a genus of delicate, primitive, gnatlike flies that spend their larval stage in ponds and slow streams. Nuclei from various larval tissues contain giant polytene chromosomes. The salivary gland chromosomes of C. thummi and C. tentans have been mapped, and the transcription processes going on in certain Balbiani rings (q.v.) have been studied extensively. See Chronology, 1881, Balbiani; 1952, Beermann; 1960, Clever and Karlson; chromosomal puff.
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