24O HABITS AND INSTINCTS IN LEPTINOTARSA. 



to avoid the extremes, a large male being on the average just as apt to 

 attempt to mate with a small female as with one more nearly his own size, 

 allowing, of course, for the greater opportunity for finding a female more 

 nearly suited to his size among the females of the general mediocre popula- 

 tion. At any rate, the attempt of a large male to mate with a small female 

 invariably results in failure, owing to his inability to force an entrance into 

 the small genital passage of the female. On the other hand, large females 

 and small males do not seem to be able to mate with any degree of success on 

 account of the lack of adaptation in the size of the reproductive organs. 



I have attempted to obtain statistical data upon this point, but the difficulty 

 of dissecting out the organs and the distortion that inevitably results are so 

 great that the error introduced is probably greater than the range of varia- 

 tion. At any rate, it is evident from the table that the extremes only very 

 rarely mate, and that when they do it is most frequently with mediocre indi- 

 viduals, as might be expected. Therefore, there is to a large extent in the 

 mating of these beetles an elimination of all but the mediocre individuals from 

 the parenthood of the following generation. Hence it follows that no matter 

 whether the characters of the extreme individuals be heritable or not, there is 

 little or no chance for transmission of them to the following generation, and 

 thus an effective although unconscious selective mating as regards size is 

 brought about in this species. The selective factor is, as has been shown by 

 Crampton in Lepidoptera, a lack of adaptation in the size of the reproductive 

 organs of the sexes ; the end result is the preservation in the population of an 

 extremely mediocre condition as regards size, with a very few only in the 

 more extreme classes. 



Assortive mating between beetles of the same general color value that is, 

 between beetles which belong in the same class of color value as regards 

 albinism or melanism has also been studied in oblongata, but with quite dif- 

 ferent results. As regards color alone there does not seem to be the slightest 

 trace of any selective influence in the mating of these beetles. Among 500 

 copulating pairs examined, it was found that a highly albinic male was on the 

 average just as apt to mate with a melanic female as with one nearer its own 

 color. Of course, if the albinism or melanism was associated with abnormal- 

 ities in size, there was a rejection of the individual, but upon the basis of size 

 and not upon that of color. From the general habits of this beetle in mating 

 and its entire lack of appreciation of color, we should not expect to find any 

 assortive mating upon this basis. Nor has there been discovered the least 

 trace of any selective mating or segregation on this basis in the dimorphic 

 males and females of this species; that is, males and females of like color 

 have not been found in copulation with any greater frequency than have those 

 of different colors. The data from 500 copulating pairs, taken from three 

 different generations, have not shown the slightest trace of preferential 

 mating in regard to these characters. 



