The Tachina Flies 



issuing from the puparium the fly breaks away the entire end of 

 the hardened larval skin. 



It used to be thought that every caterpillar upon which these 

 eggs were placed was doomed, but it often happens that the 

 mother tachina fly, with a faulty instinct, places her eggs upon 

 the back of a caterpillar which is about to cast its skin and in such 

 instances it frequently moults before the eggs have had time to 

 hatch, so that when they do hatch the young larvae find them- 

 selves out in the cold world instead of revelling in the interior of 

 a well-fed caterpillar. So frequently does this occur that a very 

 large proportion of tachina eggs are wasted by the mother flies. 

 The observations of Fernald and his assistants in their work upon 

 the gipsy moth in Massachusetts have given us exact figures in 

 regard to this matter. In one instance 250 caterpillars, each 

 bearing eggs of tachina flies, were fed and carried through their 

 transformation without the appearance of a single adult fly. In 

 another instance 235 caterpillars, each bearing from one to thirty- 

 three eggs, were fed and watched and from these, 226 moths 

 were reared and only nine were killed by the tachinas. 



An interesting point connected with the life of these flies is 

 brought out when we compare them with the parasitic Hymen- 

 optera, the ichneumon flies and the chalcis flies. In the latter 

 case we are struck by the extremely definite relation between 

 the kind of parasite and the kind of host. The parasites of a par- 

 ticular genus will attack perhaps only insects of a certain family 

 and it is a very definite rule that parasites of a given subfamily 

 will attack only insects of a certain order. With the tachina flies, 

 however, it is quite different. The same species of fly will lay 

 her eggs not only upon insects of several different families but 

 upon insects of two or even three different orders. This would 

 seem to me to indicate that the parasitic mode of life in the 

 tachina flies is one of comparatively recent acquirement and that 

 sufficient time has not elapsed since they began to take on this 

 habit for so great a differentiation, so great a co-relation between 

 the host relation and the structure of the insects, to grow up. 

 The ancestors of the tachina flies were probably flesh-flies and 

 the parasitic mode of life has come from a gradual change from 

 feeding on dead insects to feeding on live ones. 



Coquillett has pointed out that in their instincts these flies 

 appear to be much stupider than the ichneumon flies. The latter, 



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