SOME PLANT MARRIAGES 



441 



FIG. 544. MARSH-CALLA 



(Calla palustris). 



A. marsh snail is seen to be climbing t'.ie stem. 

 Snails have been observed to pollinate the flowers. 



depend for their very existence on certain 

 little white moths, belonging to the im- 

 portant group Tinseina, and to the genus 

 Pronuba. Thus the Yuccas affords an in- 

 stance apparently the only known instance 

 of plants which are dependent on a single 

 species for pollination. 



The common Yuccas of the United States 

 are forms of Yucca falamentosa, and their 

 white-winged pollinator is the commoner 

 Yucca moth, Pronuba yuccasella (fig. 512). 

 During the daytime this moth, either singly 

 or in pairs, may be found resting with 

 folded wings within the half-closed flowers. 

 After sundown it is to be seen flitting 

 swiftly from plant to plant and flower to 

 flower ; the dusky nature of the hind wings 

 and of the under surface of the front wings 



almost completely off-setting and neutralizing, when in motion, the upper 

 silvery whiteness of the latter. This is the male insect. The female is 

 busily at work most of the time in the flowers ; for on her devolves a 

 double duty, which leaves no leisure for these nocturnal flittings. As a 

 part of the maternal task of continuing her race, she must act as foster- 

 mother to the plant in order to ensure a proper supply of food to her 

 larvae, which, as we shall presently see, feed on its seeds. Her activity 

 begins soon after dark, and consists at first in assiduously collecting a load 

 of pollen. She may be seen running to the top of one of the stamens, and 

 bending her head down over the anther, stretching her maxillary tentacles, 

 which are wonderfully modified for the purpose, to the fullest extent ; and 

 using her palpi to scrape the pollen from the anthers towards the tentacles. 



After gathering a sufficient supply, and shaping it into a pellet twice or 

 thrice the size of her head, she sets off 

 for another flower, leaving the despoiled 

 blossom to be pollinated by some other 

 individual of her race. On entering the 

 new flower, she takes up a favourable 

 position, and after resting motionless for 

 a short time, plunges her lance-like ovi- 

 positor into the soft tissue of the pistil, 

 and conducts her first egg to its destina- 

 tion. Mark what follows. No sooner is 

 the ovipositor withdrawn than the moth 

 -runs up to the top of the pistil, thrusts the 

 pollen into the stigmatic funnel, and 



W 



FIG. 545. MARSH-CALLA (Calla 

 palustris). 



A single flower removed from the ?padix and 

 enlarged. 



