118 ENTOMOLOGICAL SOCIETY 



of the family. Yet the relatively greater numbers in which the 

 species occurs indicate that it has some advantage over them, and 

 this may possibly rest in the total absence of parasites which so 

 long a subterranean life insures to the larva. The imago is softer 

 and more feeble than is that of most species of the genus, and is 

 more easily captured by a host of enemies, and this fact may also 

 have significance, and would comport with Weismann's theory. 



The probabilities are that the species is a very old one, and 

 that the underground life-periods of the two races have become 

 firmly fixed through heredity. In this connection, and as bear 

 ing on their possible susceptibility to changed climatic condi 

 tions, I would call particular attention to the experiments which 

 I made and recorded in 1885, where the eggs from a septendecim 

 race and a tredecim race were interchanged as to localities (Rep. 

 Entomologist, U. S. Dept. Agr., for 1885, pp. 254-257), as it 

 is to be hoped that those living in the neighborhood of the marked 

 trees under which the eggs were placed will observe and record 

 the results between the years 1898 and 1902. 



THYSANOPTERA. In this sub-order there are few recorded 

 observations as to length of life. Most of the species that have 

 come under my observation produce several generations annually, 

 continuing to breed through the summer, and, under greenhouse 

 conditions, even through the winter. Normally the winter is 

 passed in the adult state, and there is no record of any individuals 

 living beyond a single year, while the average limit of life is 

 probably but two or three months. 



DIPTERA. In this order, with its remarkable variety of forms 

 and great diversity of life-habit, we have a corresponding diver 

 sity in individual longevity. It would require too much space 

 even to indicate the habits of the different families, and it will 

 suffice to state that, with very few exceptions, the insects of this 

 Order are also limited to a single year in individual life, and that 

 the larger number, or those which produce more than one gene 

 ration annually, have a still more limited life duration. There 

 are, nevertheless, some remarkable exceptions among the para 

 sitic forms. In the great majority of species the longest period 

 is passed in the larva state, and it is in this state chiefly that 

 hibernation is had. Yet between the extremes of long larval and 

 brief imaginal life, as illustrated in the CEstridse, and extended 

 imaginal life with the larval career so abbreviated that it is passed 

 before birth, as in the Hippoboscidae, we have examples of every 

 variation, though the law of compensation is as manifest here as 

 in the other Orders, and where the imago persists beyond the 

 average length the larval period will be generally shortened, and 

 versa. No exact records of the duration of life among the 



