80 
BRITISH APHIHES. 
of the extraordinary swarms so often noticed by 
authors. White, of Selborne, remarks on the cloud of 
smother-flies ’’ which for several days fell in August, 
1785. He says they probably were migrating from the 
hop-gardens of Kent and Sussex. Again, an immense 
cloud of winged Aphides, bred in many localities, swept 
across the river at Gand in September, 1834, and 
covered the quays and streets of that city. Morren 
tells us that a learned resident doctor, seeing that the 
visitation of cholera and of these insects was syn- 
chronous, declared that the insects were the immediate 
cause of the disease ! We also hear that the sun was 
darkened by the same swarm at both Bruges and 
Anvers. A very large -area was thus covered, their 
march being only limited, we are told, by mountains 
and elevated plains.^ 
* The following will show that Professor Huxley’s graphic illustra- 
tion, nevertheless, much under- estimates the real quantity of animal 
matter capable of elaboration from one single rose Aphis in a year. 
Basing the calculation, for simplicity, upon the supposition that every 
Aphis lives twenty days, and that at the expiration of that period each 
Aphis shall have produced twenty young and no more, then, at the 
expiration of three hundred days only, the living individuals would be 
represented by the following figures : 
Aphides. Days. Aphides. 
1 produces in 20 = 20 = A. 
A „ 40 = 202 _ 400 = B. 
B „ 100 = 205 = 3,200,000 = 0. 
C „ 200 = 20’0 = 10,240,000,000,000 D. 
D „ 300 = 20^5 32,768,000,000,000,000,000= E. 
Again, if 1000 Aphides weigh 1 grain, 
and 1 man weighs 2,000,000 grains, 
1 man weighs 2,000,000,000 Aphides. •• 
E 
2,000,000,000 = 1.638,400,000 men i 
equal, perhaps, to the population of China sevenfold. 
But a mathematical friend remarks that this calculation even does 
not express the real rate of increase, since it supposes the progeny of 
the first Aphis to be produced at once, and not to commence producing 
until the expiration of the first twenty days. To this same friend I am 
indebted for the annexed calculation. 
If we suppose the progeny of the first Aphis to equal 20 in twenty 
days, and this progeny to begin producing when five days old 20 young, 
