on the exoskeleton, or the outside skin or shell of the cricket. Spiracles are lined by small hairs, and can open and close to control ventilation or air flow into the tracheal system. They are directly connected to relatively wide tubes called the tracheae. Unlike mammals that just have one trachea, insects have a network of tracheae - tracheae is the plural of trachea.

The tracheae then branch off into many tracheoles, that further branch off into narrower and narrower tracheoles. These tracheoles extend to all the tissues of the cricket’s body. The very tips of these tracheoles are the specialised respiratory surface of the tracheal system, where gas exchange occurs directly between inhaled air and the body cell. This extensive branching of airways is so that oxygen can reach all tissues and cells of the body.

Unlike mammals and fish, insects don’t have a closed circulatory system to pump oxygenated blood around the body.

Instead, the tracheal system takes oxygen directly into the cells of the body.

In this picture you can clearly see stripes, or rings around the trachea - these rings are called chitin rings. They prevent the tracheae from collapsing. And finally there are these air sacs, which are air bag extensions of the tracheae. Flying insects like crickets have these air sacs to increase the volume of air taken in.

What's the journey of air in and out of the tracheal system like?

Usually, air flows into and out of the cricket’s body through open spiracles by diffusion. This air flow is regulated by small muscles that operate valves within each spiracle. The muscles contract to close the spiracles or the muscles relax to open the spiracles. From the spiracles, air diffuses along the tracheae, then the tracheoles. No new air can flow into the tracheal system when spiracles are closed.

Air sacs increase the volume of air that can be taken in by the cricket, which become useful when the spiracles are closed and there is no new air flow.

To increase the rate of ventilation during high activities such as flying, crickets can make rhythmic body movements to help actively ventilate their respiratory surface.

Rhythmic body movements compress and expand the air sacs like bellows (see picture to the left).

These rhythmic body movements help to draw more air into the tracheae and tracheoles at a faster rate. This increased ventilation only happens when crickets make rhythmic body movements during high energy activities. In all other times, ventilation happens via simple diffusion.

Concept 8: Tracheal System Adaptations & Limitations

Success Criteria

  • I can explain how specific adaptations of the tracheal system enable snapper to survive in their terrestrial niche.

  • I can discuss the advantages and limitations of the cricket tracheal system.


Do Now:

Do Now in your books:

List all of the tracheal system structures you would expect to see with the naked eye.

Ms. Adviento's Video on Concept 8: Cricket Tracheal System Adaptations & Limitations

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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 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 ratio is a requirement for efficient gas exchange because the more sites for gases to enter and exit the tracheole, 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 drying out the tracheoles. In fact, due to their large SA : V from being such small animals, insects are more susceptible to drying out, 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 Concept 7 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) Haemolymph

Finally the third adaptation for moisture is haemolymph. Tracheole cells produce a watery fluid called haemolymph 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/air flow through the tracheoles. Increased ventilation/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 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

There are 4 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.

Concept 4 Task 1: Complete one of this worksheet on OneNote.

Concept 4 Task 2: Complete Checkpoint #3