176 Proceedings of the Biological Society of Washington. 



these are, except in the case of the forms of general distribution 

 for which no specific records are given. All of these common 

 species have been collected on the island, except Microdon fusci- 

 pennis, Helophilus distinctus and H. chrysostomus , and these three 

 have all been taken on the Maryland shore near the island. 

 The number of additional species (including the three just 

 mentioned) that have been collected along the Potomac River 

 from Great Falls to Little Falls, the area most closely related 

 faunistically to Plummers Island, is 39. Specific records for 

 all of these are given. 



The 27 species not included in the above categories are almost 

 equally divided between Beltsville, Md., Washington, D. C, 

 and Falls Church, Va., an indication of the number of types of 

 collecting grounds in our area. 



ANNOTATED LIST OF SPECIES. 



Microdon Meigen. 



M. AURULENTus Fabrlcius. — Great Falls, Va., June 27, 1906, D. H. 

 demons. 



M. coARCTATUs Loew. — Originally described from the District of 

 Columbia. Glencarlyn, Va. , June 11, in open meadow; Falls Church, 

 Va., June 11, 1916, Banks. Larvae collected by T. E. Snyder in an 

 ant's nest under bark of chestnut oak log at Difficult Run, Va., July 4, 

 1916, pupated July 5, and adults emerged from July 10 to 12, Greene. 



M. CRAiGHEADi Walton. — Beltsville, Md. , July 26, 1914, L. O. Jackson. 



M. FusciPENNis Macquart. — Apparently the most common species of 

 the genus; has been taken in all sections of our territory from June 12 to 

 September 27. Flies low, hardly over a foot above the ground. 



M. GLOBOsus Fabricius. — Virginia, June 27, 1880, Theo. Pergande; 

 Great Falls, Va., June 25, 1915, September 26, 1913, Greene; Glencarlyn, 

 Va., July 2, 1916, Banks; Plummers Id., Md., July 6, H. S. Barber; 

 Lakeland, Md., June 25, 1906, D. H. demons. 



M. MEGALOGASTER Suow.* — The ouly definite record is Dixie Landing, 

 Va., June 1, this specimen being the type of M. bombiformis Townsend, 

 by whom it was collected. A specimen labeled District of Columbia, but 

 with no date nor name of collector, is in the collection of the U. S. 

 National Museum. 



M. RUFiCRUs Williston. — Great Falls, Va., June 25, 1915, Greene; 

 Chain Bridge, Va., June 5, Falls Church, Va., June 14, Glencarlyn, Va. , 

 June 7, Banks; Chain Bridge, Md., June 9, 1905, D. H. demons; Belts- 

 ville, Md., June 18, 1916, in copula, McAtee; Bladensburg, Md., June 



• See also p. 203. 



