44 Riley Notes on Yucca Insects and Yucca Pollination. 



From his account, as well as that of Mr. Coquillett, it appears 

 evident that Pronuba maculata, in accordance with the greater 

 tendency of the flowers of lohipplei to open during the day, is 

 more diurnal in habit than Pronuba yuccasetta, carrying on the 

 acts of oviposition and pollination during the day. Further, 

 unlike the other Pronubas so far known, this species rests with 

 the head toward the stigma, and when disturbed is very apt to 

 drop suddenly from the flower and take wing. I cannot do 

 better than quote verbatim Mr. Trelease's interesting account of 

 the act of pollination, that of oviposition being, as already stated, 

 absolutely the same as in yuccasclla : 



" Having withdrawn the oviduct, in doing which she moves up so that 

 her head is about level with the stigma, or even before this organ is en- 

 tirely freed, the moth usually proceeds to pollination ; but it is not infre- 

 quent for two eggs to be laid between each two visits to the stigma, and, 

 owing to her peculiar alertness, she appears to be even more easily 

 frightened into omitting pollination than are the other species of Pro- 

 nuba. Standing with her head at about the height of the stigma, with 

 the short tongue projecting out in front, she uncoils her long tentacles 

 from the compact mass of pollinia, which she carries similarly to the 

 other Pronubas, only that small part of her burden which adheres to the 

 bases of the tentacles being removed from it, and, raising her body on 

 tiptoe, she very slowly saws the tentacles back and forth across the top 

 of the stigma, generally following one of the three shallow grooves, and 

 very carefully working their slender tips into the more or less gummy 

 exudation over the central depression. Sometimes the operation is inter- 

 rupted long enough to admit of the tentacles being coiled back against 

 the load of pollen and again extended ; but the curious manner in which 

 her head is held back from the stigma, as a rule, prevents any of the main 

 load from reaching even the marginal papillae. 



" On first witnessing this operation I was impressed by the much slower 

 motion of the moth than usual and the evident care which she took to 

 run the ends of the tentacles into the central depression of the stigma, 

 which I then supposed to be solid. The subsequent discovery of the sty lar 

 canal, communicating with the ovarian cells, showed that it is into this 

 narrow passage that she so carefully guides the tips of her tentacles with 

 their modicum of pollen, and no doubt the abundant stigmatic secretion 

 serves not only to foster the development of the nascent pollen tubes 

 after pollination, but, wetting the tentacles, aids in the disintegration of 

 her mass of pollinia. These, if really related to her work, would seem to 

 have acquired their coherent structure as a means of facilitating their 

 collection rather than as an adaptation to their removal bodily from the 

 anther to the stigma, as is the case in orchids and asclepiads, where, how- 

 ever, special means of secure attachment to the insect accompany this 

 aggregation of the pollen grains into a large mass." 



