1919] Plath: Bloodsucking Fly Larva, in Birds' Nests 195 



the bone. Several other larvae were placed near a drop of human 

 blood, but they invariably crawled in the opposite direction as soon 

 as they came in contact with it. Similar experiments with ox blood 

 brought the same results. Larvae which had not reached their full 

 growth when taken from nests invariably died, unless they could 

 feed upon blood. On the other hand, full grown larvae always pupated, 

 even if they had no access to food. This shows that the larvae of 

 Protocalliphora azurca (Fallen) are dependent upon blood in order 

 to mature. 



The blood ingested by the larvae is stored in a kind of reservoir, 

 the diverticulum, which branches off from the esophagus close behind 

 the pharynx, and there retains its red color for three or four days, 

 gradually passing into the posterior end of the intestine as a dark 

 brown substance, which appears as a longitudinal streak in the middle 

 of the animal's body. If the larvae are not fed, this streak is gradu- 

 ally discharged as feces, so that after five to seven days more the 

 larvae become creamy white in color throughout. From this we may 

 infer that it is not necessary for the larvae to replenish their supply 

 of food at frequent intervals to attain full growth. 



When the larvae are about to pupate they crawl, anterior end 

 downward, into the feces which they deposit at the bottom of the nest 

 and there sometimes form a kind of disk, the lower side of which is 

 made up of the anterior ends of the larvae, and the upper side of the 

 posterior ends. About two weeks after pupation, the young flies emerge 

 from the lower side of the disk. In captivity the fly larvae, before 

 they pupate, usually attach themselves to objects located in dark places 

 by means of viscous fluid which solidifies a few moments after it has 

 been discharged. 



The power of resistance of the larvae was surprising. Several of 

 them, after having been immersed in a 70 per cent alcohol solution 

 for twenty-four hours, were still wriggling vigorously. Several others 

 which were being prepared for dissection were placed in a fixing fluid 

 for six hours, then washed in a 50 per cent alcohol solution and placed 

 in one of 90 per cent. They were still alive two days later and it was 

 necessary to resort to a stronger fixing fluid, such as Gilson's. to 

 prepare them for dissection. Other larvae were placed in a very 

 strong insect powder, but they remained alive in it for two or three 

 days. 



As soon as the larvae pupated the pupae were placed beneath 

 inverted tumblers. After the flies emerged from the pupae they were 



