368 INSECTS INJURIOUS TO VEGETATION. 



number of canker-worms, taken promiscuously from various 

 trees, I found that nearly one third of the whole were unable 

 to finish their transformations, because they had been attacked 

 by internal enemies of another kind. These were little mag- 

 gots, that lived singly within the bodies of the canker-worms, 

 till the latter died from w^eakness; after which the maggots 

 underwent a change, and finally came out of the bodies of 

 their victims in the form of small two-winged cuckoo-flies, 

 belonging to the genus Tachina. Mr. E. C. Ilerrick, of New 

 Haven, Connecticut, has made the interesting discovery that 

 the eggs of the canker-worm moth are pierced by a tiny four- 

 winged fly, a species of Plati/g-aster, which goes from egg to 

 egg, and drops in each of them one of her own eggs. Some- 

 times every canker-worm egg in a cluster, will be found to 

 have been thus punctured and seeded for a future harvest of 

 the Platygaster. The young of this Platygaster is an exceed- 

 ingly minute maggot, hatched within the canker-worm egg, 

 the shell of which, though only one thirtieth of an inch long, 

 serves for its habitation, and the contents for its food, till it is 

 fully grown ; after which it becomes a chrysalis within the 

 same shell, and in due time comes out a Platygaster fly, like 

 its parent. This last transformation Mr. Herrick found to take 

 place towards the end of June, from eggs laid in November of 

 the year before; and he thinks that the flies continue alive 

 through the summer, till the appearance of the canker-worm 

 moths in the autumn affords them the opportunity of laying 

 their eggs for another brood. As these little parasites prevent 

 the hatching of the eggs wherein they are bred, and as they 

 seem to be very abundant, they must be of great use in pre- 

 venting the increase of the canker-worm. Without doubt 

 such wisely appointed means as these were once enough to 

 keep within dne bounds these noxious insects; but, since our 

 forests, their natural food, and onr birds, their greatest enemies, 

 have disappeared before the woodman's axe and the sports- 

 man's gun, we are left to our own ingenuity, perseverance, 

 and nnited efforts, to contrive and carry into effect other means 

 for checking their ravages. 



Between the years 1841 and 1847, canker-worms almost 



