44 BULLETIN 885, U. S. DEPARTMENT OE AGRICULTURE. 
lots of trees under observation. Also during the exceedingly dry 
months of February (except on Feb. 15 and 16 following a shower 
on the 11th), March, and the first 14 days of April, practically no 
adults were found on the trees. But following the first heavy rain 
of 1.21 inches on April 13, 1919, the adults began to appear on the 
terminal and subsequent young growth in unusual abundance and 
continued in such numbers until the end of August. From the fore- 
going data it seems that what takes place in the laboratory in the life 
history also takes place out of doors in the dry season, and that at least 
one generation is lost during that season, making the maximum num- 
ber of generations that occur in the Canal Zone, at least on the Pacific 
side, five instead of six. 
PARTHENOGENESIS. 
Parthenogenesis occurs in Aleurocanihus woglumi just as in other 
Aleurodidse whose life history has been studied. Adults from two 
spirals obtained in the laboratory from unfertilized females have all 
been males, and in the field, especially at the end of the dry season, 
a considerable number of colonies were found, all of whose individuals 
were males. One of these colonies contained 72 male pupae and 
another contained 44. Plate VIII, C, shows a colony of 32 male 
pupse on a lime leaf. The reason that parthenogenesis occurs more 
frequently during the dry season is that many adults rarely emerge 
at any one time, so that those females that emerge during the months 
of January to April do not have as many chances for fertilization as 
those that emerge during the rainy season. As has been shown, the 
chief guiding factor of the males in finding the females and mating is 
the tendency or habit of large numbers of the sexes to congregate on 
the young or terminal growth of the trees. 
NATURAL FACTORS THAT TEND TO CONTROL THE BLACK FLY IN THE 
CANAL ZONE. 
There are two natural factors that tend to control the black fly in 
the Canal Zone and adjoining parts of the Kepublic of Panama. One 
is the drying out or lack of rains coupled with increased evaporation 
due to an increased average monthly wind movement during the dry 
season. This has already been referred to under " Seasonal history ," 
page 42. 
The other factor is the heavy rains that occur during the rainy 
season. Just after the molting periods and immediately after the 
emergence of the adults from the pupa cases, the black fly is in a very 
weak condition and heavy rains coming as they do at a decided angle 
doubtless wash off large numbers of them. That this actually does 
take place has been repeatedly observed in all the stages in the life- 
history work conducted by the authors in the field. 
