NUMBI^R OI' GENERATIONS. 243 



attractive qualities with difficulty, or not at all ; and the same difficulty must 

 be experienced in our observation and appreciation of individual differences 

 and attractive qualities in animals, but to an immensely greater extent. This 

 limitation of our appreciation of the qualities of animals should not be used as 

 a screen behind which to take refuge with some cherished hypothesis, there to 

 maintain its universal validity; for although these processes and conditions 

 may exist, their existence is not open to proof or disproof. 



NUMBER 01? GENERATIONS. 



The number of generations in Leptinotarsa each year, in both temperate 

 and tropical latitudes, is a remarkably constant character, and might well be 

 used as a generic differential. As far as I knov/, the number in all of the spe- 

 cies is limited to two. Thus, there are two generations throughout the range 

 of decemlineata, although Lugger has recorded three in Minnesota, and others 

 have supposed that there may be three in the southern United States. I have 

 not, however, been able to get decemlineata to breed more than twice in a 

 season without a period of hibernation or aestivation. In the spring decem- 

 lineata emerges from the ground, and after a period of feeding, during which 

 the germ-cells are also maturing, it breeds and la3^s the eggs for the first gen- 

 eration. These are usually all deposited at about the same tim.e, but there are 

 always for a month or more some individuals that are laying eggs, and of 

 course the larvas and imagines resulting from these eggs which are last laid 

 are much later in maturing than are the majority of the population. The 

 first brood, on emergence, feeds for a few days, and then deposits the eggs for 

 the second generation. The majority of these eggs hatch late in the summer, 

 and after the animals feed and fly around for a month or more they burrow 

 into the ground, and there hibernate until the following spring. The second 

 generation does not develop the germ-cells nor show any reproductive activity 

 until after it has passed through a period of hibernation or aestivation. Beetles 

 are found breeding even late in the autumn, but these are the belated indi- 

 viduals of either the first or second generation. As far as I can discover, the 

 life cycle in this species is that given above. In the tropics, where one might 

 think that the constant high temiperature would keep the beetles breeding all 

 the year through, there are likewise only two generations a year in all the 

 species that I have studied, it being necessary in all of them that the second 

 generation have a period of rest before resuming its reproductive activity. 

 Normally this rest is passed in the ground in estivation, but by stimulation 

 the beetles may be kept active, although they will not breed until a period of 

 at least two or three months has elapsed. The species in the genus are there- 

 fore double-brooded, the second brood undergoing hibernation or aestivation 

 before reproductive activity is resumed. Owing to the long time in each gen- 

 eration during which the beetles may breed, there are found in nature at the 



