370 



Yucca brevifolia is polliiiized by Pronuba synthetica Riley, the most 

 remarkable species of the genus, having very st(mt maxillary tentacles, 

 a very stout ovipositor, shorter than that of yuccasella, but character- 

 ized chiefly by having fuliginous and unsealed wings, and a polished, 

 naked, and flattened body — structures all well adapted for crawling 

 between and about the compact and crowded flowers, with their thick 

 and leathery petals, but very abnormal in the Lepidoptera. In fact, 

 this species strongly recalls in its general asi)ect some of the saw-flies 

 belonging to the genus Dolerus, the resemblance being heightened by 

 the rather conspicuous, cenchri-like spots, and by the conspicuous divi- 

 sion between the thorax and abdomen. It also strikingly recalls some 

 of the Neuroptera, as Sialis infnmata. 



Now, these resemblances to insects of different Orders, and to families 

 which are generally considered to be of low type within their Order, 

 can not possibly be mimetic, as there can be no conceivable cause, pur- 

 pose, or advantage in the mimicry. It is also impossible to account 

 for these resemblances upon any present genetic connection. Yet Ave 

 are hardly justified in disposing of them as merely accidental and with- 

 out meaning. They suggest a possible synthesism in the past, when 

 types were less specialized and present Orders had not become so well 

 differentiated. 



Yucca irhipplei, which occurs in southern California, has flowers dis- 

 tinguished by their relatively long and stout stamens, the pollen of 

 which is copious and glutinous, not to say mucilaginous, and a short, 

 contracted style, with the stigma, however, expanded and covered with 

 sticky threads. It is pollinized by Pronuba macidata Riley, which, as 

 already shown, has a tongue modified into an accessory pollen-gathering 

 organ. If any species of Yucca would seem not to need a special in- 

 sect for pollination. Yucca w/iipplei is that species ; for the long stamens, 

 the sticky and abundant pollen, and the peltate, hairy stigma would 

 all seem to facilitate ordinary x^olliuation. Nevertheless, the very re- 

 stricted style would seem to be purposely developed to counteract these 

 other facilities, and we find a Pronuba associated with it, with a re- 

 markably modified tongue, and with the maxillary tentacles very long 

 and attenuated at the tip — structures which doubtless enable the moth 

 to perform the act of i)ollination. I have never been able to observe 

 the act, nor has anyone yet recorded either the acts of pollination or 

 oviposition. There will be nothing peculiar about the latter, but I 

 shall be very glad to get actual experience in reference to the former, 

 as I am satisfied that the observed facts will show, still more fully than 

 in the case oi Pronuba yuccasella, that the special modifications of both 

 flowers and insects have gone on until the mutual interdependence has 

 become absolute.* 



" Since this was written Mr. D. W. Coqiiillet has, at my solicitation, carefully 

 watched the acts of pollination and oviposition in this species, and his accounts as 

 communicated show that the actions of Pronuba macidata are substantially similar 

 to those of P. yuccasella. 



