88 PROC. ENT. SOC. WASH., VOL. 24, NO. 4, APR., 1922 



females but represent races, varieties or species. The figures 

 of the anterior extremity of the mouth hook of the three types 

 show the precise nature of the differences between the three 

 varieties: while the drawings of the thoracic region show the 

 differences in the pattern of the spine bands as well as the differ- 

 ences in the dimensions of the first stage larvae. It should be 

 noted, however, that the figure of the larva of the gratiosa 

 variety was prepared from a specimen taken from a caterpillar 

 of the Borer and that consequently this specimen is probably 

 larger than are the newly born larvae of this type. Finally, 

 although the body and mouth armature of the larvae of the 

 females taken at Haslar are uniformly larger than those of the 

 larvae of the specimens from the Cambridge female, the felt- 

 chambers of the posterior spiracles are of the same size in these 

 two forms. 



' 





Figure 16. P. senilis, variety "c," thoracic region of first stage larva: 17, 

 senilis, typical form, thoracic region of first stage larva: 18, senilis, var. 

 gratiosa, from caterpillar of Pyrausta nnbilalis, thoracic region of first stage 

 larva. 



The differences between the adult flies corresponding to the 

 three forms of primary larvae are slight and have to do chiefly 

 with the color and distribution of the pollen of the head and 

 thorax. The adults collected at Haslar, which may be con- 

 sidered as the typical senilis, since they agree with a specimen 

 of this species which Dr. Villeneuve kindly supplied from his 

 own collection and with a female ^containing undeveloped 

 larvae) so determined by him and collected at Auch, have the 

 pollen of head and body yellowish or golden. The specimens 

 reared from the Corn Borer, determined by Dr. Villeneuve as 

 senilis var. gratiosa B. B., have the pollen of the head and body 

 blue or bluish grey: but as the specimen of the third variety 

 collected at Auch is also bluish grey and was also determined 



