THE YUCCA MOTH AND YUCCA POLLINATION. 173 



are at rest, would protect the flower from spoliation by useless 

 raiders. This belief is also strengthened by the fact that ane- 

 mophilous flowers, or those fertilized by the wind, never sleep, 

 and that flowers which attract insects by smell emit their odor at 

 particular hours. 



But the most interesting fact not commonly understood, that 

 has now been very fully established by the most thorough re- 

 searches, is, that a very large number of plants, even where the 

 sexes are united in the same flower, absolutely depend on insect 

 aid for pollination, and that the contrivances to induce cross-fer- 

 tilization are infinite in diversity, while the modifications in struct- 

 ure which these insects have undergone the better to fit them to 

 perform this service, are equally remarkable. Yet in most cases 

 we have adaptation of the plant only, and except in a few in- 

 stances, as, for instance, in that Madagascar orchid, Angrcecum 

 sesquipedale, where the nectary is so deep that its nectar can be 

 reached only by a moth (like Macrosila cluentius) with a very 

 long tongue, our orchids are not dependent for pollination on any 

 one Lepidopterous species, but may be aided by many which have 

 tongues of sufficient length. 



There are, in fact, few plants which are dependent on a single 

 species for pollination. So far as I know, the yuccas furnish the 

 only instance of this kind, for they 

 actually depend on some particular 

 species of little white moths belong- 

 ing to the Tineina and to the genus 

 Pronuba. The yuccas are a very in- 

 teresting genus of lily-like plants, so 

 familiar to every one in our public 

 and private gardens that I need not 

 say very much about them (Fig. 1). 

 There are numerous species and even 

 sub-genera, but they are all character- 

 ized by anthers not reaching any- 

 where near the stigma, so that fertili- 

 zation unaided can take place only by 

 the merest accident. In other words, 

 the stigmatic tube is nowhere within 

 reach of the stamens, and the pollen either remains attached to 

 the open and withered anthers or falls and remains in different- 

 sized lumps on the inside of the perianth, and can not be intro- 

 duced into the stigmatic tube without artificial aid. 



Our commoner garden yuccas, forms of ftlamentosa, depend on 

 the commoner yucca moth, Pronuba yuccasella (Fig. 2, b, c), and 

 so do all the different species found east of the Rocky Mountains, 

 so far as we yet know. During the daytime we may, by knowing 



Fig. 2. Pronuba yuccasella : a, lar- 

 va ; b, ? moth with closed wing ; 

 c, $ moth with wings expanded 

 natural sue ; d, side view of larval 

 joint ; e, head of larva, beneath ; ,/", 

 head of larva, above ; g, thoracic 

 leg of same ; A, maxilla; *', mandi- 

 ble ; y, spinneret and labial palpi ; 

 k, antenna enlarged. 



