When you still need a microscope to see Giant structures

Giant chromosomes can be observed in the salivary glands of certain two winged flies (Diptera) and were first observed by Balbiani in 1881 [1]. These chromosomes are oversized and develop from standard chromosomes when specialized cells
undergo repeated rounds of DNA replication without any cell division.

This process increases the cell volume and forms giant chromosomes, which are also known as polytene chromosomes.

Giant chromosomes have characteristic light and dark banding patterns that can be used to identify chromosomal structural rearrangements, deletions and even to locate genes.

In addition, differences in size, spacing and other characteristics on the bands are useful information to cytogenetisists. For example, giant chromosomes are used to identify particular species of Chironomid larvae that are otherwise difficult to identify.

[1] Balbiani EG (1881). "Sur la structure du noyau des cellules salivaires chez les larves de Chironomus". Zool. Anz. 4: 637–641.