APHIS. 
170 
cockle or warp in such a manner as to form one 
or more large concavities beneath, and in which 
the insects generally reside in great multitudes. 
In some years they are so numerous as to cause 
almost a total failure of hop and potatoe planta- 
tions : in other years the pease are equally injured, 
while exotics raised in stoves and green-houses 
are frequently destroyed by their depredations. 
They are also supposed to be the chief, if not the 
sole cause of that viscid exsudation or moisture so 
often observed on the leaves of various trees, and 
popularly known by the title of honey-dew j which 
is said to be nothing more than the excrementiti- 
ous substance evacuated by these insects from the 
hinder part of the body and from the two tubular 
processes at the tip of the abdomen. 
Of the British Aphides one of the largest and 
most remarkable is the Aphis Salicisy which is 
found on the different kinds of Willows, and is 
nearly a quarter of an inch in length, and of a 
yellowish^rey colour, spotted with black. When 
bruised these insects stain the fingers of a red 
colour. Towards the end of September, accord- 
ing to the observations of Mr. Curtis, multitudes 
of the full grown insects of this species, both 
winged and others, desert the willows on which 
they feed, and ramble over every neighbouring 
object in such numbers that we can handle no- 
thing in their vicinity without crushing some of 
them; while those in a younger or less advanced 
state still remain in large masses upon the trees. 
Aphis Millefnlii of Degeer, or the Yarrow Aphis 
