The Two-winged Flies 323 



practically immersed in plant-sap, the thin body-wail acts as an osmotic 

 membrane through which an interchange of fluids takes place automati- 

 cally. The Cecid larva has to eat whether it will or not, and has to eat 

 practically all of the time! These larva; may be distinguished by their 

 possession of a strange little chitin plate on the under side of the front part 

 of the body, called the breast-bone. What the exact use of this little sclerite 

 is has not yet been determined. Perhaps it helps in locomotion, perhaps 

 in rasping or lacerating the soft plant-tissue to increase the flow of sap. The 

 larvas pupate .where they lie, sometimes spinning a thin silken cocoon, some- 

 times transforming within the hardened last larval moult, sometimes with 

 no special protecting covering at all. 



The most notorious gall-gnat is the wheat-pest, known as the Hessian 

 fly, Cecidomyia destructor, and distributed over all the United States east of 

 meridian 100°, as well as in California. By the ravages of its larva?, feeding 

 as they do on the sap of growing wheat, this minute fly causes an annual loss 

 in this country of approximately ten million dollars. This enormous direct 

 tax is paid by those farmers who prefer to farm in the good old way, with a 

 strong belief in the dispensations of an erratic Providence, rather than to 

 do their farming as modified by modern knowledge and practice. The 

 tax-collecting insect, which is a tiny delicate blackish midge about one- 

 tenth of an inch long, lays its eggs in the creases or furrows of the upper 

 surface of the leaves of young wheat, and the hatching larvae wriggle down 

 to the sheathing bases of the leaves, where they lie and drain away the sap 

 of the growing plant. When full-grown they pupate withii) the outer hardened 

 brown last larval cuticle, and resemble very much a small spindle-shaped 

 seed. This is called commonly the "flaxseed" stage. The adult soon 

 issues and after a few days of flight and egg-laying dies. There may be as 

 many as four or five generations in a year, both spring and winter wheat 

 being attacked. The remedies are the late planting of winter wheat, the 

 burning or plowing in of the stubble after harvesting, and the early planting 

 of strips of decoy wheat about the field, which shall attract the egg-laying 

 females and may be afterwards plowed under with the myriad eggs it contains. 

 The Hessian fly is a European insect brought unintentionally to this country 

 about 1778, but probably not, as often said, with the straw brought by the 

 Hessian troopers of the Revolutionary War. It attacks rj-e and barley as 

 well as wheat, and has, in turn, to withstand the com.bined attacks of half 

 a dozen hvmenopterous parasites, which are said to destroy nine-tenths 

 of all the Hessian-fly larvae. Without these natural checks to its increase 

 this pest would destroy every wheat-field in this country in a \try few 

 years. 



In 1896 the Monterey pines, Pinits radiata, much grown, together 

 with the famous Monterey cypresses, as ornamental trees on the San Fran- 



