102 
NATURAL HISTORY OF 
there is only one palpus attached to each blade of 
the maxillae. These organs generally consist of four 
joints, of which the terminal one is remarkable for 
the variety of forms which it assumes in different 
insects, affording valuable characters for distinguish- 
ing genera. When an internal palpus is present, 
it is usually formed of two slender articulations. 
The palpi are susceptible of rapid and extended 
motion, and are sometimes observed in a state of 
intense vibration, similar to the antennae of Ichneu- 
mons, and other Hymenoptera, when exploring a 
decayed trunk to discover a proper nidus for the 
reception of their eggs. They are supposed to con- 
stitute one of the principal organs of touch. That 
they perform this function is rendered extremely 
probable by their structure, which is well adapted, 
by its peculiar pliancy, to the examination of the 
objects with which they come in contact. The 
joints into which they are divided likewise favour 
this explanation of their use, since they seem to 
present some analogy to the articulated extremi- 
ties which form the principal seat of the sense of 
touch in the higher animals. It is at the same time 
probable that these organs are subservient to other 
purposes. 
As the mouth is covered above by the labrum or 
upper lip, so it is closed beneath by the under lip 
or labium . This part is situate between the max- 
illae, and is composed, as it were, of two portions 
joined together by their inner edges. The part 
