42 
DR ALFRED E. CAMERON. 
of which it serves as a general sort of index. Temperature, humidity, and wind 
velocity are each and severally modified by the particular kind of plant association, 
whether it be herbage, shrubs or forest, and each kind of association harbours its 
own kind of animal life, species which are characteristic of it and of no other. 
Often the line of demarcation between one association and another can be .strictly 
delimited, so that species which may be introduced into an association which is alien 
to their habits react negatively to the prevailing physical factors of their new abode 
and tend to turn back. On the other hand, where two distinct associations are in 
juxtaposition and gradually merge into one another, as in the case of woodland and 
grassland, many species from both will intermingle, especially so at the places of 
transition. Thus the feeding habits of many larval forms will be actively pursued in 
the humus or decaying wood of forests, and will react negatively to light and to a 
dry atmosphere, whilst the winged adults will visit the herbage of meadows and 
pasture, bathing themselves in the rays of the sun. Migration, however, is more 
frequent from stratum to stratum, by which is meant the vertical divisions of a 
uniform area such as the subterranean, surface, plant, and aerial strata, for which 
Vestal* has introduced the terms subterricolous, terricolous, herbicolous, and 
aericolous respectively. It is this process of interchange of either one individual or 
the same group of indivduals from one stratum to another that lends unity to the 
association. The process may be induced by changes of light intensity, or may be a 
direct response to the different needs and activities of an insect during different 
stages of its life-history. An analysis of migration reveals the presence of four 
factors, viz. motility, agency, proximity, and topography. Not all of these are 
present in every instance of migration, and in many* cases- where the proper 
distributive agent is lacking to eke out the motile powers of an organism, the 
effective operation of the two will be profoundly modified by distance • and 
topography. In general, insects do not take long migratory flights except when 
there is some urgent necessity, such as lack of food due to enormous multiplication 
of a species, a phenomenon which occurs in the Rocky Mountain locust ( Scliistocerca 
americana). A curious example of migration, as yet unexplained, which the author 
had the opportunity of observing during a sojourn in America last summer, is that 
of two species of Salt Marsh mosquito, Culex cantator and Culex solicitans, which 
come' off the extensive salt marshes bordering the New Jersey -coast in enormous 
numbers. Specimens were taken in grasses and rank herbage as many as five 
to seven miles inland. Dr Smith t first proved (1902) that the theory which 
maintained that mosquitoes do not fly far from the point where , they breed, was not 
applicable to all the species of this family. 
In a general way it is now recognised, as has been already remarked (p. 38), that 
* Vestal, A. G., “An Associational Study of Illinois Sand Prairie,” Ball. III. State Lab., Nat. Hist., 1913, vol. x, 
art. 1, p. 67. 
+ Smith, J. B. “Mosquitoes,” Rept. New Jersey State Agric. Exper. Station, Trenton, N.J., 1904, p. 5. 
