52 Rileij — Notes on Yucca Insecls and Yucca Pollination. 



on]}^ a very few pith-lioriiig or stein-l)oring species. ^^ We have 

 in this structure, which is so exceptional in Lepidoptera, another 

 Illustration of a principle to which I have often referred in my 

 writings, namely, that hirval structure in insects has heen modi- 

 fied independently of the ultimate structure, and is, as a conse- 

 (i[uence, of very little taxonomic value. Thus we have in this 

 same family the larva' of Prodoxus, (e. g., the t^'pical declpiens) 

 which remain in their short burrows, possessing no legs, while 

 those of Pronuba. which quit their Ijurrows and penetrate the 

 ground, possess thoracic legs. Yet in the particular case of 

 Prodoxus cinereus the larva ap]iroaches Pronuba in having 

 thoracic tubercles which may be looked upon as either remnants 

 of legs or the beginnings of the development of such. This larva 

 burroAvs in the soft i^ith of Yucca vhipplei much more freely 

 than any of the other species of the genus so far studied, making 

 much longer channels, the substance of the stem being less firm 

 than that of the other species of Yucca. In so far, therefore, as 

 this particular Prodoxus larva has peculiar structures, we can 

 trace their origin to purely dynamic influences, assisted l)y 

 heredity and selection — a consequence, in other Avords, of en- 

 vironment — and repeated independently in larva^ of different 

 orders having no possible genetic connection. 



The distril)ution of the genus Pronulxi, as exemi)lified in these 

 additional observations, is extremely interesting. Pronuba yuc- 

 casella, the typical species of the genus, not only occurs over 



* I have not had time to closely scan the literature for cases of this kind, 

 but do not recall any. I am familiar, however, with three unrecorded 

 instances, two of them of. Pterophorid larv?e which bore the stems of 

 Solidago. One is the larva of Alncita kelUcollll Fish, which singularly 

 departs from the typical Lepidopterous larva in its elongated bodj' and 

 in having a pair of supra-anal spines which give the anal plate anapixuir- 

 ance so characteristic of that of many Coleopterous larvaj. The second 

 case is that of an undescribed species of the same family, Ptero2)horidfe, 

 wliich has the anal plate obliquely truncate and fringed with a row of 

 stiff hairs and witli a pair of small thorns at its ventral border, this modi- 

 fication also recalling that possessed by several wood-boring Coleopter- 

 ous larva\ The third case is that of the larva of a Noctnid, Hadena dipata 

 Morr., which burrows in the pith of young corn or maize. It has the 

 anal plate obliquely truncate and flattened along the posterior margin, 

 which is armed with a series of hornj' points, and thus again repeats the 

 structure which recurs in certain Coleopterous larvte, especially of the 

 Elaterida% which inhabit barrows in the trunks of trees. 



