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Last updated date: 02nd Dec 2023
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MVSAT Dec 2023

What is not a common feature in periplaneta and scorpions?
A. Excretory organs are malpighian tubules
B. No appendages in abdomen
C. Respiratory organs are trachea
D. Both are mostly terrestrial arthropods

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Hint: Almost all the features in Periplaneta and Scorpions are similar to each other and other organisms of their phylum Arthropoda. But they also have some characteristic features unique to a certain group of insects.

Complete Answer:
Periplaneta or cockroaches and scorpions are of the kingdom Animalia and phylum Arthropoda. They have many similar characteristics. The excretory organs in both Periplaneta and Scorpions are the malpighian tubes.
Their appendages are absent in the abdomen and present in thorax. The respiratory organs of both the organisms are different.

In cockroaches, the respiratory organ is the tracheal system. It has two important parts:
- Spiracles: When air enters into the respiratory organs through external openings, spiracles act as muscular valves opening way to the internal respiratory system. Cockroaches have 10 pairs of spiracles in their body. Their opening is regulated by spiracles. Air is drawn in when sphincters open and when they close, air does not enter.
- Trachea: The trachea is a tubular structure for conducting the air. Networks of trachea work together for respiration in cockroaches. Trachea is further divided into fine tubules called tracheoles which have tracheal fluid at their tips.
- Oxygen enters through spiracles and through trachea, reaches the tracheal fluid. The tracheal tip is connected to the body tissues and CO2 diffuses into the tracheal fluid due to partial pressure difference. Oxygen diffuses into the tissues. Carbon dioxide moves through the trachea and is released out of the spiracle.
- In Scorpions, the respiratory organs are called book lungs. Each book lung consists of a series of thin plates that are richly supplied with blood and are arranged like the pages in a book. These plates expand into an internal pouch which is formed by the external skeleton that opens to the exterior by a small slit. The folds maximize the surface exposed to the air and increases the amount of gas exchanged with the environment. Scorpions have four pairs of book lungs.

Both the arthropods are also terrestrial. Hence, the correct option is C, ‘Respiratory organs are trachea’.

Note: The book lungs in Scorpions provides a lot of surface for the exchange of gases with the surrounding air. Such organs are mostly found in arachnid arthropods. The ventilation system of the spiracles in cockroaches provides them with proper levels of gases for their body.