The Chrojnosomes in Diabrotica 467 



cytes would suggest that they might have originated in a detached 

 portion of the odd chromosome (x), but such a supposition is not 

 borne out by their later behavior in the maturation divisions, nor 

 is there any evidence of an unequal pair among the other chromo- 

 somes indicating accidental separation of a part of one chromo- 

 some. 



The only evidence I have that the supernumeraries might be 

 chromosomes in the process of development or degeneration is the 

 one individual (Diabrotica 12-punctata, No. 83 of the lot of 100 

 collected in October, 1907) in v^hich one very small supernumer- 

 ary v^as observed (Figs. 76-78). In other cases there seemed to be 

 remarkable uniformity in size without regard to the number 

 present. 



If at some period in the past history of the race before the eastern 

 and western species separated one supernumerary arose in any 

 way, its peculiar habit of division, sometimes in one, sometimes in 

 the other maturation division, may have given rise to the propor- 

 tional numbers of the different types in the two species. Or it 

 may still be possible, as was surmised earher in the study, that 

 (i) there will prove to be two distinct types (varieties or species) in 

 each of the present species, one having the large unpaired hetero- 

 chromosome only, the other having an unequal pair of hetero- 

 chromosomes hke that in Haltica, and that (2) the irregularities in 

 time of division and the consequent peculiarities in number and 

 distribution of the supernumeraries in Diabrotica are to be attrib- 

 uted to hybridism. If this should prove to be true it would indi- 

 cate little or no hereditary value for these supernumeraries or for 

 the smaller members of the unequal pair in other Coleoptera. A 

 careful biometrical study of several external characters may bring 

 to light some differences which can be associated with the pres- 

 ence or absence of the supernumeraries. The only other differ- 

 ence in the chromosomes of the two types seems to be a varia- 

 tion in the form of the odd chromosome {x). In type I it is 

 usually nearly spherical in growth stages, while in type II it is 

 more or less elongated. 



Until the material is investigated further, it hardly seems worth 

 while to discuss at any greater length the hereditary significance of 



