90 VARIATION IN LEPTINOTARSA. 



Individual Variation in the Punctation of the Pronotum. 



On the pronotum the punctations are most abundant on the posterior and 

 lateral margins. They are highly variable, both individually and geographic- 

 ally. When seriated, as in table 31, they give results that are in complete 

 accordance with those derived from the seriations in other parts. 



Individual Variation in the Glands of the Elytra. 



The beetles of this genus have in the elytra rows of compound hypodermal 

 glands situated between the dark stripes, which secrete a yellow, oily fluid of 

 strong odor and acrid taste. This fluid is strongly repugnatorial, and serves 

 the purpose of protecting these beetles from the attacks of insectivorous ani- 

 mals. As far as is known, these structures exist only in rows beneath the 

 stripes. The number of glands does not vary greatly in any given species, 

 but between the different species there are wide differences. Thus, in L. sig- 

 naticollis there are relatively few glands, while in multitcrniata and in decem- 

 lineata they are numerous. As these glands are strongly protective, species 

 possessing them in abundance would have an opportunity to multiply in num- 

 bers, whereas those with fewer glands would be less protected and more ac- 

 ceptable as food for insectivorous animals, and hence less abundant. L. multi- 

 tceniata, deccmlincata, and oblongata possess large numbers of these glands ; 

 consequently these species are active and the most common and widely dis- 

 tributed ones in the genus ; while signaticollis, dilecta, haldemani, and others 

 have fewer and less active glands, and are much less common and widely dis- 

 tributed. The correspondence between the number of glands and the numer- 

 ical abundance of certain species is suggestive of the part which these glands 

 may play in enabling a species to become a common and widely distributed 

 form. Natural selection would in this case be an important factor in the 

 increase of the species possessing the glands and in the elimination of those 

 with few of these protective structures. 



The variations of these glands when seriated give results completely in 

 accordance with those derived from other characters already studied. 



In table 32 there is a marked difference in the position of the modes 

 of the species, and also in the range of variation. Thus, in the species of the 

 lineata group, multitcrniata and decemlineata show high modes and high 

 ranges of variation, while juncta and signaticollis have modes lower in the 

 scale and a much smaller range of variability. In other groups the modes are 

 low and the range of variation limited. 



Individual Variation in Size and Shape. 



Individual variations in size and shape are of interest to a limited extent 

 in geographical and place variation. Both the variation in size and in shape 

 follows exactly the laws of the distribution of fluctuating variations found in 

 other characters, and further discussion of them is unnecessarv. 



