THEIR RELATION TO EACH OTHER 



113 



ually even on the plant lice surrounding it, and not only 

 checks increase but lessens infestation. Not all the 

 Syrphid flies are carnivorous, however; quite a num- 

 ber are scavengers, some are pollen feeders and others 

 live in the tissues of succulent leaves or stems. 



The really important parasitic group is found among 

 the Tachinid flies and their close allies resembling 

 house-flies, blow-flies, flesh-flies and others of that 

 character. Some of them — the majority indeed — are 

 inconspicuous grayish flies of moderate size ; others are 

 metallic blue or 

 green, with or with- 

 out stripes, more or 



Fig. 54. — Syrphus 

 larva eating plant 

 louse. 



Fig. 55. — A Tachinid fly: its eggs on body of 

 caterpillar, larva and pupa. 



less spiny, while a few are large, set with long bristles 

 and marked with contrasting colors, appearing formi- 

 dable and dangerous, even if they are not actually so. 



These flies lay their eggs on a variety of insects, 

 but more frequently on caterpillars, and may be said 

 to be specific checks to a great variety of cut-worms, 

 including the army-worm. The eggs are often laid on 

 the outside of the caterpillars just back of the head 

 where the insect cannot reach them, and as they are 

 white and of good size, they are easily seen. These 

 Tachinids come nearer to being able to keep up with 

 the increase of their hosts than almost any other forms 

 that I know, and I have on several occasions seen army- 



