BIOLOGICAL BACKGROUNDS FOR POPULATION STUDIES 



279 



as "crawlers" that wander over the leaves 

 for a short period and then "settle" and be- 

 gin feeding. The female scales moult twice, 

 yielding two instars; the males moult lour 

 times. It is possible to distinguish prerepro- 

 ductive and reproductive phases of imaginal 

 Hfe. Carpenter (loc. cit.) constructed a 

 death curve for Pseudaonida which is re- 

 produced with shght emendation as Figure 

 79. This graph brings out certain relation- 

 ships between mortality and developmental 

 stage. The situation is well summarized by 

 Carpenter as follows: 



based on rat populations in the postnursing 

 stage. It is known that there is ecological 

 mortality both in utero and during the 

 period of suckhng. Bodenheimer (1938) 

 estimated prenatal mortaUty as 5 per cent 

 of conceptions— a low figure. King (1929), 

 reporting on captive Norway rat colonies 

 (Rattus norvegicus), recorded a mean of 

 0.549 per cent stillbirths, but this figure 

 does not include early miscarriages. 



Wiesner and Sheard (loc. cit.) observed 

 mortality during "the span of life spent by 

 the rat in greater or lesser dependence on 



10 20 30 40 50 60 70 80 90 100 110 



DAYS 



Fig. 79. Number of deaths (approximate) in a population of the camphor scale, Pseudaonida 



duplex, over the entire life cycle. 



"These data show a very low mortality in 

 the egg, which is protected by the scale of the 

 female, followed by an overwhelming mortality 

 during the period when the newly hatched 

 nymphs or crawlers wander over the host plant 

 searching for new feeding sites. Under ad- 

 verse temperatures and wind conditions, prob- 

 ably most of the young perish at this stage. 

 The first and second stadia scales show a com- 

 paratively small mortality, the pre-reproductive 

 adults a somewhat higher rate, and the re- 

 productive adults continue this trend to the 

 median day of egg laying. Since adults of most 

 oviparous scale-insects die very shortly after 

 having deposited their eggs, the oviposition 

 records presented by Bliss were taken as in- 

 dicative of the death trend of the adults after 

 the sixtieth day. The great mortality during the 

 crawler stage and the first stadium leave too 

 few individuals to have the adult death peak 

 very large, but it appears to be associated with 

 the beginning of the egg-laying period." 



If now we return to the albino rat popu- 

 lation (see page 276 and Figure 75) and 

 examine it from the viewpoint of ecological 

 rather than physiological longevity, we can 

 add to its analysis certain points of interest. 

 It will be recalled that these data were 



its mother," a span ending about thirty days 

 after birth. Of a total of 250 htters ob- 

 served, comprising 1607 births, 492 (241 

 males, 251 females), or 30.6 per cent, died, 

 and 1115 (571 males, 544 females), or 

 69.4 per cent, survived. Thus there is a 

 high mortahty (30.6 per cent) for this early 

 period. The authors suggest that the pos- 

 sible sources of this ecological mortality are 

 low viability; accidents, such as "... 

 squashing of the young by the mother be- 

 cause the latter fails to assume the appro- 

 priate 'nursing posture;' " cannibahsm on 

 the part of the mother, and size of litter. 

 The entire cycle of mortality over the life 

 span of these rat populations clearly depicts 

 the low mortality in utero, and the exces- 

 sive mortahty of the nursing period. The 

 low mortality of the reproductive period 

 accounts for the approach to the physio- 

 logical curve and the increased mortality of 

 the post-reproductive ages. 



Some valuable observations on the ecolog- 

 ical mortality of the prereproductive span 

 were obtained by Ranson (1941) on ex- 

 perimental populations of the vole. MicrO' 



