350 
Development of Mosquito 
some time, P H readings were taken. A P H of 8-4 was registered by the 
water in each dish. By the addition of acetic acid to dish “ A 5 ’ the P n was 
altered to 4*4, when tested with methyl red. By the addition of A 7 /10 NaOIl 
to the water in dish “C” the P H was altered to 9*6, when tested by thymol 
blue. 
Sixty larvae of A. maculipennis in the pond water in which they had been 
collected were then stranded on filter paper by pouring the water through 
the paper. This was done in order that the larvae might be added to the 
water in the dishes, carrying with them the least possible water so as not to 
alter the recorded readings. Twenty larvae were then carefully removed from 
the paper with the aid of a section lifter, and placed in each of the three 
dishes. The water was again tested by withdrawing a small quantity from 
each dish to separate tubes, and the readings were found to be the same as 
before. The three dishes were then placed under equal conditions of light and 
temperature. 
On the following day all the larvae in dish “A" (P H 4-4) were dead or 
dying, while the larvae in dishes “B” and “C” were normal and very active. 
A day later all the larvae in dish “ A 5 ’ were dead, and the sporangia of Sapro- 
legnia sp. formed a dense frill around their bodies. The larvae in dishes “B” 
and “C” still appeared normal, and about ten days later began to pupate, 
emerging successfully after an interval of a further five to seven days. 
Changes in the P H affecting parasitic Saprolegnia. 
Great as is the direct effect of variations in the P H of the water in which 
mosquito larvae are living, yet there is another effect that is of the utmost 
importance to the larvae. This is the influence that variations in the P H 
have upon the parasitic fungi of the genus Saprolegnia and upon bacterial 
and protozoal parasites of mosquitoes. I think that it will not be an over¬ 
statement to say that of all “natural enemies” Saprolegnia is the greatest 
enemy that mosquito larvae have. Saprolegnia is widespread, and anyone 
who has dealt with mosquito larvae will probably have found that this 
fungus has taken a heavy toll of the specimens. More than half of the 
larvae that die in the laboratory while under apparently good conditions 
are parasitized by this fungus. Saprolegnia flourishes in an acid medium, 
therefore it is rare normally to find A. maculipennis and A. bifurcatus attacked 
in the alkaline water of ponds. If, however, the water be brought to the 
laboratory, its reaction may become reversed owing to the absorption of 
C0 2 , or if it is intentionally made acid, it will be found usually that most of 
the larvae will die from being attacked by Saprolegnia. 
Resistance to Saprolegnia attack follows the ordinary laws governing 
disease in that if the vitality of the individual be high, infection can be 
successfully combated; if on the other hand the environment in which the 
individual lives is unsatisfactory to its general health, infection is readily 
