PARTS OF MOUTH. 
17 
The point is usually black, and is armed with fine hairs, 
perhaps employed for tentative purposes. 
The three set^ are the representatives of the man- 
dibles and maxillce of the insect. Although the labial 
and maxillary palpi are wanting in Aphis, indications of 
them may be traced in certain processes existing in the 
immature embryo, which subsequently are suppressed. 
The labium is well developed in such genera as 
Lachnus and Stomaphis. Usually it is falciform. It 
acts as a partial cover to the groove of the rostrum, 
and protects the setse when the proboscis lies folded 
on the breast between the coxae. 
Aphides thus are wholly suctorial in their habits, and 
depend upon the sap of different plants and trees for 
nourishment. As the sources of their food vary, so the 
rostrum undergoes modification, to meet special require- 
ments. Whilst some genera are furnished with exceed- 
ingly short rostra, others show this organ produced to 
an extraordinary length. The most marked example 
of this peculiarity may be noticed in Stomaphis quereus, 
which seeks its sustenance in the alburnum of the 
dense trunk of the oak tree. Here the rostrum is nearly 
twice the length of the insect, and the setse are very 
much longer. By these piercers the insect burrows 
under the hard masses of the cortex, and produces, by 
their irritating and inflammatory action, a plentiful 
flow of sap.* 
The juices are drawn into the mouth by a sort of 
alternating or pumping motion, analogous to that seen 
in the proboscis of the honey-bee. 
The rostrum is often disproportionately long in the 
young of some Aphides. Thus, in Lachnus and 
Schizoneura it projects beyond the end of the abdomen, 
and it is carried as if it were the tail of the insect. 
An extreme development of the setse may be noticed 
in the young of Chermes laricis. These long and deli- 
cate piercers are coiled into a spiral, which would seem 
to act as a kind of spring cable, by which the insect 
* For a figure of tliis remarkable rostrum vide Plate B, fig. 1. 
2 
