The Ichneumon Flies 



the loosely spun silk with the individual although more of this 

 loose silk surrounding the whole mass in any given host cocoon. 

 Thus it seems as though the parasitic larvae in preparing for the 

 winter appreciated the degree of cold which they would have to 

 experience and wisely prepared for it by making their habitations 

 thicker and tighter against the inclemencies of the weather. 



As abundant and hardy as this species seems to be it does 

 not escape the attacks of enemies of its own. Certain soldier- 

 bugs have been seen to capture the adult females of the Pimpla 

 when they were engaged so assiduously in egg-laying that they 

 were blind to their surroundings. This is not surprising since 

 one can approach them during this process so closely as to be 

 able to study them well with a small hand lens. Then, too, 

 secondary parasites have been reared from their cocoons, that is 

 to say, internal parasites of their own, and still more strange to 

 relate, Hymenopterous parasites of these secondary or hyper- 

 parasites have also been reared from the Pimpla cocoons. An 

 extraordinary chain of links in the development of species is thus 

 brought about and may be studied by any one during almost any 

 summer in one of our northeastern cities. When the tussock 

 moth caterpillars appear in great numbers on our shade trees dur- 

 ing a given season, it will almost invariably be found that this 

 Pimpla is present also in great numbers and that the majority of 

 the caterpillars are stung by it. This means that the following 

 season there will be an unusual number of adults of the parasitic 

 insect which is now termed the primary parasite. So great is 

 this abundance that the first generation of tussock moth caterpil- 

 lars is practically wiped out of existence. Then comes the second 

 curious fact, that the secondary parasites become enormously 

 abundant and kill off the abundant Pimplas. At the end of the 

 same season or at the beginning of the next the tertiary parasites 

 put in their appearance and the secondary parasites are destroyed, 

 thus giving relief again to the primary parasites which once more 

 begin to be abundant and ready for the next case of super-abund- 

 ance of the host caterpillar. This little chain of species depending 

 upon species offers one of the easiest and most interesting series 

 of observations which may be made by any school class in the 

 part of the country indicated. 



