202 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



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Figure 96. — The western horntail (Sirex aerolatus) : A, Adult male, X 1.5; 

 B, female, X 1.5; C, larva, X 3; D, pupa, X 3; E, exit holes; F, larval 

 mines. (Edmonston.) 



thickened and curled, are made by bark maggots of the genus 

 Cheilosia (20). This type of defect is very prevalent in western 

 hemlock growing in western Washington and Oregon at elevations 

 below 1,800 feet and is called the "black check" of hemlock. These 

 defects do not impair the wood for structural purposes but render 

 a high percentage worthless for finishing wood, staves, or other 

 special purposes. 



The adults are small two-winged flies which lay their eggs on 

 the bark of the trees, probably on the resin which exudes from 

 feeding punctures made by the hemlock hylesinus. The maggots 

 enter the bark, making a small wound, and feed on the sap and 

 inner bark. The larvae, when full grown, are white maggots % 

 inch long, with the fore part of the body thickened and with a 

 long, telescopic, protractile tail. Feeding continues for several 

 years, then in the spring puparia are formed in the resin mass at 

 the entrance to the wound, and the adult flies emerge in April and 

 May. 



Two species have been recognized as important in the West. 

 The hemlock bark maggot (Cheilosia alaskensis Hunter) makes 



