THE OCCURRENCE OF ANABOLIA NERVOSA IN JUNE 25 



necessity and advantage are not quite so obvious, to find that 

 summer insects in similar localities are also subject to acceler- 

 ating influences. With these species, it may be desirable 

 that the young larvae should be up to the usual standard of 

 growth before their enforced earlier hibernation ; but this does 

 not explain the earlier appearance of the perfect insects the 

 following year, seeing the larvae will usually resume feeding 

 later in spring. In a, single-brooded species having a con- 

 siderable range in altitude and latitude, the time during which 

 the vital processes can be active must be shorter in the colder 

 than in the milder localities, and the loss of time in the colder 

 must be compensated for by acceleration in development. 

 This more rapid development probably accounts for the 

 relatively small size of examples of certain species of 

 TricJioptera from elevated Highland lochans compared with 

 examples of the same species from lower levels. (If no such 

 difference exist between examples of the same species from 

 low and from high levels, and if the more rapid development 

 of the high be taken as the normal, then may it not be inferred 

 that the development of the insect is retarded at the lower 

 levels by what may be termed a modified condition of 

 aestivation ?) When the acceleration is insufficient, the 

 species must have reached the limit of its distribution in 

 altitude or latitude, or, it must at least cease to be an annual 

 insect. It is well known that some boreal and alpine insects 

 fail to put in the cycle of their existence within the year. 



To any one living in a hilly district, with time sufficient 

 for the investigation of a fairly large area, this subject 

 presents a field for interesting work. Of course, it would be 

 essential to base observations on insects found on their 

 breeding-grounds, and not on mere stragglers wandering 

 " up hill and down dale." 



