I). Keilin 
431 
INTRODUCTION. 
This paper consists of two parts: (I) a complete account of the life-history 
and a description of early stages of Melinda cognata Meigen which I found here 
in Cambridge living as parasites on a common snail, Helicella virgata da Costa, 
and (II) a review of all observations made hitherto on the life-history of 
different flies feeding upon living or dead molluscs. 
Acknowledgments: I am much indebted to Mr C. J. Wainwright for the 
identification of my specimens of Melinda cognata Meig. and those of sar- 
cophagids. I owe also to him, and to Dr J. Villeneuve, valuable information 
concerning different species of Onesia flies. I am grateful to Dr Th. Becker 
for the identification of some of my specimens of Phorids and the description 
of a new species of Phora. The identification of different snails I owe to the 
kindness of Mr H. H. Brindley. Finally I have to acknowledge the very 
friendly assistance rendered by Mr F. Balfour-Browne in revising my manu¬ 
script for the press. 
I. SYSTEMATIC POSITION OF THE GENUS MELINDA ROB.-DESV. 
AND ITS RELATION TO THE GENUS ONESIA ROB.-DESV. 
Before we begin the study of the life-cycle of Melinda cognata Meig. it is 
important to deal with the systematic position of this fly. 
The genus Melinda was founded by Robineau-Desvoidy in 1830 for a few 
species of flies which he placed near the genus Calliphora in his group of 
Diptera ovipara. Later on, Schiner (1862) and almost all other entomologists 
placed the genus Melinda as a synonym of the genus Onesia which was also 
created by Robineau-Desvoidy. In 1901, Hendel came back to the old idea of 
Robineau-Desvoidy in separating from Onesia a few species under the generic 
name of Melinda , but most entomologists although agreeing that Onesia con¬ 
tained two groups of species, those of the cognata type and those of the 
sepulchralis type, nevertheless continued to place all these species in one 
genus. 
As we shall see presently, the separation of the genus Melinda from Onesia 
is sustained not only by differential characters of the adult flies but also by a 
comparative study of their life-history and larval morphology. 
As on the other hand, these two genera were confused, the different 
characters of the species of one genus were often wrongly used in making 
generalisations regarding species belonging to the other genus. 
Therefore, to avoid confusion, we should examine separately and in 
chronological order all we know of the life-history of species belonging to the 
genera Onesia and Melinda. 
Parasitology xi 
28 
