Insects and Parasitic Diseases. 39 



was curved, tapered very abruptly, and ended in a finely pointed 

 tip. 



Fig. 22 shoAvs a parasite evidently in a slightly more advanced 

 state of development. It measured about 0.15 mm. long by 0.025 

 mm. wide at the anus, and was enclosed in a spherical cyst. The 

 process at the anterior end, previously referred to, was still to be 

 seen, although a change in the general outline of that end was 

 observed. The clear space seen at the anterior end of the body in 

 the preceding stage (Fig. 21) was not made out. The whole of 

 the body was composed of nuclei, those about the middle being the 

 largest. Excepting the rectum, which was large, there was no evi- 

 dence of an alimentary tract. The rectum was much distended, 

 but the anus, which was 0.026 mm. from the tip of the tail, was 

 closed as in the preceding stage. 



The larva seen in Fig. 23 showed still further development. In 

 this the outline of the anterior end was still more irregular than 

 in the preceding stage. A well-defined mass of large nuclei occu- 

 pied the greater part of the body from near the anterior end pos- 

 teriorly for a length of 0.082 mm. From the posterior end of this 

 mass to the rectum tliese nuclei were somewhat smaller and more 

 scattered and the rectum was rather smaller than in the preceding 

 stage. The anus w^as 0.026 mm. from the tip of the tail, and was 

 closed by a rounded projection as in earlier stages. The length 

 of the parasite was about 0.175 mm. 



Reference to Experiment No. 13, page 32, shows that these five 

 parasites were ten days old when examined, and that the period of 

 their existence in the Stcmioxys larva could not have exceeded 

 eight days. Presumably the two least developed forms found their 

 way into the body of the intermediate host later than the three more 

 advanced ones. 



Larval stages comparable with those shown in Figs. 21, 22 and 

 23 are not known in H. muscae, but probably they do occur between 

 the stages illustrated by Fig. 4 and Fig. 5. 



The earliest stage in Avhich a definite oral opening is known to 

 occur is shown in Figs 24 and 25. This larva, together with several 

 others in the same stage of development, was found on January 23rd 

 in a newly-formed pupa from the same culture as the larvae shown 

 in Figs, 21, 22, and 23 (i.e., Experiment No. 13). It was therefore 

 nine days old, not more than seven days of which period had been 

 spent in the body of the fly larva. It measured 0.24 mm. long. 

 The oral cavity was apparently closed at its junction with the 

 pharynx (?). The body, which was 0.24 m.m. long, was narrowest 



