610 A Cosmopolitan Butterfly. II. Its History. (October, 
ing a cyclone, and other similar though not so striking instances 
might be added. Several accounts have also been given of the 
migration or simultaneous movement of this insect in swarms. 
Dr. Hagen records two instances ;! on October 26, 1827, Prevost 
saw such a moving swarm, composed of a stream of butterflies 
from ten to fifteen feet broad, passing from south to north for . 
two hours. On April 26, 1851, Ghiliani saw, near Turin, a great 
` flight of these insects ; according to Bouquet,? the day was fine, 
after continued rain, and a strong breeze blew from the west ; 
commencing at eleven A. M., the swarm came from the south- 
southeast and continued with a precipitate flight for five hours 
towards the north-northwest. 
“ In England and on the continent of Europe,” says Trimen,’ 
“ cardui sometimes appears in great abundance, and then, perhaps 
for several seasons, will be uncertain in appearance and restricted 
to particular localities. I have not heard of this irregularity of 
appearance being noticed in other parts of the world.” This is, 
however, the universal testimony- of observers in America, and is 
probably due to the action of parasites. It was one of the first 
phenomena that drew my especial attention to butterflies. This 
butterfly, indeed, is one of the best subjects of study for those 
who wish to investigate the causes of irregular apparition ; and 
only such as spend much time in the field can hope to solve the 
problem. A close observation of the comparative abundance of 
the butterfly for several consecutive years in the same locality; 
(selecting only those which are nearly full grown, and recor ing 
the proportion of healthy and infested ones), will probably show 
whether the attack of parasites is a vera causa. 
As regards the parasites, Prof. A. E. Verrill has reared from 
caterpillars of this insect a species of Microgaster called M. fi ruct- 
uosus by Cresson. Mr. Riley has also bred a dipterous parasite ; 
its larva usually issues from the victim while the latter is m the 
caterpillar state, though sometimes not until it has transformed 
to chrysalis ; and in one instance the Vanessa completed its meta- 
morphoses with the parasitic fly yet in its abdomen ; it did not, 
however, properly expand its wings. Mr. Riley has also bred 
Ichneumon rufiventris Brullé from this insect ; this hymenopteron 
issues from the anterior extremity of the chrysalis, infested spec 
1 Stett. Entom. Zeit., xxii. 
80. 
2 Annales Soc. Ent. Fr. [2] ix. Bull., 56- 
8 Rhop. Afr. Austr., 121. 
