M. E. MacGregor 349 
had given place to acidity, and the readings of the tree-hole water ranged 
well below P H 4*4. 
This significant result led me to collect a large batch of Finlaya genicuiata 
larvae for the following experiment. The tree-hole water in which the larvae 
were brought to the laboratory gave a reading of P tI 4-4. 
The water was poured into jars A and B, an equal number of larvae being 
placed in each jar. 
On the first day of the experiment the P H of “A” was 4-4, and the 
P H of “ B ” was driven well below 4*4 by the addition of acetic acid. 
Ten days later the P H of jar “A” had risen to 4*6—probably by the 
absorption of ammonia from the laboratory atmosphere. The P H of jar U B” 
was still well below 4*4. By this time the larvae had reached the fourth 
instar, pupated successfully, and later emerged as very robust mosquitoes. 
Moreover none had shown the suspended development condition , and not one had 
died out of either hatch . This was unusual in my experience, as I had hitherto 
regarded the dilution of the tree-hole water with tap water as of no conse¬ 
quence provided that dead insects and old leaves were added in abundance. 
However it has since been found that while fresh dry beech leaves if placed 
in distilled water will very soon render the water distinctly acid, old beech 
leaves that have soaked in water for some time lose their acid content. 
Old leaves were used in the previous experiments, and it has become 
evident that they were incapable of neutralising the alkalinity of the added 
tap water, and by the addition of only a small quantity to tap water the 
normal acid environment of the larvae was changed to an alkaline environ¬ 
ment much to the detriment of the larval development. 
These investigations in regard to the development of Finlaya genicuiata 
have led me to conduct similar experiments with Anopheles plumbeus another 
tree-hole breeding mosquito. Similar results were obtained with the latter 
species, i.e. the larvae living in acid water flourished, whereas their develop¬ 
ment in alkaline water was inhibited. 
The influence of the P H on pond and ditch breeding mosquitoes. 
I have mentioned that the average P H of the local ponds and ditches is 
8*4—that is, unlike the tree-hole water, the reaction is alkaline. 
Experiments were undertaken to determine what effect changes in the 
hydrogen-ion concentration would have in the case of Anopheles maculipennis, 
A. hifurcatus and Ochlerotatus nemorosus . It was again found that changing 
the reaction of the water had a profound effect: acidity affecting develop¬ 
ment adversely. The following is a typical result of the series of experiments: 
About two pints of pond water in which A. maculipennis was flourishing 
under natural conditions was divided equally and placed in three clean glass 
dishes, marked “A,” “B,” and “C” respectively. After a small quantity of 
pond weed had been placed in each dish, and the whole allowed to stand for 
