146 NORTH AMERICAN DIPTERA. 



leaves or flowers, in such places as are frequented by the hosts 

 which they parasitize. Not a few will be caught with the 

 beating net. 



The larvae of the more typical Tachinidae are thickened, 

 cylindrical and flattened below, the segments indistinctly 

 differentiated, with transverse and lateral swellings, bare or 

 provided with fine, short spines. They are amphipneustic, 

 the anterior spiracles small and point-like or multipartite, the 

 posterior stigmatic plates large, strongly chitinized, and each 

 with three, internally convergent grooves. The antennae are 

 wart-like, with two, ocellus-like, chitinous rings, one lying 

 below the other. There but two mouth-hooklets, porrect and 

 but little curved. The puparia are oval, with the segments 

 slightly differentiated ; the skin is finely wrinkled, and both 

 ends are rounded. 



In Ocijptiiva and Gymnnsonia, and probably in allied forms, 

 the larvae have a chitinous, anal stigmatic tube ; and the pu- 

 paria have six or two, similar processes. The larvae of Hyal- 

 omi/ia are translucent, smooth and metapneustic, the mouth- 

 hooklets very large; there are two, short, divergent, anal tubes. 



The larvae of this group are all parasitic in habit, so far as 

 is known, and the parasitism is probably confined to the early 

 stages of other insects. Their usefulness in kee])ing injurious 

 insects in check is immeasurable. 



By far the largest number of species are parasitic upon 

 Lepidoptera, of which not less than four hundred have been 

 recorded. About seventy species are known to be parasitic 

 upon Hymenoptera, less than forty upon Coleoptera, eighteen 

 upon Orthoptera, five u[)on Hemiptera, and as many upon 

 other Diptera. 



More particularly the observed parasitism of some of the 

 North American genera is as follows : 



AcHAKTONEUKA, Culoptenus, Dcinais, Citheronia, Datana, 

 Pt'odenia, Spli'uix, Vanessa, Anisota, Athens, ClisiocamjM, 

 Jle/iotis, J'nx/f'in'.a.efr.: AKUVKopuvr-AX : Eurka-tes, Srhlz- 



