156 



INFLUENCE OF TEMPERATURE OX BIOLOGICAL SYSTEMS 



In conclusion on this portion of the paper, we have eliminated numerous 

 possibilities as to why the hatching threshold is several degrees above the 

 temperature at which the developmental rate approaches zero. To a first 

 approximation there is a good correlation between this threshold and the 

 depletion of stored food reserves due to the different effect of temperature 

 on development versus energy expenditure below 20°C. But recent data 

 suggest that this explanation is too simple and will require further elabora- 

 tion. 



Turning to the second question posed at the beginning of this paper, we 

 are faced with the puzzling situation that with the change in developmental 

 rate below 20°C (fig. 6» we can still get hatching, even very good hatching. 



/-■.'S^V 



250 H 



Fig. 8. Low magnification photomicrographs of Oncopeltus larvae incompletely- 

 hatched after incubation at 15°C. Show presence of large mass of albuminous yolk in 

 the midgut (these semi-hatched larvae had died and partially dried prior to place- 

 ment in the fixing fluid; hence tissue details are poor). A, cross section through mid- 

 gut region; B, longitudinal frontal section. Iron hematoxylin stain. 



but the larvae are in some way debilitated so that none, or only a much 

 lower percentage than normal, grow to maturity even when they are im- 

 mediately transferred to optimum conditions. It seems that we have here 

 a case where there is some biological significance to what Crozier would 

 have called a 'critical temperature' in the developmental rate curve. This 

 phenomenon of a 'viability threshold' was discovered in connection wdth an 

 unsuccessful attempt to isolate a strain of Oncopeltus with lower tempera- 

 ture thresholds (8j ; it has been considerably extended by Hodson and 

 Al Rawy (5). Their extensive data are summarized in figure 9. In examin- 

 ing figure 9 one must remember that the plotted percentages show survival 

 under favorable conditions of larvae from eggs incubated at the indicated 

 conditions of temperature and humidity. Another way of saying this is 

 that the temperature and humidity values given beneath the bars represent 



