HIBERNATION STAGE. 23 
mean temperature of the months reaches 13° G. iu May, 17° C. in June, 
19° C. in July, 17° 0. in August, and 14° C. in September. 
But it has now long been proved that plant physiology does not 
accept the simple heat-amount of Boussingauit^ and we have besides 
to consider the period of suulight (duration of light) during which alone 
the chlorophyll-containing parts are assimilated, as well as the mean 
temperature reached in the sun — at best measured by an actinometer. 
However, in animals the transformation of tissue depends much less 
on the amount of light than in plants, hence simply the total heat- 
amount can scarcely be sufficient to explain the differences in the ani- 
mal developmental processes, especially if we only take into account 
the temperature of the air. It would be much better to take into con- 
sideration the temperature of the soil throughout their larval life of 
insects living in the earth, and in insects living in wood the temperature 
of the tree, L e., the portion of the tree concerned. Compare the exact 
researches of Krutzsch.* Such researches should determine what is the 
minimum temperature at which generally an advance in development 
would be possible. Also the optimum temperature, i. e., the tempera- 
ture which is most favorable to any process should be noted. 
For example, these optima would require to be different for the dif- 
ferent developmental stages in the insects, as would the temperature- 
minima supportable to the same. We also know, through the re- 
searches of Semper, t that as in the germination, growth, and flowering of 
plants, so also in animals; i. e., in our common fresh water snails, the 
temperature- optima for the different function, i. e., for the ripening 
of the sexual products and for growth, are different, a thesis which by 
Semper has been applied to a striking attempt at an explanation of the 
occurrence of wingless, larval-like, but still sexually developed Ortho- 
ptera in southern lands, i. e., the so-called "stick insect n (Judeich and 
Nitsche). 
Hibernation stage. — The developmental cycle of two species of insects 
with similar generations may, under similar climatic relations, produce 
a very different shape, namely, in the cases where they pass the winter 
in different stages of development, since the hibernation-stage is always 
the longest, and hibernation is possible in the egg, as in the larva, pupa, 
or imago, stage. But under normal relations a given species of insect 
always hibernates in the same stage, i. e., many moths as pupa?, some 
butterflies as imagines. 
It is not possible, then, to predicate in general for a single order of in- 
sects as to what stage they may hibernate in, since species of the same 
family differ in this respect. Thus, for example, according to an estimate 
*Untersuchungen iiber die Temperatur der Baume im Vergleiche zur Luft und 
Boden-Teinperatur. Forstwirthscbaftlickes Jahrbuch der Akadeinie Tbarand, x, 
1854, 214-270. 
tAnimal life as affected by tbe natural conditions of existence. Tbe Internationa] 
Scientific Series. New York, 1881. 
