’ 224 NATURAL HISTORY OF WASPS. 
of the larval legs of the caterpillar. Indeed, the 
name of legs may seem to be scarcely at all ap- 
plicable to them. Thus, within the circle, and below 
the level of the last abdominal ring, a tubercle rises, 
nearly in the centre of the depression. This tu- 
bercle is divided at its base, and so gives origin, on 
either side, to a fold of the imtegument, which 
sweeps round, concentric with the outline of the last 
abdominal ring, so as to almost to meet its fellow on 
the opposite side. These two half rings can be 
protruded or withdrawn at the will of the larva. 
When they are withdrawn a vacuum is formed within 
the abdominal ring which is applied to any surface, 
and the sucker holds. When they are protruded 
the sucker is set free. A firm outer ring, with a 
softer central piston capable of voluntary protrusion 
and retraction, is the instrument generally employed 
for frequent temporary adhesion throughout the 
animal kingdom. We have seen this in the simplest 
form in the fly’s foot. The organ now under exami- 
nation is much more powerful and more complex. 
Though unequal to the support of the large heavy 
larvee, on the rough surface of the cell-wall, for any 
length of time, it is quite sufficient to sustain the 
weight of the smaller grubs. By its means these 
will cling to any object, such as a pencil or a finger, 
and they may be held suspended thus for a half a 
minute or longer. It is quite perfect for the tempo- 
rary purpose of maintaining a hold on the cell-wall 
during the alternate movements of the body, as it 
can be applied or disconnected in a moment; and 
during the elongation or retraction of the larva seems 
almost to act of itself. 
