4 86 



Saw-flies, Gall-flies, Ichneumons, 



comstockii, which infests various caterpillars. Its larvae are external para- 

 sites clinging to the skin of the caterpillar. The chalcis-flies may usually be 

 recognized by the characteristic branched single vein of the fore wings (Fig. 



673)- 



The economic importance of the hymenopterous parasites is obvious; 

 from the point of view of the economic entomologist there are no other 



FIG. 686. Larva of Xylina lacticinerea, green-fruit worm, killed by the parasitic grub 

 of Mesochorus agilis, which has spun its cocoon beneath the caterpillar, fastening 

 the latter to the leaf. (After Slingerland; natural size.) 



insects outside of the pests of such interest as these natural pest-fighters. 

 Attempts have been made to make allies of them in man's warfare against 

 injurious insects by artificially disseminating them, even to the extent of 



colonizing by importation from foreign 

 countries various new species in partic- 

 ularly pest-ridden localities. In Cali- 

 fornia a constant and aggressive war has 

 to be maintained by the fruit-growers 

 against many insect pests, and particu- 

 larly against the scale-insects. In this 

 warfare a number of attempts have been 

 made to introduce from other continents 

 parasitic enemies of the scales. Unques- 

 tionably considerable success has attended 

 some of these importations, although as 

 yet no other such signal overcoming of an 

 insect pest by the use of these Hessians 

 has occurred as attended the importation 

 from Australia, several years ago, of the predaceous ladybird-beetle (Vedalia), 

 enemy of the once dreaded fluted scale (see p. 189 for account of this). 

 Any discussion of the parasitic families of Hymenoptera would be incom- 



FIG. 687. A caterpillar of Xylina 

 lacticinerea, green-fruit worm, from 

 which the parasitic larva of Meteorus 

 hyphantrice has just emerged and 

 is spinning its cocoon. (After Slin- 

 gerland; natural size.) 



