Light micrograph of the large polytene chromosomes from the salivary gland of a blowfly, Calliphora. Polytene chromosomes consist of many parallel identical chromatids resulting from repeated duplication without division of the chromosomes, the chromatids remaining exactly paired. Polytene chromosomes occur in the salivary glands of dipterous insects, their multistranded structure making them unusually thick & long (up to 0.2mm). They are widely used for research into genetics and developmental biology since careful staining allows the positions of individual genes to be found with an ordinary optical microscope. Magnification: x400 at 35mm size.