264 



ENTOMOLOGY 



FIG. 256. 



Yucca. An extraordinary example of the interdependence 

 of plants and insects was made known by ^iley, whose 

 detailed account is here summarized. The yuccas of the 

 southern United States and Mexico are among the few plants 

 that depend for pollination each upon a single species of insect. 

 The pollen of Yucca Ulamentosa cannot be introduced into the 

 stigmatic tube of the flower without the help of a little white 

 tineid moth, Pronuba yuccasella, the female of which pollen- 

 izes the flower and lays eggs among the ovules, that her larvae 



may feed upon the 

 young seeds. While 

 the male has no un- 

 usual structural pecu- 

 liarities, the female is 

 adapted for her special 

 work by modifications 

 which are unique 

 among Lepidoptera, 

 namely, a pair of pre- 

 hensile and spinous 

 maxillary " tentacles " 

 (Fig. 256, A) and a 

 long protrusible ovi- 

 positor (B) which 

 combines in itself the 

 functions of a lance 

 and a saw. 



The female begins to work soon after dark, and will con- 

 tinue her operations even in the light of a lantern. Clinging 

 to a stamen (Fig. 257) she scrapes off pollen with her palpi 

 and shapes it into a pellet by using the front legs. After 

 gathering pollen from several flowers she flies to another' 

 flower, as a rule, thrusts her long flexible ovipositor into the 

 ovary (Fig. 258) and lays a slender egg alongside seven or 

 eight of the ovules. After laying one or more eggs she ascends 



Pronuba yuccasella. A, maxillary tentacle 

 and palpus; B, ovipositor. After RILEY. Fig- 

 ures 256-258 are republished from the Third 

 Report of the Missouri Botanical Garden, by 

 permission. 



