PHYSIOLOGY AND DISEASES OF VEGETABLES. 
113 
for this variety of insect, which feed on particular vegetables. 
There are seventy-three species named by Linnaeus, who had 
placed them in his order Hemiptera, each variety choosing a 
particular vegetable to exist upon ; and some varieties live 
only on particular parts of plants ; one variety, aphis brassica , 
will exist on the leaf of turnips, and another variety will find 
food only on the stalk of the turnip blossom. 
The aphis tribe, according to the modern division of insects, 
is included in the class five, Homoptera, (four-winged, with 
sucking mouth.) This class is divided into three orders or 
sections, in accordance with the number of joints of the 
tarsus, or foot. The wings, when folded on its back, form a 
ridge, like the roof of a house. 
This variety of insects offer some very anomalous features 
of structure and habits, and they depart very wddely from the 
general type of the division, but the mouth or rostrum of this 
class are uniform, as they are all adapted to suck juices from 
vegetables; the tongue being channelled like a gutter, and 
having lance-like piercers to form punctures on the surface of 
plants. 
Before any further description of these insects is given, 
it would be well to speak of their appearance on the 
hop and other plants, and what are their general habits, as 
evinced to a casual observer; and in particular, what has 
been the circumstances attendant on their attacks in general. 
The first appearance of the aphis is the winged insect 
resting on the underside of partly developed leaves, general^ 
at the extremity of hop shoots, the young shoots of roses, 
beans, peas, &c. A few flies only at first make their appear- 
ance, perhaps one or two on some young leaves of hops ; 
these leave globular eggs between the folds of the unde- 
veloped leaves, which by the time the leaves are expanded, 
have increased into lice (an unwinged progeny). This first 
flight of winged insects is often followed by an increased 
quantity, and twenty or more winged insects may be seen on 
the fully developed leaves of hops, and these leaves may, in 
the course of ten days, be covered with the vringed and 
unwinged aphides, with also some of them on the stalk at the 
extremities of the lateral shoots. Their first attack is generally 
about the beginning of June, but during the year 1846, and 
on two occasions since, I have observed them so early as the 
middle of May. In the year above named, the season became 
hot and dry, and by the 25th of June they had all disap- 
peared. It will sometimes happen, wdien the attack is early 
and they disappear suddenly, in consequence of a peculiar 
state of the atmosphere, that there will be another and late 
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