Chapter 31, Problem 1Q

Summary Introduction

To analyze:

The limiting factor for the size of the insects, among the tracheal density of the leg or the tracheal density of the entire body by describing the graph shown below.

Introduction:

Insects belong to the phylum Arthropoda. The respiratory system of insects comprises spiracles and trachea. Spiracles are the small openings present on the exoskeleton of the insect. These openings are connected to the network of dense tubes called trachea or windpipe, which acts as the internal respiratory system for all the insects connecting pharynx and larynx to the lungs.

Explanation of Solution

The oxygen diffuses through the spiracles present on the exoskeleton and is further passed to the underlying tracheal tubes. The tracheal tubes branch into tracheoles that deliver oxygen to tissues and cells via diffusion. A study showed that level of oxygen in the atmosphere plays a crucial role in the body size of arthropods.

This leads to more tracheal density inside the insect’s body, which differs in legs as compared to the body, because larger the body of an organism, more is the tracheal network and tracheal density required to supply oxygen to all the internal tissues. However, from the graph, it is clear that as the body length of the organism increases, the tracheal density also increases in both body (TrD_tot) and legs (TrD_leg) but was found to be decreased or absent as both reaches the upper limit. Between both legs and body, density in legs was found to be more as compared to density in the body in small-sized organisms but its levels were found to be absent as the body length of organism increases.

The reason for the above observation is the concentration of oxygen, which is less in the atmosphere (21%). Thus small organisms is more efficient in intake of oxygen due to their small body size and tracheal density is sufficient to supply appropriate amount of oxygen. In large organisms, enhanced tracheal network is required to supply air to each organ of the body. But this does not cope-up with the present available oxygen levels as more oxygen in air (during Paleozoic times) allow minimum metabolism to transport air to the farther trachea in insect and results in a decrease density of trachea in legs, causing a shortage of oxygen in the legs.

Conclusion

Hence, it can be concluded that tracheal density in the legs is most limiting factor in the size of insects. This is so because large body length and size requires more tracheal network to transport air to all the tissues of body. Absence or less amount of trachea in legs of an organism shows the lack of oxygen supply in legs due to large body length.