100 
Psyche 
[June- August 
In Miastor as in all cyclorrhaphous diptera, the pupa is 
formed within the last larval skin. After the pupa-larva 
comes to rest the color of its skin changes from white to yellow, 
later to orange red and dark brown. 
Under optimum conditions a period of about five days is 
required for the metamorphosis of pupae into imagines. 
The causes which lead to the occurrence of the various types 
of larvae within a single species of Miastor are not known. I 
have observed all the various types occurring side by side in 
nature and in the laboratory. Springer’s^ belief that the yellow 
forms and pupa-larvae are produced by the action of light upon 
typical paedogenetic mothers does not seem tenable, since I 
have observed all types occurring in the laboratory in cultures 
reared in the dark as well as in those reared in the light. 
Method of Culture in the Laboratory. 
In order to carry on investigations concerning the factors 
regulating the life-cycle of Miastor and those causing polymor- 
phism among the larvae of a single species, it seemed immediately 
desirable to find a suitable artificial medium in which cultures 
might be reared in the laboratory under controlled conditions. 
With this in view I attempted to rear Miastor larvae on various 
media, finally meeting with success on a mushroom bouillon- 
agar innoculated with yeast. 
The mushroom bouillon is made by taking equal parts by 
weight of mushrooms and water. The mushrooms are boiled 
in the water for 20-30 minutes. At the end of this time the mush- 
rooms are withdrawn from the bouillon to which is added suffi- 
cient agar to make 4 per cent of the bouillon weight. The whole 
is heated until the agar is dissolved, when it is poured into small 
mouthed stock bottles stoppered with cotton, and sterilized; 
in this way the culture medium may be kept indefinitely. When 
it is needed for use, the medium is liquified in a hot water-bath 
and while still hot poured into culture dishes, and allowed to 
2Springer, Fritz. Polymorphismus bei den Larven von Miastor metraloas. Zool. Jahrb 
Abth. f. Anst, vol. 40 p. 57 (1917). 
