400 A. J. NICHOLSON 
Warm weather acts merely as a stimulus to the activity of 
the insects and causes them to go out and seek food. The 
stimulus which gives rise to egg development appears to be 
a good meal of blood. Numerous experiments have been 
carried out to determine whether blood is necessary for the 
production of eggs in mosquitoes. To the best of my knowledge 
in only one case have mosquitoes been induced to lay when 
fed on any substance other than blood. $. K. Sen (26) sue- 
ceeded in inducing Stegomyia scutellaris to oviposit 
by feeding with milk or peptone sweetened with cane-sugar, 
and in two instances was successful when the insect had fed 
on nothing but cane-sugar. I carried out a number of feeding 
experiments on A. maculipennis, feeding them on sugar 
and water, with and without the addition of peptone, and on 
dates, bananas, and other fruit, all of which the mosquitoes 
consumed very greedily, but im no case did any development of 
eges take place. In all my sections of abdomens in which the 
eges are developing, the gut is found to contain blood. with 
the exception of the final stage, in which the eggs are fully 
developed, when the gut is alwaysempty. As all these speci- 
mens were collected in cowsheds, this does not prove that blood 
is always necessary for the production of eges, but it appears 
to me certain that this normally is the case. 
In hibernating mosquitoes the abdomen is very narrow and 
flattened dorso-ventrally, but when they take their first meal 
of blood in the sprig the abdomen becomes almost globular 
and distended to its limits with blood. In an insect which has 
recently fed, the abdomen shows a large semi-transparent 
uniform mass of blood, while a small whitish mass is seen 
through the cuticle at the anal extremity. ‘his consists prin- 
cipally of Malpighian tubules but also contains the ovary. 
On the second day the posterior portion of the blood is very 
dark red and opaque, while the remainder is as before. In 
sections the dark-red portion is seen to consist of partially- 
digested blood containing very distorted corpuscles, while the 
remainder appears to be quite fresh and might easily be 
mistaken for a fresh meal of blood. The white mass at the 
