40 Journal of the Mitchell Society [December 



or 10 per cent of the whole number, uninfested, yet 61 larvae were able 

 to pupate. From these 61 pupae, however, only 7 adult moths 

 emerged, showing a total mortality among the Army Worm from larva 

 to adult of 98.6 per cent. And since 90 per cent of the larvae were in- 

 fested with the eggs of this parasite, it would seem to indicate that, in 

 this case at least, the parasitic fly was decidedly the most important 

 factor in causing the high mortality of the Army Worm. In a few 

 cases it was found that where only a single parasitic egg was attached 

 to a larva, that the host was able to complete its transformations. 



"The greatest number of parasitic eggs observed on a single larva 

 was 12, with an average of 3 for the entire number (442) infested. The 

 442 infested larvae yielded 709 parasitic puparia, or an average of 

 nearly two for each infested larva. The 709 puparia yielded 556 

 adult parasites. The greatest number of adult flies from a single 

 Army Worm was four. These figures show that the mortality of the 

 parasitic fly from egg to puparium was 52 per cent, and from puparimn 

 to adult 22 per cent, making a total mortality from egg to adult of 73 

 per cent. 



"This shows that the tendency would be for the fly to continually 

 gain in relative numbers, owing to the lighter mortality, and easily ac- 

 counts for the complete subjugation of the Army Worm in normal 

 years by this one natural enemy. No other parasites were found in 

 the course of these experiments." 



Since we are so dependent upon these insect friends perhaps it 

 would not be amiss to inquire a little more closely into their life econ- 

 omy. For convenience we may divide the friends of man into two 

 groups, predacious insects and parasitic insects but this is merely a 

 matter of convenience and the line separating the two is by no means 

 a sharp one. For example we might inquire whether the larvae of the 

 solitary wasps mentioned in another connection were parasitic or pre- 

 dacious and while in general their behavior is that of a predacious in- 

 sect there is little doubt that this is simply a further development of a 

 strictly parasitic habit. Likewise the hne separating the predacious 

 insects from the plant feeding forms is not great. Some species seem 

 to feed predaciously if insect hosts are abundant, but if not available 

 they turn their attention to plants. Hence we feel safe in saying that 

 the evolution of these habits have been from plant feeding to pre- 

 dacious insects and thence to parasitic insects. 



Predacious insects are many and they have varied habits. Some 

 are active and search far and wide for their prey like the ground bee- 



