DIMORPHISM IN BLISSUS LEUCOPTERUS. 33! 



For convenience in calculation the polygon was considered as 

 two, the first having six and the other five classes, the small 

 connecting class being divided between the two polygons. Both 

 polygons are skew, running down very rapidly on their outer 

 slopes and shading off gradually toward each other to be con- 

 nected by a very small class. The skewness of the polygon with 

 the mode at 1.5 mm. is -f .0235 and that of the one with the 

 mode at 2.7 mm. is .018. 



An examination of short-winged specimens from California and 

 from Long Island kindly loaned from the National Museum by Dr. 

 L. O. Howard and others from New York State loaned by Dr. 

 C. E. Felt gave polygons with the same mode as that obtained 

 from short-winged material from Ohio. Similar results were ob- 

 tained by a comparative study of long-winged insects sent from 

 Urbana, 111., by Professor S. A. Forbes. This indicates that the 

 tendency of a given form is toward the same mode from whatever 

 region taken or whether the two forms are mixed or separate. 



DISCUSSION OF RESULTS. 



The significance of these results is by no means easy to deter- 

 mine. Looking at the polygons only it seems reasonable to 

 suppose that the present dimorphic species has been derived from 

 a parent stock with a mode lying somewhere between the two 

 present ones. In that case it may be assumed that differences of 

 environment have permanently impressed themselves, dividing the 

 parent stock into two evolutionary lines one of which at present 

 has wings longer and the other wings shorter than the parent stock. 



The evidences of geographic distribution appear to negative 

 this view. The genus is almost cosmopolitan, having been re- 

 ported from every continent save Asia and from many islands of 

 the sea. So far as known, it is most abundant and certainly 

 most destructive in the United States. Nevertheless there are 

 good reasons for regarding the chinch bug not as a native but as 

 an immigrant. In his very reasonable hypothesis about the origin 

 and distribution of the chinch bug in North America, Professor 

 Webster (1898) l assumes that our stock of chinch bugs has come 



i Webster, F. M., "The Chinch Bug," U. S. Dept. of Agriculture, Bulletin No. 

 15, New Series. 



