158 DISPERSION OF POLLEN BY ANIMALS. 



moth abandons the despoiled flower and seeks another forthwith. Having found 

 one, it circles nimbly round it, making a sudden spring off and on, and ends by 

 settling on two of the thick reflexed filaments, sprawling its legs out upon them. 

 It then seeks to reach a favourable spot on the surface of the pistil with its 

 ovipositor and there deposits its eggs. The ovipositor is composed of four horny 

 bristles, and is adapted to pierce through the tissue of the pistil. After the eggs 

 are laid and the ovipositor is withdrawn, the moth darts to the top of the infundi- 

 buliform stigma (fig. 240 3 ), unrolls its trunk-like palpi, and stuffs the pollen into 

 the stigmatic funnel, moving its head to and fro repeatedly during the operation 

 (fig. 240 2 ). It is alleged that the same moth repeats the processes of alternately 

 laying eggs and stuffing the stigma with pollen several times in the case of the 

 same flower. 



Most of the eggs introduced into the pistil are deposited in the vicinity of the 

 ovules. They are of oblong shape, narrow and transparent and increase rapidly in 

 size, soon revealing in each a coiled-up embryo. On the fourth or fifth day the 

 larva is hatched and at once begins to devour the ovules in the cavity of the ovary. 

 Each grub requires from 18 to 20 ovules to nourish it during the period of its 

 development. When it is grown up, it bites a hole in the still succulent wall 

 of the ovary, crawls out through the aperture, lets itself down to the ground by a 

 thread, burrows into the earth and spins an oval cocoon underground in which it 

 remains till the following summer. Fourteen days before the time of flowering of 

 the Yucca, it begins to show signs of life, and the moment the flowers of that 

 plant open the silvery moths escape from their pupal envelopes. 



An important element in the interpretation of the relations subsisting between 

 the Yucca and the Yucca-moth is the fact that without the assistance of insects 

 the sticky pollen of the plant in question could not get to the stigma. In the case 

 of Yucca aloefolia alone there seems to be sometimes a transfer of pollen to the 

 stigma through the instrumentality of the petals or of the elongating filaments; 

 but in most species of this genus, that is to say, in those wherein the fruit is 

 capsular, this certainly does not take place. With the exception of the moth 

 referred to, insects but seldom fly to them, and those which alight by chance on 

 the flowers do not cause a deposition of pollen on the stigmas. If it were not for 

 the transport of the pollen by Pronuba yuccasella the ovaries and ovules of Yucca 

 would not ripen into fruits and seeds. As a matter of fact, all the fruits of the 

 capsular species are rendered abortive if moths are kept away from the flowers by 

 means of a gauze covering. Also, in gardens where there are no Yucca-moths, the 

 production of fruit is suppressed. Yucca Whipplei, which in California, its native 

 land, is visited by a particular moth and develops an abundance of dehiscent 

 capsular fruits, has repeatedly flowered in the Botanic Gardens of Vienna, but the 

 moth does not exist in the gardens, and, in consequence of its absence, not a single 

 fruit has ever ripened there. On the other hand, it is ascertained beyond a doubt 

 that the grub of the moth in question lives exclusively on the young seeds of these 

 species of Yucca, so that one is forced to the conclusion that the moth stuffs the 



