7: Dissect an insect to show the structure of the gas exchange system.

Question 1

What are the aims of this practical?

Answer 1

• To discover how insects are adapted for gas exchange.

- To consider the ethical issues of using organisms in the lab.

Question 2

Describe the gas exchange system of an insect (locust)

Answer 2

• A tracheal gas exchange system that carries oxygen directly to all parts of the body.
• Gas enters through spiracles.
• The gas then diffuses through tracheae, supported by chitin to prevent the tubes collapsing with pressure changes.
• The trachea then subdivide to form smaller tracheoles, which lead directly to the insect’s tissues.
• Oxygen diffuses out of the tracheoles into cells.
• Carbon dioxide diffuses out of the cells, into the trachea and out of the spiracles.

Question 3

What are the ethical considerations when using animals in scientific work?

Answer 3

• The benefits in terms of advancing knowledge must be balanced against any potential harm to a living being.
• Dissection provides personal experience of the nature of fresh tissues and of the variation between individual animals.
• Knowledge of whole organisms is advanced inn a way that cannot be done without dissections.
• Manual skills are developed.

As a life has been taken, it is the responsibility of the student to derive the maximum possible learning benefit from any use of the animal. Live locusts must be treated with care and not subjected to unnecessary pain or stress.

Question 4

What are the ‘Three R’s’ of ethical animal use inn scientific work?

Answer 4

Replacement:
Avoiding or replacing the use of animals defined as ‘protected’ under the Animals (Scientific Procedures) Act. This can include using human volunteers, mathematical or computer models, cell and tissue lines, or alternative organisms such as invertebrates.
e.g. locust is experiment is a good example of replacement as locusts are used as opposed to frogs or mice…

Reduction: using fewer animals per experiment or study. (e.g. working in groups, not wasting specimens)

Refinement:
Methods that minimise pain, suffering, distress or lasting harm that may be experienced by the animals.

Question 5

Method:

Answer 5

1. Observe live locust in a clear syringe… locate external features of gas exchange system: spiracles…
2. view rhythmic ventilation movements of underside of abdomen. (tilt locust)
3. Take dead locust. View from side using dissecting microscope. look for position and shape of spiracles.
4. place dead locust in upright position in the middle of a dissecting dish. cut off wings. fix it to the wax or cork using dissecting pins.
5. using fine-pointed dissection scissors, make cuts along each side of the locust exoskeleton, from abdomen tip to front of antennae. avoid cutting through body tissues. remove exoskeleton using forceps and a seeker to separate tissues and leave organs intact within insect body.
6. flood dissection with water-causes air filled trachea to stand out (silvery appearance under water)
7. use a dissecting microscope and hand lens to examine the structures of the gas exchange system and make labelled diagrams. look for trachea. observe position and structure of air sacs.
8. use the scissors to cut around spiracles and remove with a piece of trachea attached. place it on a slide inn a few drops of water, cover with a coverslip and observe under microscope. Add a drop of methylene blue at one side of the coverslip, drawing it through using the corner of a paper towel at the other side. Observe again under microscope and record observations.
9. Lift mid gut using forceps. Observe attached tracheae. using scissors, remove a small section of mid-gut tissue with trachea and place in drops of methylene blue in a watch glass for 5 mins. Rinse and mount a small section in water on a slide. Observe first under Low power, then under high power, using the microscope. look for the narrowing of trachea as they branch until the form tracheoles.

Question 6

Risk:

Answer 6

cuts from dissection tools.

• keep implements orderly in tray when not in use.
• be cautious when using.

Spread of pathogens from locusts/ allergies to locusts:
-wash hands with soap after handling insects.

Question 7

Analysis of results:

Answer 7

-create labelled diagrams of the gas exchange system.