192 University of California Publications in Zoology [Vol. 19 



contained nine young birds, four of which died shortly before they 

 were full fledged. The fly larvae were creamy white in color, about 

 1.5 centimeters in length, and 0.5 centimeters in width. A number of 

 them contained a bright red substance which looked like blood. From 

 this I concluded that the larvae were blood-sucking parasites, and that 

 the young birds had died from loss of blood. Both larvae and pupae 

 were destroyed, as I supposed that this form of parasitism was no 

 dmibt generally known to zoologists. 



Later in the summer I met Dr. C. A. Kofoid. head of the Depart- 

 ment of Zoology of the University of California, and told him about 

 the larvae and pupae. He regretted that they had been destroyed and 

 suggested that I look into this matter more closely the following 

 summer. It was impossible for me to do this, however, until the 

 summer of 1917. 



The period of investigation which is now about to be discussed 

 extended from the latter part of June, 1917, until about the middle 

 of September. The work was carried out under the supervision of 

 Professor W. W. Cort of the Department of Zoology of the University 

 of California, to whom the writer is indebted for a number of important 

 suggestions. 



The first nest examined, that of a Nuttall sparrow (Zonotrichia 

 h ucophrys nuttalli Ridgway), contained thirty-six, full grown fly 

 larvae, and thereafter the larvae were found at various stages of 

 development, ranging from 0.3 centimeter in length and 0.1 centi- 

 meter in width to 1.5 centimeters in length and 0.5 centimeter in width, 

 in about two out of every three nests (the exact figures are shown in 

 tables 1 and 2 of this paper). Nearly all of the small and half grown 

 larvae showed a bright red substance in the anterior part of their 

 intestine, while, on the other hand, this was the case with only a very 

 few of the full grown larvae. Smears made from this red substance 

 and examined under a high power microscope showed that it was 

 vertebrate blood, which the maggots could have obtained only from 

 the birds inhabiting the nests. To prove, this beyond a doubt a number 

 of experiments were carried out. Several dozen fly larvae were placed 

 in a number of bird's nests containing young and the latter observed 

 from day to day. Some of these nestlings died and the others seemed 

 to be retarded in their growth ; but otherwise these experiments fur- 

 nished no absolute proof that the vertebrate blood found in the larvae 

 was avian blood, for I had never seen any of the fly larvae attached 

 to the young birds. 



