Megasthĕnes in Harpers Dictionary of Classical Antiquities
（Μεγασθένης). A Greek historian, who stayed for a considerable time, as ambassador of King Seleucus Nicator, at the court of the Indian king Sandracus or Sandracottus (B.C. 315- 291), at Palibothra on the Ganges. From information about the country and the people, obtained while he occupied that position, he compiled a historical and geographical work about India (τὰ Ἰνδικά), the chief treatise on that country left us by the ancients. On it are founded the accounts of Diodorus and Arrian; beyond this only fragments are preserved. His record of the state of India at the time has been discredited; but recent investigations have, to a great extent, shown its trustworthiness. The remains of Megasthenes have been edited by Schnaubeck (Bonn, 1846), and cf. Müller's Frag. Hist. Graec. (Paris, 1868-74).
Megasthenes in Wikipedia
Megasthenes (Μεγασθένης, ca. 350 – 290 BC) was a Greek ethnographer in the Hellenistic period, author of the work Indica. He was born in Asia Minor (modern day Turkey) and became an ambassador of Seleucus I of Syria to the court of Sandrocottus, who possibly is Chandragupta Maurya in Pataliputra, India. However the exact date of his embassy is uncertain. Scholars place it before 288 BC, the date of Chandragupta's death.
Arrian explains that Megasthenes lived in Arachosia, with the satrap Sibyrtius, from where he visited India:
"Megasthenes lived with Sibyrtius, satrap of Arachosia, and often speaks of his visiting Sandracottus, the king of the Indians." Arrian, Anabasis Alexandri 
We have more definite information regarding the parts of India Megasthenes visited. He entered the country through the district of the Pentapotamia, providing a full account of the rivers there (thought to be the five affluents of the Indus that form the Punjab region), and proceeded from there by the royal road to Pataliputra. There are accounts of Megasthenes having visited Madurai (then, a bustling city and capital of the Pandya Kingdom), but appears not to have visited any other parts of India. At the beginning of his Indica, he refers to the older Indians who know about the prehistoric arrival of Dionysus and Hercules in India. A story very popular amongst the Greeks during the Alexandrian period. Particularly important are his comments on the religions of the Indians. He mentions the devotees of Hercules (Shiva) and Dionysus (Krishna or Indra), but he does not write a word for Buddhists, something that gives ground to the theory that the latter religion was not widely known before the reign of Asoka.
His Indica served as an important source to many later writers such as Strabo and Arrian. He describes such features as the Himalayas and the island of Sri Lanka. He also described a caste system entirely different from what exist today, showing that the caste system is fluid and evolves. His description follows:
The first is formed by the collective body of the Philosophers, which in point of number is inferior to the other classes, but in point of dignity preeminent over all. The philosopher who errs in his predictions incurs censure, and then observes silence for the rest of his life.
The second caste consists of the Husbandmen, who appear to be far more numerous than the others. They devote the whole of their time to tillage; nor would an enemy coming upon a husbandman at work on his land do him any harm, for men of this class, being regarded as public benefactors, are protected from all injury.
The third caste consists of the Shepherds and in general of all herdsmen who neither settle in towns nor in villages, but live in tents.
The fourth caste consists of the Artizans. Of these some are armourers, while others make the implements that husbandmen and others find useful in their different callings. This class is not only exempted from paying taxes, but even receives maintenance from the royal exchequer.
The fifth caste is the Military. It is well organized and equipped for war, holds the second place in point of numbers, and gives itself up to idleness and amusement in the times of peace. The entire force--men-at-arms, war-horses, war-elephants, and all--are maintained at the king's expense.
The sixth caste consists of the Overseers. It is their province to inquire into and superintend all that goes on in India, and make report to the king, or, where there is not a king, to the magistrates.
The seventh caste consists of the Councillors and Assessors,--of those who deliberate on public affairs. It is the smallest class, looking to number, but the most respected, on account of the high character and wisdom of its members; for from their ranks the advisers of the king are taken, and the treasurers, of the state, and the arbiters who settle disputes. The generals of the army also, and the chief magistrates, usually belong to this class.
Later writers such as Arrian, Strabo, Diodorus, and Pliny refer to Indica in their works. Of these writers, Arrian speaks most highly of Megasthenes, while Strabo and Pliny treat him with less respect. Indica contained many legends and fabulous stories, similar to those we find in the Indica of Ctesias.
Megasthenes with his pioneering work has been rightly recognized as the father of Indian history. He is also the first foreigner Ambassador to be mentioned in the Indian history.