468 N. M. Stevens 



the supernumerary chromosomes or the possible results of their 

 irregular distribution. It however seemed advisable to pubhsh 

 the results which have been obtained, as considerable time must 

 elapse before more material can be worked over; and it is to be 

 hoped that another summer's work in California with breeding 

 experiments and collections from different localities may furnish 

 the data which are now lacking, and clear up the whole matter. 



SUMMARY 



1 Diabrotica vittata has twenty-one chromosomes, ten pairs 

 and an unpaired heterochromosome which behaves like the odd 

 chromosome in other Coleoptera and in the Orthoptera and 

 Hemiptera homoptera, dividing in the second spermatocyte divi- 

 sion, but not in the first. Synapsis occurs at the close of the syni- 

 zesis stage. A chromatin nucleolus is present in all of the sper- 

 matids. 



2 Diabrotica soror and Diabrotica 12-punctata both have in 

 all cases nineteen chromosomes, nine pairs and an unpaired hetero- 

 chromosome, which divides hke that in Diabrotica vittata. 

 About 50 per cent of the individuals examined have only nineteen 

 chromosomes, the remaining 50 per cent have from one to four 

 additional or "supernumerary" chromosomes which divide in 

 either spermatocyte division, not in both, and may therefore give 

 rise to from four to ten different kinds of spermatozoa with refer- 

 ence to their chromatin content, in the same individual. The 

 percentage of individuals containing no supernumerary chromo- 

 some, one, two, three, or four supernumeraries, is nearly the same 

 for the two species — 48, 33, 15, 3, i for Diabrotica soror at Moun- 

 tain View, Cahfornia, and 51, 35,11,2, 1 for Diabrotica 12-punctata 

 at Bryn Mawr, Pa. It has not as yet been possible to associate 

 the different nuclear types with variations in any external char- 

 acter. 



Biological Laboratory of Bryn Mawr College 

 Bryn Mawr, Pa. 



Note — A part of the facts concerning the chromosomes in Diabrotica soror were given at the Inter- 

 national Congress of Zoologists in Boston, August 21, 1907. 



