John the Baptist lived on a diet of locusts and wild honey, which was indicative of his wilderness ministry. Matthew 3 tells us twice that John was preaching "in the wilderness" (3:1, 3:3) and explains that "his food was locusts and wild honey." (3:4) Mark's gospel does precisely the same (1:2-6). Luke's Gospel never defines the particular diet. Luke does, however, contain a prophecy from an angel to John's father before his birth that he would "drink no wine or liquor," (Luke 1:15) and later records Jesus saying:
"For John the Baptist has come eating no bread and drinking no wine," (Luke 7:33, see also Matt. 11:18).
Luke also records that John "lived in the deserts until the day of his public appearance to Israel" (Luke 1:80) and that the word of the Lord came to him "in the wilderness," (Luke 3:2). Both Luke and Matthew record Jesus asking the crowd regarding John, "What did you go out into the wilderness to see?" (Luke 7:24, Matt. 11:7). Indeed, all four gospels explain the ministry of John the Baptist as the fulfillment of the words of Isaiah 40:3:
"The voice of one crying in the wilderness, make ready the way of the Lord, make his paths straight," (Matt. 3:3, Mark 1:3, Luke 3:4, John 1:23).
Locusts and other grasshoppers and crickets were permitted as food under the Mosaic law in Leviticus 11:22. That John would be living on such as these and wild honey rather than eating normal, comfortable foods like bread and wine is a clear expression in the Gospels of his distinct desert wilderness life. It is also a powerful testimony of the prophetic fulfillment that his ministry represented, one of many such fulfillments that testified that the promised Lord and Messiah had come in Jesus Christ.