42 



ECONOMIC ENTOMOLOGY 



Tragus, which parasitizes the chrysalids of Papilio; and Ophion, a 

 form with a compressed body, which lives on the Polyphemus moth 

 and yellow-necked caterpillar. 



The Braconids (Braconidce) are smaller and are also parasitic. The 

 most common genera are Microgaster, whose cocoons are often found 

 on the backs of sphinx and cabbage butterfly larvae; and Aphidius which 

 parasitizes plant-lice. 



Chalcids or chalets flies iChalcididcB) are minute metallic insects. 

 One species, Pteromalus puparum, is a parasite of the chrysalids of the 



Fig. 41. — Female of Megarhyssa (Thalessa) ovipositing. 



cabbage butterfly. Another, Aphelintis, is parasitic on scale insects 

 (see also p. 354). 



Proctotrypids are very minute parasitic hymenoptera often prey- 

 ing upon the eggs of other insects. 



Besides these parasitic forms there are predaceous Hymenoptera, 

 such as the mud-wasps, digger-wasps, wood-wasps, etc., that feed their 

 larvae on insects. They fill the brood cells with caterpillars, grass- 

 hoppers, plant-Hce, or grubs of small beetles or flies. Sphecius makes 

 use of cicadas as food for its young. 



Several families of the Diptera are either parasitic or predaceous. 

 The two best known are the Tachinids and the S>Tphids. 



Tachinids or tachina flies {TachinidcB) are bristly flies closely related 

 to the ordinary house-flies. They parasitize many kinds of caterpillars 



