YUCCA MOTH AND YUCCA POLLINATION. 117 
Further accurate and careful observations are very de- 
sirable as to the dates of appearance of Pronuba in the 
Gulf States region. The general result of my observations 
in the southern States as to the presence of the insect would 
indicate that its appearance is adapted there to the greater 
irregularity of the blooming of the Yuccas; but as a gen- 
eral thing, Y. aloifolia is seldom fructified except when 
blooming at the same time as filamentosa, and the same is 
true of gloriosa, which more rarely produces fruit. 
No one can observe our Yuccas closely without noticing 
a certain irregularity in blooming. The same root-clump 
of filamentosa will some years throw up several flower- 
stalks and in other years none. ‘This is particularly no- 
ticeable in the Rocky Mountain region, and the fact that 
the flowers of Yucca brevifolia are uniformly scarce over 
large stretches of country has been repeatedly observed 
and reported to me. I observed it on my first visit to 
these tree Yuccas in 1887 and I remember very well that of 
Yucca angustifolia — which is quite common on approach- 
ing Denver and was found all along the Denver & Rio 
Grande R. R. to Colorado Springs, growing even in the 
side-walks of that town—not a single flower-stalk was 
noticed when I first visited those parts in 1873. This 
irregularity in blooming would prove fatal to insects de- 
pending upon the flower and fruit, were there not some 
provision to meet it, and it is one of the noticeable char- 
acteristics of the Yucca moths that they are equally irregu- 
lar in development; for, as we shall presently see, a very 
large percentage of the moths fail to issue the year follow- 
ing the development of the larva, but show a great tendency 
to retardation in development, some of them not issuing 
until the second, third or fourth year. This is equally true 
of both Pronuba and Prodoxus. 
NEED OF PRONUBA. 
We have already seen that all the pods do not contain 
Pronuba lJarvee, though we rarely find any on the fila- 
