24 NORTH AMERICAN DIRTERA. 



ows in the Eoeky Mountains the writer lias seen them rise at 

 nightfall in the most incredible numbers, producing a hum- 

 ming noise like that of a distant waterfall, and audible for a 

 considerable distance. While at rest they usually raise tlieir 

 forelegs in the air and keep them constantly vibrating. 

 Aquatic larvae may be frequently met with in standing water, 

 often extremely delicate little creatures, so transparent as to 

 be hardly distinguishable; they have been dredged from 

 nearly one thousand feet below the surface of Lake Superior. 



Most of the species are inoffensive, or actually beneficial as 

 scavengers. There are some, however, belonging to the genus 

 Cevatopogon, and its allies, and knoAvn generally as midges, 

 or punkies, which have the power of sucking blood and are 

 extremely annoying. In the White Mountains, at the sea- 

 shore, along mountain streams generally, and in the West 

 Indies they are especially troublesome. The larvai live in the 

 flowing sap of trees, in decaying vegetation or under fallen 

 leaves. 



The following table, largely adopted from V. d. Wulj), 

 contains many genera heretofore known only as exotic, not a 

 few of which will doubtless be found in the United States 

 when the family has received more attention with us. 



TABLE OF GENERA. 



1. Antcnniu of tlio J long-plumose or ponic'ilhito 2 



Anterinffi of the Z with short hairs. l;^> 



2. Second posterior eell wanting and hence no second basal cell. . ."> 

 Second posterior cell present, the second hasal cell coni])lete. . 12 



o. Fourth longitudinal vein furcate. ...... 4 



Fourth longitudinal vein not furcate; antenniB with an unequal num- 

 ber of joints in the two sexes; antennal joints of the Z plumose to 

 the tip; thorax ])rojecting in front over the head. ...;'> 



4. "Palpi conii)osed of three joints". . . Tersesthks Townsend. 



I'alpi com])()sed of four joints ; antennte of the male penicillate, 



the last joint bare ; dorsum of the thorax not produced over the 



liead Ceuatopogox Meigen. 



