PKRIODICITY IN Till: Al'PEARANCE OF BUTTERFLIES. 1015 



It is none the less true, liowcver, of other insects and among tlicni of but- 

 terflies. Indeed there are comparatively few butterflies which appear in 

 similar numbers every year. There is always more or less fluctuation 

 in this regard, but we notice it onl}' when their excessive abundance, 

 especially with such swarming butterflies as Anosia plexij)pu8 and Van- 

 essa cardui, or their great rarity causes general comment, at least among 

 entomologists. Sometimes we can directly tell the cause of a scarcity, 

 rarely that of a superabundance ; for in the former case, the scarcity may 

 involve several species, and the plain cause some excessive or exceptional 

 meteorological condition. 



Now though the massive meteorological conditions which we term cli- 

 mate have undoubtedly very much to do with the distribution of butterflies 

 and determine, indeed, in very many cases, whether or not a given kind 

 shall or shall not live in a certain place, the indirect results of meteoro- 

 logical conditions have undoubtedly more to do with the abundance or 

 scarcity of a given bxUterfly in a given season. For the very existence 

 of the butterfly shows its capability of withstanding the excesses of meteoro- 

 ogiclal conditions in the spot in which it lives, and the greatest stresses 

 under which it lives are those more active forces, like insectivorous creatures 

 and parasites, which find their own life dependent on taking its, or its 

 neighbors. The activity of these is governed largely by temperature and 

 storm conditions and hence the indirect influence of meteorological condi- 

 tions on the life of the butterfly may be more important than the direct. A 

 caterpillar which could withstand any amount of cold or of warmth in itself 

 considered might not be able to battle against the foes which a mild winter 

 kept in unusual activity and need of sustenance. It does not appear that our 

 butterflies suffer particularly from an exceptionally cold or long winter, 

 but rather from unusual warmth, sufficient to arouse insects from torpor 

 at times when hibernation should be expected ; or, in the fair season, 

 directly from long continued storms and moisture. 



The fluctuation therefore in the numbers of our butterflies is probably 

 due in large measure to the activity or inactivity, the abundance or rarity of 

 their active enemies, and especially, considering how extensive their depre- 

 dations, to the abundance or otherwise of their parasites. It is the strik- 

 ing of the balance which exists between a creature and its enemies in the 

 struggle of each for its own existence. Let some event, untoward to it, de- 

 crease the ratio of the parasite, — the butterfly flourishes ; but its very con- 

 sequent superabundance the following year only gives a better jjasturing 

 ground to the parasite, reduces the butterfly below the normal, and causes 

 the parasite to abound inordinately, only to find its food supply cut off by 

 its own voracity and incontinence and the scales again to be turned. It 

 is then this perpetual warfare, this unending, inexorable struggle for ex- 

 istence, testing the fitness to survive, which is the prime cause of perio- 

 dicity in the abundance of a given species. 



