INSECTS IN RELATION TO PLANTS 



205 



FIG. 256. A wasp, Sphex ichneu- 

 monea, with pollinia of milkweed 

 attached to its legs. Slightly en- 

 larged. 



hoods of a flower; first to remove the pair of pollinia and then again 



to introduce one of them into an empty stigmatic chamber. 



Yucca. An extraordinary example of the interdependence of plants 



and insects was made known by Riley, 



whose detailed account is here summa- 

 rized. 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 filamentosa 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 pollenizes the flower 



and lays eggs among the ovules, that 



her larvae may feed upon the young 



seeds. While the male has no unusual 



structural peculiarities, the female is adapted for her special work by 



modifications which are unique among Lepidoptera, namely, a pair of 



prehensile and spinous maxillary 

 " tentacles" (Fig. 257, A) and a 

 long protrusible ovipositor (B) 

 which combines in itself the func- 

 tions 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. 258) she scrapes off 

 pollen with her palpi and shapes 

 it into a pellet by using the front 

 legs. After gathering pollen from 

 several flow r ers she flies to another 

 flower, as a rule, thrusts her long 

 flexible ovipositor into the ovary 

 (Fig. 259) and lays a slender egg 

 alongside seven or eight of the 

 ovules. After laying one or more 



eggs she ascends the pistil and actually thrusts pollen into the stigmatic 



tube and pushes it in firmly. The ovules develop into seeds, some of 



FIG. 257. Pronuba yuccasella. A , maxillary 

 tentacle and palpus; B, ovipositor. After 

 RILEY. Figures 257-259 are republished from 

 the Third Report of the Missouri Botanical 

 Garden, by permission. 



