D. Keilin 
435 
along the Hills Road near the ‘'Gogs 1 .” In all these places, and especially 
along the footpath mentioned above, a great number of the snails contained 
Melinda larvae. It is interesting to note that in all these places, and especially 
along the Grantchester Road, this snail was found in the company of Helicella 
itala, H. cantiana, Hygromyia rufiescens, Helix nemoralis and H. hortensis 
but in no case were any of these species parasitised by the fly. Further, a white 
non-banded variety of H. virgata found in abundance along the iron fence 
between the Cains ground and Church Rate Walk, seemed to have escaped 
the attack of the fly, as I collected several hundreds of specimens on different 
days and never found Melinda larvae. On the other hand I found the larvae in 
the same variety of snails collected in other places. 
(b) Melinda cognata Meig. and its life-cycle. 
As previously mentioned, Robineau-Desvoidy placed this fly (M. cognata) 
among the oviparous Diptera. Most probably he relied upon the fact that 
dissection of the female never revealed the existence of larvae as one would 
expect to find in a viviparous insect. But the real proof of oviparity or 
viviparity is to see the eggs or larvae at the time they are laid. I have often 
found the eggs of Melinda in ones or twos or very rarely threes in the mantle 
cavity of H. virgata near theBojanus organ or kidney of the host(ov. PI. XXIII, 
fig. 3). They were usually in different stages of development, one being newly 
laid while the other contained a well-developed embryo, or there might be 
an egg and one or two larvae showing that .all the eggs had not been laid at 
the same time. Unfortunately I was never able to observe the actual de¬ 
position of an egg, but I imagine that the fly oviposits near the pneumostome, 
or directly in the mantle cavity, as the pneumostome is often widely open 
especially when the snail is in motion, but even in the snail contracted within 
its shell it opens and closes rhythmically. It is probable that the most favour¬ 
able time for infection is during the copulation of the snails, when the latter 
are incapable of rapid contraction, but infection certainly takes place at other 
times also, as I have found Melinda larvae in immature snails. 
From the egg there emerges a small larva which by means of its strongly 
chitinised mouth-parts bores its way into the kidney of the host where it lies 
with its posterior end protruding into the mantle cavity or pulmonary cavity 
of the snail ( L . PI. XXIII, fig. 3). This position is obviously connected with 
the fact that the larva is metapneustic and requires to keep its only pair 
of spiracles exposed to the air. 
During this stage the larva feeds upon the fluid of the kidney and destroys 
the renal epithelium the cells of which contain crystals of ammonium urate. 
Thus the alimentary canal of the parasite becomes filled with these crystals. 
So far, the snail shows no signs of distress, one cannot distinguish a healthy 
snail from an infected one without removing it from its shell. If this is done, 
1 T have to express here my best thanks to Capt. J. Munio for kind help in collecting the snails. 
