238 HABITS AND INSTINCTS IN I^EPTINOTARSA. 



glands of the reproductive organs, which are active on the maturing of the 

 genital products. It appears, further, that as far as can be determined there 

 is no selective influence exerted upon mating by the variations which probably 

 exist in the strength of the glandular secretions and in the ability of the dif- 

 ferent males to detect the scent. If such selective influence be present, it is 

 so slight that it is not to be detected by any method known to me. 



ASSORTlVie MATING. 



Assortive mating, which im.plies the tendency of certain conditions to be 

 brought together in reproduction with greater frequency than other condi- 

 tions, is believed by some of the modern selectionists to be a constant and a 

 rather important factor in the mating of animals. This idea is a further and 

 perhaps more logical extension of the sexual-selection principle which has 

 been used to account for many ornaments found in animals. In the genus 

 Leptinotarsa assortive mating is found in the habit that extreme individuals 

 have of showing a strong tendency in mating to select partners of a more 

 mediocre condition than themselves, whereas the more ordinary individuals 

 usually select partners that are their exact counterparts ; and this is true of 

 each pairing, whether it be a first or a subsequent one. There seems to be 

 inherent in the species of this genus the habit of rejecting in reproduction 

 those individuals which possess extreme or unusual characters. I have 

 investigated this phenomenon in decemlineata, oblongata, imdecimlmeata, and 

 dilecta, all of which species have furnished evidence of the existence of this 

 selective influence. 



The most satisfactory material in the genus for the investigation of this 

 problem is the species oblongata, in which the assortive mating as regards 

 size, general albinic or melanic tendencies, and the dimorphic red and yellow 

 conditions in both sexes affords an excellent opportunity to test the action and 

 existence of this phenomenon. 



In respect to size there is a most distinct assortive mating in this beetle, a 

 fact that has been determined both statistically and by a close study of the 

 beetles in nature. In these observations it was apparent that the extreme 

 individuals were not, as a rule, able to obtain partners in the reproductive act. 

 Thus the largest and smallest of the individuals of both sexes were far more 

 apt to be found unmated in nature than were the more mediocre individuals. 

 This is much more clearly shown in the tables of the seriations of these beetles 

 in respect to size. In these seriations the classification has been made upon 

 the basis of ten classes, in which i represents the smallest condition known 

 and 10 the largest. The unit used in the measurement of these beetles was 

 the perpendicular distance between parallel lines passing through the apex of 

 the elytron and the median posterior side of the pronotum when the beetle 

 was at rest. The entire length can not be used because of the great variation 



