TRIBES OF APHIDIDAE 49 
forms which are not necessarily closely related. The cornicles are the 
most characteristic structures of the family. Nothing homologous 
to them has been found in related families of the Homoptera, though 
such organs probably occur. Several distinct types of cornicles may 
be distinguished. First, the tuberculate type in which the short 
cornicles are situated on cone-shaped and hairy tubercles of the body, 
as present in the Lachnina. Second, the truncate type in which the 
cornicles are usually not longer than broad and not found on hairy 
tubercles; this type is characteristic of the Chaitophorina and the 
Callipterina. Third, the cylindrical type as present in the Aphidina. 
The Pemphiginae have the cornicles reduced to mere pores or are 
wanting. Attention has often been called to the correlation of great- 
ly developed cornicles with the corresponding great development of 
the cauda. In the Lachnina the cauda is inconspicuous; the 
Chaitophorina and Callipterina have the cauda triangular and pointed at 
apex or sometimes enlarged (globate); the triangular and pointed 
cauda is also characteristic of the Aphidina, where the cauda reaches 
its greatest development in the Macrosiphini correlated with the great 
development of the cornicles of this group. The Pemphiginae have 
inconspicuous cauda correlated with the reduction or loss of the 
cornicles. 
Biological characters. Aphids are among the most defenseless of 
insects, their soft bodies make them an easy and coveted prey to their 
many enemies. Their strength lies in their great power of reproduction, 
by which they more than hold their own against all their enemies. 
The parthenogenetic mode of reproduction has become characteristic 
of the family, probably more so than in any other group of insects. 
Generation will follow generation in rapid succession through the sea- 
son. Aphids are among the first insects to become active in spring 
when they have the field all to themselves before their enemies make 
their appearance. During the height of the season, when their enemies 
have all but cleaned them out of their old haunts, the winged genera- 
tions have appeared and they are carried by their wings and the wind 
far and wide to establish new colonies in innumerable places. When 
the cold of the fall has laid low their enemies, the aphids continue to 
spread and multiply, and among the last insects of the season are the 
sexes which then develop and deposit their winter eggs. 
Protective resemblance is present to a marked degree in some 
species, by which they escape their enemies. In the Pemphiginae 
special protective structures are of common occurrence. The wax-like 
secretions, so common in this group, envelop the individual and the 
