THE CANADIAN ENTOMOLOGIST. 117 



insulidens, Bird, the only example not bred, shows the insect in a good 

 light, and should be an aid to western collectors. 



A few remarks further argumentative to the results of these studies 

 may be pertinent. The extent to which variation proceeds with this 

 genus has long been a subject receiving attention. That affecting the 

 imago is such as to cause little uneasiness, even though extended in a very 

 few cases. When it is a question of size or wing outline we can explain 

 this in a partly mechanical sense. When their larvce have lived in an 

 especially rank or vigorous plant an excessive subsequent development 

 may be expected, while those larvae that leave their burrows from one 

 cause or another and suffer from a lack of food until locating in some 

 substitute, produce the undersized or dwarfed specimens so frequently 

 met with. Thus the rank growth of Ambrosia trifida, on the Jersey 

 meadows, yields a giant race of nitela, whereas the dwarfs so often seen 

 among marginidens and cerussata are the result of the larvae leaving their 

 original abode and taking up with some substitute like Burdock. The 

 wing outline is influenced by the quarters occupied by the pupa ; when 

 this is formed in a gallery having insufficient room for a normal develop- 

 ment, the resultant imago has the primaries narrower and more acute at 

 the apex in proportion to this previous condition. Impecuniosa and 

 duovata best illustrate this feature. The colours of the imago here as else- 

 where are subject to their peculiar vagaries, but it is not found that they 

 are in any way exceptional. Just what produces colour is not definitely 

 known, of course, but it pertains no doubt to a chemical rather than any 

 mechanical process. So the slight disparities at times noted in the same 

 species, as arising from differences in the food-plant, might be explained 

 on this ground, though it does not meet the question properly. Further 

 than citing that colour is most susceptible to change in the depth of 

 powderings, in the hue or even suppression of the ordinary white spots, it 

 may be needless to proceed. We may apply the general biological law to 

 the effect that commoner and more widely distributed species are apt to 

 show a greater variation, and not meet with any incongruities. The 

 common fiitela best shows the departures ever taken in the colour scheme, 

 but they are positively not due to locality, food-plant, sex, or even the 

 varying conditions that might assail different broods, and the form to 

 which the varietal name applies is merely the extreme in the opposite 

 direction. But varietal studies have not been confined to the imago 



