FLOWERS AND INSECTS 131 
flowers. In these flowers self-pollination is a necessity, and 
is found to be very effective in producing seed. 
91. Yucca and Pronuba. There can be no doubt, also, 
that there is a great deal of self-pollination effected in 
flowers adapted for pollination by insects, and that the in- 
sects themselves are often responsible for it. But in the 
remarkable case of Yucca and Pronuba there is a definite 
arrangement for self-pollination by means of an insect (see 
Fig. 136). Yucca is a plant of the southwestern arid regions 
of North America, and Pronuba is a moth. The plant and 
the moth are very dependent upon each other. The bell- 
shaped flowers of Yucca hang in great terminal clusters, with 
six hanging stamens, and a central ovary ribbed lengthwise, 
and with a funnel-shaped opening at its apex, which is the 
stigma. The numerous ovules occur in lines beneath the 
furrows. During the day the small female Pronuba rests 
quietly within the flower, but at dusk becomes very active. 
She travels down the stamens, and resting on the open 
pollen-sac scoops out the somewhat sticky pollen with her 
front legs. Holding the little mass of pollen she runs to 
the ovary, stands astride one of the furrows, and pierc- 
ing through the wall with her ovipositor, deposits an egg 
in an ovule. After depositing several eggs she runs to the 
apex of the ovary and begins to crowd the mass of pollen 
she has collected into the funnel-like stigma. These actions 
are repeated several times, until many eggs are deposited 
and repeated pollination has been effected. As a result of 
all this the flower is pollinated, and seeds are formed which 
develop abundant nourishment for the moth larvae, which 
become mature and bore their way out through the wall of 
the capsule (Fig. 136). 
92. Securing cross-pollination. In very many ways flow- 
ers are adapted to the visits of suitable insects. In ob- 
taining nectar or pollen as food, the visiting insect receives 
pollen on some part of its body which will be likely to 
come in contact with the stigma of the next flower visited. 
