Giant chromosomes. Light micrograph of giant (polytene) chromosomes of the fruit fly Drosophila melanogaster. These chromosomes are found in the salivary glands of D. melanogaster and many other fly species. The giant structure occurs when successive replicate strands of chromosome substance fail to separate from one another, instead lining up precisely in parallel. With careful microscopy, the minute structure and function of the chromosomes may be studied. Magnification unknown.