THE BLACK FLY OF CITRUS. 37 
In the field the males are readily distinguished from the females by 
their noticeably smaller size and on closer observation by the differ- 
ence in the structure of the end of the abdomen (see " Technical 
description/' p. 41). 
Humidity seems to be a very important factor in the time that 
emergence of adults from the pupa cases takes place, the maximum 
emergence in the field occurring in the early morning between 6 and 
9 o'clock and dwindling down toward noon. Considerable emer- 
gence may take place in the late afternoon, following a rain, or at 
any time on cloudy, humid days. 
After the bodies of the adults become hardened, the adults may rest 
by the pupa cases for anywhere from 3 hours to 5J days in labora- 
tory experiments. In the field the longest time observed was 1 day. 
Females often lay eggs within an inch or less of the pupa cases from 
which they have emerged, especially on lightly infested leaves. 
When they have left their original postion, after the hardening of the 
body, it seems to be an instinct of both sexes to congregate on the 
young growth of an infested tree probably both to obtain food and 
to insure fertilization of the females. Since it has been observed 
that females will lay eggs within a short distance from the pupa cases 
from which they have emerged it is probable that in the miscellaneous 
gatherings of adults on the young shoots three kinds of females are 
present: Those that have laid no eggs; those that have laid part of 
their eggs ; and those that have laid all of their eggs. 
Numerous counts of the number of males and females on the young 
growth of heavily and moderately infested trees were made in order 
to determine the ratio of the sexes present. On the heavily infested 
trees the number of males and females was usually equal. At times, 
however, the females would outnumber the males as much as 8 to 3. 
On the moderately infested trees there was a wide difference in the 
ratio of the sexes present, the proportions ranging from 5 males to 1 
female to 7 females to 1 male. For the most part the females were 
more abundant. In counts made of the ratio of the sexes present 
in the pupal stage out of 25 colonies selected at random from mod- 
erately infested trees, 230 were females and 159 were males. On 
miscellaneous orange leaves, heavily infested, 450 were females and 
471 were males. Out of 20 life-history spirals, 91 pupae were males 
and 152 were females. This would tend to show that the females 
were more numerous than the males, and that the wide range in the 
numbers of each sex present at different times is probably due to the 
variation in the duration of the pupal stage even in the same colony. 
Mating has been observed both in the field and in the laboratory. 
In the field it may take place at any time of the day, but seems to be 
more frequent in the late afternoon than at other times, due to the 
constantly increasing number of adults that gather on the young 
