322 NATURAL SCIENCE. May. 



less assist fertilisation by scattering the pollen. Dried fruits of the 

 previous season showed the perforations made by the escaping larvae. 

 Yucca australis, studied in south-west Texas, shows seeds 

 tunnelled in the manner characteristic of the moth, the pulp being 

 perforated by the escaped larvae. No flowers were observed, but 

 large fruits, gathered some three weeks after fertilisation, bore none of 

 the constrictions or indentations which so commonly mark the 

 puncture of the moth, a fact which suggests that the eggs are deposited 

 in the upper part near the stigma, as was found to be the case m 

 the large tree yucca, Y. bvevifolia. The agent in the flowers of the 

 latter is the dark-coloured Pyonuba synthetica, which is more active 

 through the day than its eastern congener, and, unlike the other 

 known species, slow to take flight. 



This apparent disinclination to leave the flowers may, perhaps, 

 be connected with the almost constant occurrence of high winds m 

 the desert, and may restrict cross-fertilisation to flowers of the same 

 plant more closely than in other Yuccas, though there must be 

 frequent flights from plant to plant in quiet weather, and especially 

 at night, when the wnnd sometimes falls. Moreover, the development 

 of the stigma two days before the stamens of any one flower renders 

 close fertilisation, in the strictest sense, impossible. Owing to the light 

 yellow colour of the pollen, in marked contrast with the smoky tint of 

 the moth, laden females can be seen from a considerable distance. 



The female of most species of Pronnba seeks a fresh flower 



Avherein to lay her eggs, preferring one in the first night of expansion, 



doubtless to ensure for her offspring a sufficient food-supply, the 



younger flowers being less likely to be already overstocked with eggs. 



This instinct is especially marked in P. synthetica, which was never 



seen to use any but the youngest flowers, while pollen-laden moths 



were repeatedly noticed forcing their way in the very narrow clefts 



between the rigid sepals of an opening bud, their flattened form 



facilitating this. When about to deposit an egg, the female runs to 



the bottom of the stamens much as yuccasella does, makes a rapid, 



more or less complete, circuit of their bases, and then quickly mounts 



to the very top of the pistil, and with her short, strong ovipostor cuts 



through the thin wall into the channel of the style close below the 



tip of the stigma, holding fast to the pistil meanwhile, the stamens 



V)eing below her reach. The long extensile oviduct is then passed 



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



cell. The operation usually takes from two-and-a-half to three 



minutes longer than in other observed species. Sometimes two or 



more eggs are laid before the stigma is pollinated, but commonly, 



after laying each egg, the moth retreats to the bottom of the flower, 



and then again mounts the pistil till her head is even with the stigma, 



when she uncoils the large tentacles from their resting-place against 



her load of pollen and passes them back and forth in the stigmatic 



-chamber with almost the same motion as yuccasella. The deposition 



