66 



for the cases of occasional abundance by purely local influ- 

 ences, we must find some irregularity in the working of 

 nature. One theory that has found considerable favour is the 

 laying over of the insect in one of its quiescent stages for an 

 unusually prolonged period. With regard to the first or ^%'g 

 stage, what little evidence we have is not favourable to such a 

 conclusion; we find that if ova do not hatch at their appointed 

 time, they do not do so afterwards ; and experiments that I 

 have tried gave a confirmatory result. But it is known that 

 many species do pass a prolonged period as pupae ; of this 

 a multitude of instances are on record ; of the Sphingidse 

 and Bombyciidae, a portion of the brood frequently remain in 

 pupa until the second and sometimes the third year. Erio- 

 gaster lanestris, L., is particularly prone to doing so, and 

 often remains several years ; but it has been found that the 

 number of emergences is smaller each year, and indeed the 

 only case that I have been able to discover tending in an 

 opposite direction is that of a portion of a brood of Astero- 

 Scopus jmbeailosa, Esp. ; of these some twenty larvae went to 

 earth at the end of June, 1881, and no moths emerged until 

 March, 1884, when one male and four females were bred ; but 

 it must be borne in mind that these were kept under artificial 

 conditions, and therefore protected from their natural enemies, 

 and that under such favourable circumstances the percentage 

 produced can hardly be regarded as large; and that in a 

 state of nature the longer an insect remains in pupa so much 

 greater are the risks of its falling a victim to the numerous 

 enemies that beset it ; therefore the probability of any ex- 

 ceptional abundance being due to such a contingency is 

 exceedingly remote. 



But perhaps the most important suggestion with regard to 

 the local causes theory is that of the possible effect of 

 meteorological conditions. We are told that mild winters 

 and cool summers are detrimental to Lepidoptera, while hard 

 winters and sunny summers are favourable, and that excessive 

 wet is fatal to many species in their earlier stages, and so on. 

 Observation appears to prove that such is the case ; but 

 would not such causes affect Lepidoptera generally, rather 

 than a few species ? It is worthy of remark that some of the 

 cases of greatest abundance, Vanessa carditis L., and Plusia 

 gamma, L., for instance, have occurred in cold wet seasons, 

 when other species have been actually much less common 

 than usual. I propose to refer to this again hereafter, and 

 in the meantime to dispose of a class of cases that it appears 

 to me will be more aptly taken here. There are few of us, 



