246 



ELEMENTARY ENTOMOLOGY 



Horn-tails (Siricidae). The ovipositor of the horn-tails is cylin- 

 drical, more like a borer, and, as it projects from the abdomen, has 

 given the family its name. The eggs are laid within the stems of 

 grasses and various plants, such as berry canes and alder, while 

 some of the larger species deposit them in the solid wood of various 

 shade and forest trees, usually when the tree is beginning to die. 



FIG. 



a 



The pigeon tremex, or horn-tail ( Tremex columba) 



a, larva with young larva of Thalessa fastened to its side ; , its head ; <:, d, female and male 

 pupae ; <?, female. (After Riley) 



The larvae feed within these plants, tunneling out burrows, and 

 are difficult to combat. Fortunately but few are of considerable 

 economic importance. 



2. GALL-INHABITING HYMENOPTERA 



Gall-flies (Cynipidae). The gall-flies lay their eggs in the leaves 

 and stems of plants, and the injury done by the developing larvae 

 causes the formation of a characteristic gall by the plant tissues 

 surrounding them. The adults are small insects resembling wasps, 





