46 Riley Notes on Yucca Insects and Yucca Pollination. 



PRONUBA SYNTHETICA. 



Mr. Trelease was also fortunate enough to be able to study the 

 operations of Pronuba synthetica on the flowers of Yucca brcvifolia. 

 This Pronuba is slower in its movements and slower to take 

 flight than the other species observed, 'though he found it more 

 active during the day than is Pronuba yuccasella. It takes wing 

 less readily and then merely sails down to the ground. This 

 indisposition to leave the flower may be connected with the 

 almost constant high winds on the Mojave desert, where this 

 Yucca most abounds. The fertilized pistils of this Yucca are 

 quite noticeable, by comparison with those of other species, by 

 their symmetry and lack of constriction or indentation so uni- 

 formly present in the Yuccas that are punctured by Pmnulxi, 

 yuccasella and P. maculata. The explanation is found in the 

 fact that Pronuba synthetica pierces " the uppermost part of the 

 style, conveying its eggs down to the ovary through the stylar 

 channel, the course followed by the pollen tubes." This fact 

 interested me very much, for I recollected- very well in my first 

 studies of Pronuba yuccasella, before the act of oviposition had 

 been witnessed, that, puncturing for the purpose of oviposition 

 being unrecorded and therefore quite exceptional among Lepi- 

 doptera, I was strongly of the opinion that the egg would be 

 thrust through the stigmatic opening down the stylar channel. 

 The instinct to oviposit only on the youngest flowers is particu- 

 larly marked in synthetica, which Trelease frequently saw forcing 

 itself into the narrow clefts between the rigid sepals of the open- 

 ing bud, the flattened form of the insect facilitating the opera- 

 tion. This habit also suggests the cause of the looseness of the 

 wing scales and the ease with which they are lost. Mr. Trelease's 

 observations in detail on the actions of this Pronuba cannot well 

 be condensed, and I quote them entire : 



" When about to deposit an egg, having selected a suitable flower, the 

 female of synthetica runs to the bottom of the stamens nfuch as yuccasella 

 does, makes a rapid, more or less complete circuit of their bases, and then 

 quickly ascends to the very top of the pistil, her thorax rather higher 

 than the end of the stigrna, and with her short but strong ovipositor cuts 

 through the thin wall, into the stylar channel, rarely as much as 2 mm. 

 below the tip of the stigma, meantime holding fast to the pistil, the sta- 

 mens being below her reach. The long extensile oviduct is then passed 

 through the puncture, the egg being laid apparently within the ovarian 

 cell, along the funicular end of the ovules. In removing the oviduct the 



