358 ENTOMOLOGY 



Concerning the time of incubation of the eggs of a water beetle, 

 Acilius sulcatus, Krogh found that "when the reciprocal values of the 

 hatching times at the three temperatures are plotted against the tem- 

 peratures, they are found to lie in a straight line." (See also Krogh's 

 results on Pupal Development, p. 359.) 



From all that has been said, then, it appears that there is a threshold 

 of development, which varies for different species and under different 

 conditions of existence, and that there is such a thing as an accumulation 

 of effective temperatures, or thermal constant. This constant is lim- 

 ited, however, to a certain range of temperature, below which growth or 

 development is faster than the constant requires, and above which it is 

 slower. This fact has an economic consequence; for basing predictions 

 upon the thermal constant alone, the spring brood of the codling moth 

 or other insect would appear earlier than would be expected, and the 

 autumn brood later. 



Reproduction.- Among plant lice parthenogenesis (reproduction 

 without fertilization) is apparently continuous and uninterrupted under 

 favorable environmental conditions; amphigony (reproduction by fertili- 

 zation) occurring only under the influence of low temperatures and, 

 as certain authors claim, inadequate food supply. Aphids in tropical 

 and other warm climates appear to have the tendency to reproduce 

 exclusively by parthenogenesis. The same condition apparently obtains 

 among greenhouse aphids in temperate climates. Aphids in colder 

 climates undergo heterogony (reproduction both by parthenogenesis 

 and amphigony) as an adaptation to adverse environmental conditions. 

 In certain species, the appearance of the amphigonous generation seems 

 to be a rhythmic process, which continues to occur at definite cyclical 

 intervals for some time after the influence of low temperature has been 

 eliminated. . (L. B. Uichanco.) 



Incubation. The length of the egg period varies greatly according 

 to surrounding conditions, chiefly those of temperature. First brood 

 eggs of the codling moth in Michigan hatched at outdoor temperatures 

 in 4 to 10 days, average 8 days, at an average mean temperature of 

 about 67.6 F.; eggs not hatching readily, however, during extremely 

 dry weather. (A. G. Hammar.) 



In regard to the eggs of this species, C. B. Simpson says: " (i) Under 

 a low temperature the length of the egg stage is longer than at high 

 temperatures. (2) Under normal field conditions a small difference 

 in temperature causes but little change in the length of the stage. (3) 



