THE SALIVARY GLAND CHROMOSOMES OF THREE NORTH AMERICAN SPECIES OF TWINNIA (DIPTERA: SIMULIIDAE)

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Abstract
The salivary gland chromosomes of the three known North American species of Twinnia Stone and Jamnback are described, and compared with those of Prosimulium Roubaud. Twinnia tibblesi S. and J. has the standard arm association and the standard sequence in IS, II, IIIL. It differs from standard in inversions IL-1, IIIS-2,3, and a repatterning of the centromere region of chromosome III. Twinnia nova (Dyar and Shannon) and T. biclavata Shewell share these T. tibblesi traits and, in addition, have, in common, inversion IIIL-1 and a whole arm interchange giving the combinations IIL, IIIL and IIS, IIIS. Twinnia biclavata differs from T. nova by inversion IS-1 and a nucleolar shift. Both are good species. Evidence is presented that Twinnia is phylogenetically intermediate between Gymnopais Stone and the subgenus Helodon Enderlein of Prosimulium.