THE GREGARIOUS HABIT IN BUTTHRFLY LARViE. 



11 



struggle for the perpetuity of the species. Should disaster befall the 

 advance guard which has not halted by the way, the sluggards can take 

 up the work, and the chances of survival are greatly increased. This 

 may be so in the warmer areas in which these partially double-brooded 

 species occur, but, in exceptionally warm summers in the colder areas of 

 their distribution, it may often lead to their undoing, for, although in such 

 seasons imagines of the partial second-brood may emerge safely, they 

 do this so late in the season that the larvae have hardly time to reach 

 the normal resting-stage before the winter is upon them, and, if they 

 fail to do this, they are killed off to a larva, for the hybernating stage 

 of partially double- or triple-brooded species, e.g., Brenthis selene, 

 Melitaea cinscia, Colias Ityale, etc., is fixed and constant in a 

 definite stadium, and, although that of certain Satyrids, whose 

 winter-rest is much less complete, may be less fixed, and spread 

 over two instars, and even in the case of Pararge egeria two different 

 stages, yet the larva of Colias hyale must rest in its fourth stadium, Dryas 

 papilla and Argynnis aglaia in their first stadia, Brenthis selene and B. 

 eaphrosyne in their fourth stadia, or die, and the progeny of the partial 

 second-broods of these species must reach the proper hybernating 

 stage, or annihilation must result. The inability of Colias hyale, C. 

 edusa, Pyrameis cardui, and possibly other species, to regulate their 

 hybernating period to our winter, is possibly the cause of their repeated 

 extermination and absence for long periods (sometimes extending to 

 many years) in our Islands. 



It will be seen from this short chapter that there is much variation 

 in the hybernating habits of butterfly larvae, and many difficult inci- 

 dental unsolved puzzles raised, even by the consideration of so simple 

 a subject. 



CHAPTER II. 



THE GREGARIOUS HABIT IN BUTTERFLY LARV.E. 



The gregarious habit is exhibited by the larvae of certain species of 

 such widely different superfamilies of lepidoptera, whose immediate 

 relations show little or no sign of the peculiarity, that there can be no 

 possible doubt that the habit has arisen quite independently in almost 

 every instance. In the Pierids it is exhibited in Pieris (brassicae), 

 Aporia (crataegi), and Eucheiva (socialis), in quite varying degrees, 

 although most of the Pierid larvae live solitarily ; in the Vanessids, 

 and in the Melitaeids, similar cases occur, in each case with solitary 

 relatives ; whilst the species of the far-distant Lymantriids, Lachneids, 

 Hyponomeutids, etc., whose larvae exhibit similar social traits, must 

 have developed the habit quite independently of one another. In 

 the same manner there is no doubt that the various butterfly groups 

 have developed the habit separately, although, in the same group, 

 there is probably some show of reason for a different opinion, e.g., the 

 Melitaeid species — aitrinia, cinxia, etc.; the Vanessid species — io, 

 antiopa, polychloros, urticae, etc. 



The various degrees of gregariousness, in different species, is very 

 marked and noticeable. Among our Palaearctic butterflies, a very 



