DYNAMICS OF FIELD POPULATION O F P I N E LOOPER
253
highest larval density experienced in the field. Food has always been
present in excess.
The causation of the interference among larvae is now under study by Mr. P.
Gruys of the Institute for Biological Field Research, Arnhem, Netherlands. In
one of the Annual Reports of that Institute he has suggested that growth is inhibited by mutual contact between larvae (Gruys, 1963, p. 26).
3. Adult Fecundity
as illustrated by the following ranges
The fecundity of individual moths is rather variable within each year
1954
153-266 eggs
1958
161-299 eggs
1963
117-267 eggs
These numbers have been determined with females reared from the
pupae collected in the field in April (see p. 240). Individual females were
caged in an outdoor insectary each with a male, the pairs being composed independently of pupal size. After mating the females readily
deposit the bulk of their eggs on the needles of a pine twig placed in the
cage, retaining only 0-10 in the oviduct as they do in the field. These
eggs have been included in the data on fecundity. Immature eggs without chorion which were always present in the ovarioles after death were
excluded.
The variation of fecundity proved to be related to pupal diameter.
This was readily shown because pupae were reared per
mm diameter
classes and the females caged separately. Thus for each female the size
of its pupa and its fecundity were known. The relation is shown in
Fig. 14. The coefficient of regression is of the same order of size for all
years studied, and always differs significantly from zero. It indicates
that fecundity increases by 100 eggs with an increase of 1 mm size on
average. This relationship has already been shown to exist in the same
species by several other authors quoted by Klomp (1958). Figure 14
shows that the great variability of fecundity within one pupal diameter
class makes pupal size a poor index of aduli; fecundity. This might be
the result of several causes, but the only ont: we checked was the variability of egg size.
Egg size has been determined by measuring the greatest diameter of
the egg. (For method see Klomp, 1958.) An index for the mean egg size
of an individual female was obtained from the arithmetic mean of the
diameters of all or of the majority of the eggs laid by her.
Undoubtedly egg weight would have been a more logical index and less timeconsuming in its determination than mean egg diemmeter. But a suitable balance
was not available and the less appropriate index had to be used. Later we were
I*
253
highest larval density experienced in the field. Food has always been
present in excess.
The causation of the interference among larvae is now under study by Mr. P.
Gruys of the Institute for Biological Field Research, Arnhem, Netherlands. In
one of the Annual Reports of that Institute he has suggested that growth is inhibited by mutual contact between larvae (Gruys, 1963, p. 26).
3. Adult Fecundity
as illustrated by the following ranges
The fecundity of individual moths is rather variable within each year
1954
153-266 eggs
1958
161-299 eggs
1963
117-267 eggs
These numbers have been determined with females reared from the
pupae collected in the field in April (see p. 240). Individual females were
caged in an outdoor insectary each with a male, the pairs being composed independently of pupal size. After mating the females readily
deposit the bulk of their eggs on the needles of a pine twig placed in the
cage, retaining only 0-10 in the oviduct as they do in the field. These
eggs have been included in the data on fecundity. Immature eggs without chorion which were always present in the ovarioles after death were
excluded.
The variation of fecundity proved to be related to pupal diameter.
This was readily shown because pupae were reared per
mm diameter
classes and the females caged separately. Thus for each female the size
of its pupa and its fecundity were known. The relation is shown in
Fig. 14. The coefficient of regression is of the same order of size for all
years studied, and always differs significantly from zero. It indicates
that fecundity increases by 100 eggs with an increase of 1 mm size on
average. This relationship has already been shown to exist in the same
species by several other authors quoted by Klomp (1958). Figure 14
shows that the great variability of fecundity within one pupal diameter
class makes pupal size a poor index of aduli; fecundity. This might be
the result of several causes, but the only ont: we checked was the variability of egg size.
Egg size has been determined by measuring the greatest diameter of
the egg. (For method see Klomp, 1958.) An index for the mean egg size
of an individual female was obtained from the arithmetic mean of the
diameters of all or of the majority of the eggs laid by her.
Undoubtedly egg weight would have been a more logical index and less timeconsuming in its determination than mean egg diemmeter. But a suitable balance
was not available and the less appropriate index had to be used. Later we were
I*