136 ETHEL NICHOLSON BROWNE 



pieces ever become entirely separated from the chromosome prop- 

 er in N. indica, it is difficult to tell. They are often attached 

 only by a thin fiber, no thicker than the fiber connecting the 

 chromosome with its mate Y, and they often appear to be en- 

 tirely separate. If these pieces are separate in N. indica, or if 

 they eventuall}^ become separate in this species or in one derived 

 from it, X may be considered as a complex of chromosomes 

 rather than as a single chromosome. This complex is however, 

 represented in the diploid groups by a single chromosome. N. 

 indica is, therefore the first link in the chain, from a simple X 

 which occurs in many forms either with or without a mate Y, 

 through those cases where X is composed of two elements some- 

 times combined, sometimes separate (Phylloxera, Agalena, Sy- 

 romastes) , to the cases where X consists of two or more elements 

 always separate, but acting as a unit in the maturation divisions, 

 e.g., Thyanta calceata (Wilson '11), Galgulus and some of the 

 reduvioids (Payne '09), Ascaris lumbricoides (Edwards '10) and 

 finally Ascaris incurva, where X consists of eight elements (Good- 

 rich '14). Perhaps the case next in series after N. indica is Phyl- 

 loxera caryaecaulis, where the two parts of X are very unequal, 

 and are indistinguishably fused during part of their history. 

 We have additional evidence from N. indica for Wilson's ('11) 

 assumption that X is essentially compound in nature with a 

 tendency for the parts to segregate as separate chromosomes, and 

 strong morphological evidence for sex-linked characters. 



It is also possible that the small pieces of the X chromosome 

 represent the starting point in the appearance of supernumerary 

 chromosomes, such as are found in Banasa calva (Wilson '07), 

 Metapodius (Wilson '07, '09), the Diabroticas (Stevens '08), and 

 Ceuthophilus (Stevens '12). These chromosomes divide in only 

 one of the maturation divisions, as one should expect if they 

 have originated from X, and they frequently accompany the 

 XY pair in some forms. Other hypotheses have been suggested 

 for the origin of the supernumeraries ; by the passage of both X 

 and Y to the same pole (Wilson '09), by the replacement of Y 

 by a second m-chromosome (Wilson '10) and by an abnormal 

 division of the X chromosome in one of the maturation divisions 

 (Stevens '12). 



