22 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION. 



What combination of different climatic influences in reality causes 

 that one and the same insect either in different years in the same locality, 

 or in different localities in the same year, needs a time different in 

 length for the completion of a generation, could not be determined in 

 advance. Ratzeburg was inclined in this matter to follow the similar 

 relations established by Boussingault as regards the duration of vege- 

 tation of plants. According to the views of this French observer each 

 plant needs a definite amount of heat; i. e., the sum of the mean daily 

 temperature of its time of vegetation should be a constant one, while 

 the duration of the time of vegetation may itself vary. It is alsa theo- 

 retically assumed that a plant needs heat amounting to 2000^ C, so that 

 it can develop in one hundred days, with an average mean tempera- 

 ture of 20O C; also as well in one hundred and eleven with 18° C, and in 

 ninety-one days with an average mean temperature of 22^ C. 



Eatzeburg* applies this to the case of the May beetle. He says : 



luterestiug and important is, moreover, the behavior of the May beetle. In mid- 

 dle and northern Germany its generation is a quadrennial one, in southern Germany a 

 triennial one. The reason of this plainly lies in the climatic features of those 

 regions. In the south the season o^jeus much earlier and closes later, which must 

 exert some influence on animals of a pliable nature, such as the May beetle, as well 

 as on plants. The grub there has, in three years, a start of at least three mouths, 

 in comparison with those in the north ; also, even in the third summer, its develop- 

 ment may be ready, though we should consider that with us in the fourth summer, it is 

 usually in July ; it eats no more, and in August pupates. Erichson found that the 

 pupation sometimes occurs even in May ; it fails only a little of a three-years' genera- 

 tion. Finally, everything depends, as in plants, on the amount of heat in the soil 

 and air which a genus or species needs for its development. If the May beetle does 

 not find this in the third summer, it requires it in the fourth, and can shorten the 

 time in an especially favorable year, but with us can never complete it in three years. 



Should we, for example, add together the mean temperature of Berlin for twelve 

 months it would amount to 106° C, and for four years AxlOGo—i-ii^; on the other 

 band Carlsruhe would iu three years give 375°, and beyond the Alps there is fully 

 424°. Should we also take into account the temperature of the soil, the amount in 

 the south would be still better for the May beetle. In north Germany in humous 

 sandy soil (iu the Waldschutten), the thermometer in the hybernation stage of the 

 May beetle in one mouth, from the end of March to the end of April and beginning 

 of May, rises from -4-6^ to +9° C. How is it now in the south? All other insects 

 which iuhabit both the north and south must have a "heat surplus;" but since this 

 lasts only one, but at the most two years, it follows that such results as in the case 

 of the May beetle, which requires so loug a time to develop, can not occur there. 



Accurate researches on this problem are still very rare. Herr Uhlig 

 in Tharand found by observations on the temperature made three times 

 daily during a generation of Tomicus typographus, from May 30 to July 

 21, a heat-amount of 145° C, or divided, a daily amount of 22.02°; dur- 

 ing the second generation, from August 4 to October 3, an amount of 

 1228.50, or divided, a daily amount of 20.48° (Thar. Tagebuch, 25 Bd., s. 

 256). 



Ratzeburg's statement should also be noticed. A double brood of 

 Tomicus typographtis appears if, as is usual in central Germany, the 

 "Die Waldverderber und ihre Feinde; ti°, p. 360. 



