12 
BRITISH APHIDES. 
and definite a nature as we find in tlie Hemiptera 
Heteroptera. 
The body is divided into three greater divisions, 
viz. the head, the thorax, and abdomen. 
THE HEAD 
comprises the face, the vertex or crown (which is either 
pointed, convex, or flat), the rostrum, the two com- 
pound eyes, and the ocelli. In Siphonophora and like 
genera, two thick frontal tubercles rise from the 
vertex, near the upper margin of the compound eyes. 
These basal tubercles sometimes present peculiar gib- 
bous or dentate characters. As they are immovably 
articulated to the head, they must not be confounded 
with the antennae joints, which are seated upon 
them. 
THE ANTEraJE 
are of various shapes and lengths, and differ in the 
number of their component joints. These joints furnish 
reliable characters for constructing the genera. 
In Aphis proper there are seven joints. In Pern- / 
phigus and Schizoneura there are six joints, and in I 
Phylloxera the number of joints is reduced simply to 
three. It is important to refer to the adult insect for 
these details, since considerable discrepancy often 
occurs in the larval forms. These discrepancies 
slowly disappear from the insect, as it passes from 
moult to moult. The normal number may not even be 
found in the pupa. This fact has been overlooked by 
some early authors, and has accordingly produced 
confusion in their descriptions. 
The abnormal number of antennae joints often may 
be seen on comparing the imago with the pupal and 
larval conditions of the subterranean Aphides. 
The terminal joints of the antennse afford good dis- 
