14 



somewhere about 1S78 or 1SS0, it suddenly almost, or perhaps com- 

 pletely disappeared, and no more was heard of it for some fourteen 

 years, when it began to be found again, sparingly at first, but gradu- 

 ally becoming commoner, until at the present time it appears to be 

 fairly well distributed along our south-eastern coasts. 



Why it snould have so completely disappeared immediately after 

 the time when it had been most abundant is difficult to understand. 

 Certainly it could not have been from over-collecting. Nor is it any 

 easier to say why it has recently again increased in numbers. In 

 1896 a committee was formed for the protection of Lepidoptera in 

 danger of extermination, and Porthesia chrysorrJuza was scheduled 

 as one of the species needing attention, but I fear they can take no 

 credit for having preserved it as a British insect. It has been sug- 

 gested that its disappearance was due to a series of wet summers, 

 notably those of 1882 and 1883; but this hardly coincides with the 

 exact time of its disappearance. It is, however, probable that the 

 meteorological conditions prevailing at a critical period in the 

 species' existence might be the chief cause ; but this is not the only 

 probability — -animal or vegetable parasites might just as probably 

 cause its destruction. 



Being gregarious throughout its earlier stages, larger numbers of 

 individuals would be likely to suffer by the attack of an enemy than 

 would be the case with a species of solitary habits ; and it is quite 

 conceivable that ichneumons might wipe out a whole colony in a 

 single season, or that if a few individuals became affected by a 

 deadly disease it might easily spread with similar results. Our 

 information on such matters is lamentably meagre, but the following 

 case, which came under my own observation, although somewhat 

 artificial in character, may serve as an illustration of what might take 

 place under natural conditions. In November, 1898, 1 received from 

 a friend a couple of the winter nests of the larvae and tied them on a 

 whitethorn bush in my garden. Now, my garden is infested with a 

 dipteron that attacks some species of lepidopterous larvae to such an 

 extent as to keep them very much in check. As spring came on the 

 chrysorrhcea larvae came out of their nests and fed on the freshly 

 developed leaves of the whitethorn, and appeared to be doing well 

 until about half grown, when they suddenly took to wandering, and, 

 not being confined in any way, they strayed all over the garden. I 

 was anxious not to lose them, and therefore collected all I could find 

 and put them in a breeding cage. None, however, pupated, but all 

 died when just full grown, and hosts of Dipterons came forth from 

 their dead bodies. In this case I have not the least doubt that the 

 Lepidopterous larvae were brought to the Dipterous parasite ; but is 

 it not quite possible that in a state of nature the parasite might seek 

 out its host with equally destructive results ? Such a calamity over- 

 taking a gregarious species would, without doubt, affect it most 

 injuriously, even if it came at a time of its greatest strength ; but if 

 it came at a time when it had already been reduced by some other 



