388 ROBERT W. HEGNER 



Metschnikoff's ('66) studies on Miastor indicated that the 

 germ-cells of this fly are set aside very early in embryonic devel- 

 opment, and led me in 1907 to attempt to obtain material of 

 this genus. I was informed at that time by Prof. Samuel W. 

 Williston that no paedogenetic Diptera were known to occur in 

 this country. On October 5, 1910, however. Dr. E. P. Felt 

 discovered great numbers of the larvae of Miastor americana 

 Felt under the partially decayed inner bark and in the sapwood 

 of a chestnut rail near Highland, New York, and kindly sent me 

 an abundant supply of material. 



Habitat and life history. Dr. Felt found the larvae of Miastor 

 americana lining 



. . . . in the moist, partly rotten inner bark and punky sap- 

 wood which has not been invaded to any considerable extent Ijy other 

 Dipterous larvae or Coleopterous borers. They exhibit a tendency to 

 occur in segregated masses, frequently between loose flakes of bark or 

 in rather broad crevices. These colonies contain in autumn old empty 

 skins of mother larvae; a number of yellowish mother larvae with 

 approximately five to fifteen young within; very numerous, small 

 yellowish larvae showing no trace of embryos; a number of white, 

 various sized larvae, frequently white, sometimes semi-transparent; 



and a few quiescent white larvae containing young embryos 



The mouth parts of the larvae, though the anterior portion of the head 



is strongly chitinized, appear to be comparatively weak 



The ahmentary canal contains little that can be discerned with the 

 aid of a compound microscope, and we are inclined to believe that a 

 considerable portion of their nourishment is absorbed by osmosis after 

 escaping from the mother larva, as well as before. It would appear 

 as though the several types of larvae occurring in a colony are possibly 

 only modifications due to the relative amount of nourishment obtained 

 by the individual. 



Normally, reproduction by paedogenesis occurs throughout the 

 warm months of the year and even into late fall, and commences in 

 early spring, the cold weather of winter simply causing a suspension 



of activities The adults of Miastor and Oligarces occur 



in midsummer, a season when midges of most of these forms are prob- 

 ably abroad (Felt, '11). 



Methods. A number of fixing and staining methods have been 

 employed in an endeavor to determine the origin and history of 

 the 'polares Plasma' which plays such an important role in the 

 differentiation of the germ cells of Miastor. Perhaps the easiest 



