274 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Sek. 



upon the pollen and nectar of flowers and are of no economic 

 importance. 



The larvae are typically slug-like in appearance. As a 

 group they are considered beneficial and feed upon plant lice. 

 As with the adults there is great variation in appearance 

 among the less typical forms. In the genus Microdon the 

 larvae resemble molluscs and live in the nests of ants. Others 

 are found in the nests of bumblebees and wasps. A large 

 group, termed rat-tailed larvae, have a slender posterior pro- 

 longation with terminal spiracles. These forms may live in 

 stale pools of water or tunnel deeply into the sap or decay 

 of trees. A few forms are injurious, tunneling into live bulbs 

 and woody plants. These include such forms as the narcissus 

 bulb maggot, Merodon equestris; the onion maggot, 

 Eumerus strtgatus; and Chilosia alaskensis, causing black 

 check in Western Hemlock. 



434. Microdon cothurnatus Bigot 



Hood River, V-19 to 21 (Cole). Found mostly in an 

 ant's nest under the bark of an old pine log, ten adults being 

 taken and a number of pupae. There is reason to believe that 

 species of Microdon return to the old nest year after year 

 and this certainly appeared to be the case here, as some of 

 the old pupa cases had been there three or four years. The 

 type of this species was taken on Mt. Hood. 



435. Microdon marmoratus Bigot 



Hood River, VI- 14 (Cole); Mosier, VI- 16 (Cole and 

 Childs). 



436. Microdon piperi Knab 



Corvallis; Hood River, VI (Cole) ; Mt. Jefferson, VIII. 

 Listed from Oregon by Williston as tristis Loew. On March 

 10, 1915, Childs collected larvae in a colony of ants, under 

 heavy bark of an old fir stump. 



437. Chrysotoxum derivatum Walk. 

 Mt. Jefferson; Mary's Peak, VI- VIII (Lovett). 



