The Two-winged Flies 323 
practically immersed in plant-sap, the thin body-wall ^cts 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 larvae 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 
larvae 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 ioo°, as well as in California. By the ravages of its larvse, 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 within 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 rye and barley as 
well as wheat, and has, in turn, to withstand the combined attacks of half 
a dozen hymenopterous 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 very few 
years. 
In 1896 the Monterey pines, Firms radiata, much grown, together 
with the famous Monterey cypresses, as ornamental trees on the San Fran- 
