HIBERNATION STAGE. 23 



mean temperature of the months reaches 13° C.in May, 17° C. in June, 

 19° C. in July, 17° C. in August, and 14° 0. in September. 



But it has now long been proved that plant physiology does not 

 accept the simple heat-amount of Boussiugault, and we have besides 

 to consider the period of sunlight (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, i. 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 diflerent 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, «. 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" (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 i)ass 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 pupte, 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 



*Uuter8ucbungeu iiber die Temperatur der Baume im Vergleiche zur Luft und 

 Boden-Temperatur. Forstwirthschaftliclies Jahrbuch der Akademie Tbaraud, X, 

 1854, 214-270. 



tAnimal life as affected by tbe natural conditions of existence. Tbe International 

 Scientific Series. New York, 1881. 



