BIOLOGICAL BACKGROUNDS FOR POPULATION STUDIES 277 



life cycle, a point stressed by Shelford in It is conventional and meaningful to com- 



1915. For Schistocerca gregaria populations pute the death rate as well as death cases 



in the outdoor cages the approximate total (see p. 290). This is usually expressed as 



mortaUty by stages is: rate of mortality per 1000, or the number 



£gg 23QO , dying in a particular interval of age among 



Nymphal (five instars) . 675° /Z ^^^^ ^^ve at the beginning of that interval. 



Imaginal (total) 195 %„ Bodenheimer treated the Schistocerca data 



Prereproductive 73° /^^ in this way, and the resulting curve is 



pSre'^roductive " 35V°° ^^"'"''^ ^^ ^'S"^® ^^' ^^^^^ '^ ^ considerable 



ep uc ive /oo parallel between Figures 77 and 78 from 



These figures show that in the nymphal egg stage through the prereproductive 



stages, especially the first and second in- period, for about 150 days. In short, the 



en 

 q: 

 o 

 > 



> 



q: 

 z> 

 if) 



100 



200 



300 



DAYS 



Fig. 76. Approximate survivorship curve for the locust, Schistocerca gregaria (A), compared 

 with the idealized physiological curve ( B ) . ( From Bodenheimer. ) 



stars, as seen from Figure 77, the number 

 of deaths is high. Bodenheimer does not 

 clarify this point. Presumably, it is caused 

 by a combination of factors, among which 

 hazards of postembryonic development, 

 vulnerability to predation, and denser and 

 less motile populations can be discerned. 

 The imaginal mortality is lowest for the 

 postreproductive period and is substantially 

 lower than that of the egg and nymphal 

 stages. Bodenheimer does list certain factors 

 that decimate Schistocerca populations in 

 nature. The eggs particularly are subject 

 to insect parasitism and fungus disease, 

 while the nymphs and imagoes are preyed 

 upon by hzards, birds, small mammals, and 



number of deaths and the death rate per 

 1000 follow similar patterns. However, the 

 death rate jumps towards the end of repro- 

 ductive Ufe and becomes extremely high, as 

 would be expected, during old age. 



The table fisting ecological mortaUty for 

 the locust Schistocerca gregaria is concerned 

 with each stage as a discrete unit. It is ob- 

 vious that there may exist differential mor- 

 tafity between early or later phases within 

 any one stage of development. Birch and 

 Andrewartha (1942) have shown that the 

 susceptibility of the eggs of the grasshopper 

 Austroicetes cruciata to dryness varies with 

 the state of development, being greatest for 

 newly laid eggs and least for eggs in the 

 summer diapause. The eggs became pro- 



