41 6
N. BALAKRISHNAN NAIR AND M. SARASWATHY
1965). In higher latitudes with rising temperature the breeding cycle
is initiated and continues during the summer. Some attack almost
continuously throughout the season, others vary in abundance in a way
which suggests that successive generations are coming to maturity.
The number of generations which would be produced in a single season
varies with the time required for the species to grow to maturity and
with the length of the period during which suitable conditions persist.
In temperate regions, a single generation may be produced in the course
of a short summer. In warmer regions development is more rapid and
many generations may be produced each year. The seasonal breeding
behaviour may be divided into a number of categories such as (1)
settlement continuous throughout the year without definite seasonal
fluctuations, (2) settlement continuous but with increased frequency
during a definite portion of the year, (3) settlement limited to some
definite portion of the year and (4) settlement occurring at two separated
periods of the year.
The first type occurs in areas such as the tropics where seasonal
conditions vary only slightly and the normal breeding rhythms are
apparently uninfluenced by them. This and the second category differ
only in a quantitative way, the species concerned exhibiting definite
variations in the frequency of settlement at different seasons. The
second type may be expected wherever conditions during one portion
of the year are less suitable for reproduction than at others. On the
other hand in temperate regions where the annual change in temperature is relatively great the third type is usual, the duration of
breeding depending on the time during which the temperature remains
above the critical level for reproduction.
Sometimes breeding may be interrupted in summer owing to temperatures rising too high, the species breeding in autumn or winter.
This seems to be the case for Teredo norvegica in Western Norway (Nair,
1959, 1962). Again the season at which a given species breeds may differ
in different parts of its range, e.g. in Teredo furcifera on the east and
west coasts of India. Examination of Fig. 37 reveals this clearly for
Teredo navalis, Bankia gouldi or B. setacea. The seasonal settling behaviour, therefore, depends on the time of the year at which conditions are favourable at a given place. It is not a fixed characteristic
of a species throughout its range. Certain physiological races of Bankia
setacea and Teredo navalis on the other hand are so limited in the range
of temperature at which reproduction takes place that at some places
they can breed only during a brief period during the yearly cycle when
the conditions are favourable. This type of breeding occurs in places
where seasonal changes in temperature are considerable.
N. BALAKRISHNAN NAIR AND M. SARASWATHY
1965). In higher latitudes with rising temperature the breeding cycle
is initiated and continues during the summer. Some attack almost
continuously throughout the season, others vary in abundance in a way
which suggests that successive generations are coming to maturity.
The number of generations which would be produced in a single season
varies with the time required for the species to grow to maturity and
with the length of the period during which suitable conditions persist.
In temperate regions, a single generation may be produced in the course
of a short summer. In warmer regions development is more rapid and
many generations may be produced each year. The seasonal breeding
behaviour may be divided into a number of categories such as (1)
settlement continuous throughout the year without definite seasonal
fluctuations, (2) settlement continuous but with increased frequency
during a definite portion of the year, (3) settlement limited to some
definite portion of the year and (4) settlement occurring at two separated
periods of the year.
The first type occurs in areas such as the tropics where seasonal
conditions vary only slightly and the normal breeding rhythms are
apparently uninfluenced by them. This and the second category differ
only in a quantitative way, the species concerned exhibiting definite
variations in the frequency of settlement at different seasons. The
second type may be expected wherever conditions during one portion
of the year are less suitable for reproduction than at others. On the
other hand in temperate regions where the annual change in temperature is relatively great the third type is usual, the duration of
breeding depending on the time during which the temperature remains
above the critical level for reproduction.
Sometimes breeding may be interrupted in summer owing to temperatures rising too high, the species breeding in autumn or winter.
This seems to be the case for Teredo norvegica in Western Norway (Nair,
1959, 1962). Again the season at which a given species breeds may differ
in different parts of its range, e.g. in Teredo furcifera on the east and
west coasts of India. Examination of Fig. 37 reveals this clearly for
Teredo navalis, Bankia gouldi or B. setacea. The seasonal settling behaviour, therefore, depends on the time of the year at which conditions are favourable at a given place. It is not a fixed characteristic
of a species throughout its range. Certain physiological races of Bankia
setacea and Teredo navalis on the other hand are so limited in the range
of temperature at which reproduction takes place that at some places
they can breed only during a brief period during the yearly cycle when
the conditions are favourable. This type of breeding occurs in places
where seasonal changes in temperature are considerable.