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 tin- same locality, 
or in different localities in the same year, needs a time different in 
length for the completion of a generation, conld not be determined in 
advance. Ratzebnrg was inclined in this matter to follow the similar 
relations established by Bonssinganlt as regards the duration of vege- 
tation of plants. According to the views of this French observer each 
plant needs a definite amonnt 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 als > theo- 
retically assumed that a plant needs heat amounting to 2000° 0,, SO that 
it can develop in one hundred days, with an average mean tempera- 
ture of 20° C; also as well in oue hundred and eleven with 18° C, aud in 
ninety-one days with an average mean temperature of 22° C. 
Katzeburg* applies this to the case of the May beetle. He says : 
Interesting and important is, moreover, the behavior of the May beetle. Iu mid- 
dle and northern Germany its generation is a quadrennial one, iu southern Germany a 
triennial one. The reason of this plainly lies in the climatic features of those 
regions. In the south the season opens much earlier and closes later, which mnst 
exert some influence ou animals of a pliable nature, such as the May beetle, ;i- 
as on plants. The grub there has, in three years, a start of at least three months, 
iu comparison with those in the north : also, even iu 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 uo more, and in August pupates. Eriehsou 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 iu 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, aud 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 
mouths it would amount to 106° C, aud for four years 4 x 10ii 3 — 4*24-; on the other 
hand Carlsruhe would in 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 (in the Waldsohutten), the thermometer in the hybernation stage of the 
May beetle in one month, from the end of March to the end of April and beginning 
of May, rises from -p-b to -J-9° C. How is it now iu the south? All other insects 
which inhabit 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 ou this problem are still very rare. Herr Uhlig 
iu Tharaud 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° O., or divided, a daily amount of 22.02°; dur- 
ing the second generation, from August 4 to October 3, an amount of 
1228.5°, or divided, a daily amount of 20.48° (Thar. Tagebuch, 25 Bd., s. 
25G). 
Katzeburg's statement should also be noticed. A double brood of 
Tomicus typographic appears if, as is usual in central Germany, the 
^Die Waldverderber uud lhre Feinde; tt°, p. oGO. 
