DYNAMICS OF FIELD POPULATION O F PINE LOOPER
243
Therefore, it is logical to start this analysis of fecundity with the study
of larval growth, then to assess the effects of larval growth on pupal
size, and finally study the relation between pupal size and adult
fecundity.
1. Larval Growth
The eggs are deposited in May/June and the bulk of the larvae hatch
in the second half of the latter month. Then there is a slow growth up
to mid October. The head capsules were measured (for method see
Klomp, 1968) and Fig. 11 shows the 1957 results. From the first instar
to the nymphal stage there was a steady rise in variability which has
been ascribed to several causes. These causes aim partly genetic, and can
be attributed to sex as well as to genetic difixences within the sexes;
they are partly environmental, arising from factors which induce a
variable number of larval instars, and from differences in microhabitat
and an effect of larval density on size.
Differences in sex were studied only in the nymphal stage and exclusively in the years 1956, 1957, and 1958. The difference between
mean head capsule-widths of females and males were: 2-105-1.957
= 0.148 mm in 1956; 2.242-2.078 = 0.164 nim in 1967; 2.252-2.079
= 0.173 mm in 1968.
The effect on size of other genetic differences is difficult to analyze
because the various genotypes cannot be separated probably because of
the great number of genes involved coupled with environmental effects
superimposed upon gene action producing continuous variation (see
later, p. 248).
The bottom graph of Fig. 11 suggests that t$e larvae have 7 instars,
but this is incorrect. In rearings in 1957, where the development of
individual larvae was checked, about half of the insects had five whilst
the others had six instars. Both these groups are represented in the
histograms 1, 2, and 3, which are clearly separate units, compiled exclusively of larvae in the fist, second, and third instar. Histogram 4 is
trimodal due to the fact that 4a and 4c represent the fourth and fSth
instar of individuals which had six instars, and 4b represents the fourth
instar of individuals with five instars. Consequently, histogram 6,
representing the final larval instar, is composed of two groups: individuals
pupating at the end of the fifth, and those pupating at the end of the
sixth instar. Individuals which have six instag give pupae of larger
size than those with five instars. Thus, this difference contributes to size
variation in the final larval instar.
We have no evidence concerning the environmental factors influencing the number of larval instars. According to the evidence obtained from other species both temperature and nutrition may have rtn
243
Therefore, it is logical to start this analysis of fecundity with the study
of larval growth, then to assess the effects of larval growth on pupal
size, and finally study the relation between pupal size and adult
fecundity.
1. Larval Growth
The eggs are deposited in May/June and the bulk of the larvae hatch
in the second half of the latter month. Then there is a slow growth up
to mid October. The head capsules were measured (for method see
Klomp, 1968) and Fig. 11 shows the 1957 results. From the first instar
to the nymphal stage there was a steady rise in variability which has
been ascribed to several causes. These causes aim partly genetic, and can
be attributed to sex as well as to genetic difixences within the sexes;
they are partly environmental, arising from factors which induce a
variable number of larval instars, and from differences in microhabitat
and an effect of larval density on size.
Differences in sex were studied only in the nymphal stage and exclusively in the years 1956, 1957, and 1958. The difference between
mean head capsule-widths of females and males were: 2-105-1.957
= 0.148 mm in 1956; 2.242-2.078 = 0.164 nim in 1967; 2.252-2.079
= 0.173 mm in 1968.
The effect on size of other genetic differences is difficult to analyze
because the various genotypes cannot be separated probably because of
the great number of genes involved coupled with environmental effects
superimposed upon gene action producing continuous variation (see
later, p. 248).
The bottom graph of Fig. 11 suggests that t$e larvae have 7 instars,
but this is incorrect. In rearings in 1957, where the development of
individual larvae was checked, about half of the insects had five whilst
the others had six instars. Both these groups are represented in the
histograms 1, 2, and 3, which are clearly separate units, compiled exclusively of larvae in the fist, second, and third instar. Histogram 4 is
trimodal due to the fact that 4a and 4c represent the fourth and fSth
instar of individuals which had six instars, and 4b represents the fourth
instar of individuals with five instars. Consequently, histogram 6,
representing the final larval instar, is composed of two groups: individuals
pupating at the end of the fifth, and those pupating at the end of the
sixth instar. Individuals which have six instag give pupae of larger
size than those with five instars. Thus, this difference contributes to size
variation in the final larval instar.
We have no evidence concerning the environmental factors influencing the number of larval instars. According to the evidence obtained from other species both temperature and nutrition may have rtn