Megasthenes, a Greek ambassador to the Mauryan Empire, mentioned six subcommittees in his book Indika. These subcommittees were responsible for various aspects of governance and administration in the Mauryan Empire.

The six subcommittees mentioned by Megasthenes are:

1. Council of Ministers: This was the highest body of administration in the Mauryan Empire, consisting of the king and his ministers. They were responsible for making important decisions related to the governance of the empire.

2. Civil Service: This committee was responsible for the recruitment of officials and bureaucrats who served the empire in various capacities. They also oversaw the training and evaluation of these officials.

3. Revenue Department: As the name suggests, this committee was responsible for the collection and management of revenue for the empire. They oversaw the taxation system and ensured that the revenue collected was used for the welfare of the people.

4. Military Department: This committee was responsible for the recruitment, training, and deployment of the army. They also oversaw the defense of the empire and the maintenance of law and order.

5. Department of Spies: This committee was responsible for gathering intelligence and maintaining surveillance on the activities of the citizens. They also kept an eye on the neighboring kingdoms and reported any potential threats to the empire.

6. Department of Public Works: This committee was responsible for the construction and maintenance of public infrastructure such as roads, bridges, and buildings. They also oversaw the irrigation system and the construction of water reservoirs.

In conclusion, Megasthenes mentioned six subcommittees in his book Indika, each having its own specific responsibilities, which were crucial for the effective governance and administration of the Mauryan Empire.