THE CANADIAN ENTOMOLOGIST. Ill 



spiracles, also a normal position. But at this point of larval existence a 

 rather striking point of so-called specialization occurs, and this happens 

 with a few other species also, which, though a trifling matter apparently, 

 serves to differentiate it at once and forever from its near ally, margini- 

 dens. At the close of the stage a faint trace of a corneous plate appears 

 on joint seven, in the exact position where IV. is on the preceding ones. 



With the cast of skin that brings the larvae into the penultimate 

 stage, it is found there is a large, well-developed tubercle here on 

 abdominal joint seven in the same position and of similar outline to IV. 

 on the preceding joints. It has not been that IV. has been raised to its 

 corresponding position on the others, for it is still occupying its previous 

 normal position lower down, and we have to do clearly with an acquired 

 character, which we may designate as IVa. The larva attains to a length 

 of about 34 mm. in this stage. 



Mature larva : The colour now becomes a soiled translucence, 

 without traces of the usual lines. While the head and shield have 

 increased, the plates situated at the true tubercles or elsewhere do not 

 show a corresponding enlargement. The plate IVa is as large as the 

 true IV., though both are of slightly less size than IV. on the preceding 

 joints. This feature has been constant in a large series of examples, and 

 is the principal feature of a structural nature by which it may be differen- 

 tiated from its ally. There exists a very perceptible difference in size, 

 colour and general appearance in their immature stages, obvious enough 

 to one familiar with these borers, but it seems sufficient to separate our 

 species in this matter of the acquired plate alone, as by it we can 

 distinguish the larvae of such dissimilar species as inqucesita and necopina, 

 nitela and limpida. Considering the development of this additional plate 

 IVa as pointing to a higher specialization, and that those species possess- 

 ing it represent a more recent evolution from the earlier type, permits us 

 to look with some degree of assurance for this older form in such widely 

 diffused species as immanis of our fauna, and micacea of Europe, whose 

 common parentage seems unquestioned, and whose larva?, at least the 

 latter, have the normal Noctuid arrangement on the seventh abdominal 

 segment. Continuing in this line, we might expect in the ornamentation 

 of the imago the more rigid, straighter transverse posterior line as a 

 primitive marking and a tendency with our later, specialized species as 

 having the orbicular, claviform and reniform white-marked and contrast- 

 ing — a feature not common to the Noctuids as a whole. Stress has been 



