268 



MR. B. F. CUMMINGS ON 



Stage III. — Measurements {millimetre scale). 



The Metamorphosis of Polyplax spinulosa {Barm.) 

 for Comparison. 



From a large amount of material from this common parasite 

 of the Rats Mus norvegicus and M. rattus, presented to the 

 British Museum, along with other species, by the Lister Institute, 

 it has been a simple, if laborious, matter to sort out all the 

 immature forms, which fell into three stages as in P. oxyrrhynchus. 



Larva, Stage I. (Text-fig. 15.) — This is a tiny, delicate insect, 

 with a rounded head, the postero-lateral angles being very 

 slightly developed. On the dorsal surface of the head there is a 

 suture between the two epicranial plates, which in front bifurcate 

 and so divides the two epicranial plates from the single plate — ' 

 frons — which roofs in the anterior part of the head. Sclerites 

 on the abdomen absent. 



Chaetotaxy of head and thorax as in the imago. On the 

 abdomen two bristles in each tergum. Sterna bare except the 

 last, which, as in P. oxyrrhynchus, has two bristles. Pleurae bare, 

 except that in the pleural region on each side of the last segment 

 are two very elongate hairs — one dorsal and one ventral. Spiracles 

 relatively large. 



