A. GLENN RICHARDS 161 



Obviously there is something fundamentally different about develop- 

 ment of Oncopeltus embryos and larvae at temperatures above and below 

 19°-20°C. But we have no idea what this difference is. Since for viability 

 only a small portion of either actual or developmental time is needed at 

 temperatures above 20° — provided such temperatures are supplied fre- 

 quently and repeatedly throughout development — we are postulating as 

 a working hypothesis that some vital factor (a 'vitality factor' if you wish 

 to call it such) is not synthesized in adequate amounts at temperatures 

 below 20°C. This would make the debility due to a biochemical deficiency 

 in the metabolic machinery. However, it has been pointed out by others 

 that the available data do not eliminate alternative possibilities such as 

 the production or inadequate destruction of an inhibitor, or even physico- 

 chemical changes such as sol-gel transformations (compare article by 

 Marsland in this volume). 



A final warning should be made in connection with any serious discussion 

 of the possibilities listed in the above paragraph. The present paper has 

 been concerned exclusively with temperature effects throughout, and hence 

 it may have given the impression that these effects are known to be due 

 specifically to temperature. However, data in figure 9 show that mortality 

 can be produced by other suboptimum conditions, such as humidity. It 

 would be completely gratuitous to assume that the physiological effects 

 of low temperature and low humidity are the same when the data are 

 only mortality percentages. But it would also be unwise to assume that 

 the debilitation is specifically and solely an effect of suboptimal tempera- 

 tures — the debility could conceivably be a result of some general stress 

 that in turn might be produced in various ways. 



SUMMARY 



Extending our previous work, we find that to a first approximation the 

 occurrence of a finite temperature threshold for hatching of Oncopeltus 

 eggs is correlated with depletion of food reserves brought about by the 

 fact that temperatures below 20°C retard development much more than 

 they retard expenditure of energy (fig. 6) . Experiments with varying tem- 

 peratures suggest that the above explanation is too simple and will re- 

 quire further elaboration. Variation in hatching percentage determinations 

 at minimal temperatures is correlated with and presumably due to vari- 

 ation in individual egg sizes and hence to variation in amounts of food 

 reserves in different eggs. In addition to the 'hatching threshold' there is 

 what may be called a 'vitality threshold' which is independent of hatching 

 and which requires temperatures of 20°C or higher. The requirements of 

 this threshold can be satisfied with a small percentage of developmental 

 time at temperatures above the threshold, provided the warmth is supplied 

 as frequent short treatments. Possible bases for this effect are discussed. 



