The Canadian Spoeitsma^ and Hatur^lis 



No. 3. 



MONTREAL, MARCH, 1883. 



Vol. III. 



WILLIAM COUPER, Editor. 



We cannot supply complete sets of vol. I 

 of this journal — some of the numbers are 

 exhausted. 



CAUSES OF RARITY IN SOME SPECIES 

 OF INSECTS. 



Entomologists know that some species of 

 insects are generally few in individuals, while 

 others are numerous. Those which are useful 

 to man, and have been, so to speak, domesti- 

 cated by him, are, of course, kept up in as 

 large numbers as possible, \>y the care and 

 protection bestowed upon them. But the iest 

 are left to the care of nature, and in the bal- 

 ancing of the great system of life, are subject 

 to various influences, which affect them injur- 

 iously or otherwise. An enquiry into the 

 causes which act in reducing their numbers 

 would be both interesting and instructive, par- 

 ticularly in the case of those species which are 

 always rare. Of course, some of these causes 

 are easily discovered, but others, which ma}' 

 still be important, are obscure and difficult to 

 trace out. For instance, a species may be 

 rare, owing to the scarcity of its food plant. 

 We cannot expect to find an insect, which may 

 be confined to a single food plant, abundant 

 where that plant is scarce. And an abundant 

 species may be rendered rare in a given local- 

 ity by the diminution of its food plant, say by 

 the increase of cultivated ground, or by fire, 

 &c. The following illustrates the point : The 

 Gomin swamp,a well known collecting ground, 

 close to the city of Quebec, is, or was, a breed- 

 ing place of Chionobas jutta,& Labrador butter- 

 fly, which is not found in any other place 

 within twenty miles of that locality. During 

 the past fifteen years the swamp has been 

 largely trenched and drained, and the butter- 

 flies have become scarce, no doubt owing to 

 the loss of the food plant, which is probably 

 some lichen or moss growing there. Another 

 cause of the scarcity of some insects is their 

 liability to parasitic attacks. The beautiful 

 moth, Samia Columbia, might be given as an 

 instance of this. Mr. S. I. Smith, the describer 

 of the species, says in his paper, " This spe- 



cies seems to be infested by an unusually 

 large number of parasites, since, out of more* 

 •than twenty cocoons, I have succeeded in rais- 

 ing but three, nearly all the rest having been 

 destroyed by ichneumons and other parasites. 

 Its remarkable rarity is, perhaps, due to this 

 fact." I may add that collectors in Canada 

 have had a similar experience with this moth. 

 Another case is that of Pieris rapse, which 

 threatened to be a real scourge to the country, 

 but has been reduced within reasonable bounds 

 by the assistance of the insect parasites prey- 

 ing upon it A third cause, particularly in 

 the case of noxious insects, is the efforts made 

 by man to extirpate those which destroy his 

 crops or injure him in other ways. These ef- 

 forts sometimes make an injurious insect rare, 

 but no insect has become extinct from this 

 cause, as far as we know, nor is it probable 

 that such will ever be the case. Among causes 

 n ore remote than the above mentioned, and 

 more difficult to trace, are variations in clim- 

 ate, and in the seasons, as compared with one 

 another. The way in which insects are atfect- 

 ed by different conditions of the atmosphere, 

 and by hot and cold seasons, has not been 

 thoroughly studied as yet. In some years, for 

 example, the Lepidoptera are much more 

 numerous than in other years. Their abund- 

 ance, or the opposite, is no doubt dependent in 

 a great measure on the weather of the preced- 

 ing summer and winter,as well as on that ot the 

 passing season. And what may he favorable 

 weather for the Lepidoptera may be unfavor- 

 able for insects of other Orders. The know- 

 ledge of what constitutes favorable conditions 

 for the increase of noxious insects would be ot 

 practical value to the agriculturist, and is a 

 subject worthy of the study of our entomolo- 

 gists. Another cause of the intermittent 

 abundance and scarcity of some species is to 

 be found in their migratory habits. In the 

 case of the locust, this is evident to all. Some 

 of the butterflies, belonging to the families 

 Pierip-e NYMFiiAi.in.K and Daxaid.e also 

 have this habit. Some years Pt/ramtis cardui 

 or Vanessa J-album will be plentiful about 

 Montreal, and then for a series of years will be 

 exceedingly rare. The explanation of the mys- 

 teiy probably is that a large number of the 

 insects have migrated to the locality during 



