April, '10] SANDERSON: TEMPERATURE AND INSECT GROWTH 135 



temperature was less the longer they were held at 2°C., while the 

 pupie of E. alniaria showed no development at 0°C. and their subse- 

 quent development was retarded by it. Evidently the former species 

 was slightly above, and the latter slightly below its respective mini- 

 mum. It is known that short exposures to temperatures below the 

 minimum retard the development of some species and hasten that of 

 others, depending upon the species, the temperature and the time 

 exposed. 



The influence of low temperatures has also been nicely shown in the 

 studies of Hunter and Glenn (11) on the Green Bug (Toxoptera 

 graminum) and its parasite Lysiphlebus tritici. The rates of growth 

 of these insects are shown in figures 24 and 25, which are plotted from 

 the records given, showing that development may take place at a 

 mean 1.65°C., while the point of cold- rigor is slightly higher for 

 Lysiphlebus. which shows no activity below about 4° or 5°C., while 

 the ultra-minimum or "critical point" of Baclunetjew, at which death 

 occurs, is about - — 8.33°C. or 17°F. Similar phenomena are shown 

 by the rate of reproduction of Toxoptera as plotted in figure 25, in 

 which it is seen that reproduction may actually occur at a daily mean 

 of — 7.8°C. and that it is frequent at a daily mean of 0°C. This 

 curve is of additional interest, however, because it gives the optimum 

 of the species, about 20°C., from which the rate of reproduction grad- 

 ually decreases, though it is known to occur at about 29 °C. until the 

 aphides cease feeding at 32°C. and death ensues at 37.5° to 40°C. 



It is interesting to contrast these points of cold-rigor hovering 

 around 0°C. with those of the Boll worm which is about 10°, as well 

 as several others previously cited, and that of the Cattle Tick which 

 is about 5°C. It is evident, then, that the point of cold-rigor and the 

 minimum must be determined for each species, and for each phase 

 of its growth. Dr. L. 0. Howard (10) has given these points for 

 several houshold pests and has shown how such a knowledge may have 

 most practical value in the prevention of insect injury in cold storage. 

 Similar data has been given by Duvel (6) for weevils affecting cow- 

 peas. Recently a tobacco manufacturer has applied to us for aid in 

 the destruction of a pest of stored tobacco, which we believe may 

 possibly be accomplished by the use of low temperature. 



From the above data it is evident that any accumulation of tempera- 

 ture to secure a thermal or physiological constant cannot be based on 

 a mere addition where variable temperatures are involved, for it is 

 evident that every degree has a different value in relation to the time 

 factor. Thus as the mean temperature rises with the advance of the 

 season both the time for the pupal stage and the total accumulated 



