Introduction 



light through the researches of some of Doctor Howard's associates 

 upon the parasites of such pests as the Gypsy and Brown Taif 

 Moths. Among the most interesting of these parasites are certain 

 flies called Tachina flies. It was supposed that the life histories 

 of the various species of these flies were similar and that the mother 

 fly simply deposited an egg upon the skin of the caterpillar, the egg 

 hatching into a tiny larva that burrowed through to the inside of 

 the host and there lived until the latter died. But it was found 

 that this life history would not fit the facts in the case of some of 

 these parasites imported from foreign lands. A careful study of 

 the literature of the subject showed that a Japanese Entomologist 

 named Sasaki had made a curious discovery in regard to a Tachina 

 parasite of the silk worm in the Orient. He found that in this 

 case the mother fly laid the eggs upon the leaves of the food plant 

 and that these eggs were swallowed by the silk worms with the leaf 

 particles and hatched into larvae within the bodies of the worms. 

 Here they developed until they killed their hosts, and themselves 

 changed to chrysalids to change later to adult Tachina flies. 

 Thinking that some such habit might prove true of these gypsy 

 moth parasites, Dr. C. H. T. Townsend made a careful study of 

 the subject and discovered that this same habit held good in the 

 case of the imported Tachinids. As a consequence he was able 

 to trace the obscure life histories of other flies and to develop meth- 

 ods of distributing and colonizing the parasites which were of great 

 practical use. 



Probably history will record that one of the greatest achieve- 

 ments in Doctor Howard's long career as Government Entomolo- 

 gist has been the working out of practical methods of such natural 

 control of injurious insects. He has sent his agents all over the 

 habitable globe in search of the enemies of various pests and he 

 has had a host of workers actively colonizing them in many parts of 

 America. The results have already been of incalculable value, 

 with the prospect that in the future this method will become one 

 of the most useful ways in which the injuries of destructive insects 

 may be generally prevented. 



While the study of the discoveries already made by others is 

 of greatest value and interest to everyone, the chief zest in the 

 pursuit of knowledge by the human mind is found in the learning of 

 facts never known before. One of the notable features of this book 

 is that it not only gives typical life histories in nearly all of the 



