INSECTS IN RELATION TO PLANTS 265 
the pistil and actually thrusts pollen into the stigmatic tube and 
pushes it in firmly. The ovules develop 
into seeds, some of which are consumed 
by the larve, though plenty are left to 
perpetuate the plant itself. Three species 
of Pronuba are known, each restricted 
to particular species of Yucca. Riley 
says that Yucca never produces seed 
where Pronuba does not occur or where 
she is excluded artificially, and that 
artificial pollination is rarely so success- 
ful as the normal method. 
Why does the insect do this? The lit- 
tle nectar secreted at the base of the pistil 
appears to be of no consequence, at pres- 
ent, and the stigmatic fluid is not necta- 
rian; indeed, the tongue of Pronuba, used 
in clinging to the stamen, seems to have 
lost partially or entirely its sucking power, 
Pronuba yuccasella, fe- 
male, gathering pollen 
from anthers of Yucca. 
Enlarged. 
and the alimentary canal is regarded as functionless. Ordina- 
Pronuba moth ovipositing in flower of Yucca. 
Slightly reduced. 
rily it is the flower which has become adapted to the insect, 
which is enticed by means of pollen or nectar, but here is a 
