We had a problem with Spotted Wing Drosophila last year when we caught some in traps and noted damage in our raspberries. Today, I harvested some early berries and put them in a solution of 1 cup sugar in a quart of water. After 1/2 hour, Drosophila larvae floated to the surfact. According the “Biology, Identification and Management of Spotted Wing Drosophila” FSA7079 by Donn T. Johnson and Joseph O’Neill – University of Arkansas – “Legless SWD larvae cannot be distinquished from other Drosophila larvae – with two spiracles on one end and two black mouthparts on the other.”
This bulletin is located at http://www.uaex.edu/publications/PDF/FSA-7079.pdf