182 
Dipterous Larvae 
perishes in large numbers after egg-laying at the beginning of the rainy 
season and during dry spells in and at the end of that season....” 
6. Sciomyza dubia Fin. 
In his report on land and fresh water Mollusca observed in Hertfordshire, 
C. Oldham writes (1912, p. 288): “Among some Mollusca collected in February 
on a hedge bank at Tring, I found dead shells of Vitrea rogersi, V. nitidula, 
V. cellaria and Pyramidula rotundata, each of which had in its last whorl a 
small reddish pupa. From these several specimens a dipteron, Sciomyza dubia, 
hatched out in April. Almost nothing is known of the life-history of this fly, 
and I cannot say whether the larva is parasitic on the living Mollusc, kills 
and subsequently devours it, eats the body of the snail which has died from 
some independent cause, or merely chooses the empty shell as a convenient 
place in which to pupate.” 
In a letter to me, dated 26. xii. 1919, C. Oldham mentions that in the 
autumn he often finds on the chalk downs near Berkhamstead, snails ( Helicella 
and others) which harbour fly larvae of an undetermined species; these larvae 
are sometimes seen free in the collecting tubes containing the snails. 
7. Phoridae. 
Several species of Phorids which usually inhabit the dead snails have been 
recently carefully studied by Lundbeck (xn. 1919) and as a result of his study 
the following changes have to be introduced in my previous list of these in¬ 
habitants of snails: the Phora sp. No. 2, the pupae of which I have found in 
Helix nemoralis collected in the Bernese Oberland, Switzerland (1911, p. 60, 
and 1919, p. 450), have been identified by Lundbeck as Phora or Paraspini- 
phora notata Zett. 
Phora bergenstammi Mik. The Phorid, the complete life history of which I 
described under this name (1911, pp. 31-56, and 1919, p. 449), was proved by 
Lundbeck to be Phora domestica Wood (1906). 
Phora domestica Wood was considered by Malloch (1910) to be synonymous 
with Ph. bergenstammi Mik., and this was accepted in 1912 by Wood himself. 
Lundbeck has proved, however, that these two species, although closely allied, 
differ in several respects. 
Phora bergenstammi Mik. has “4 dorsocentral bristles in both sexes, and 
the male palpi almost nude”; “the costa (in both sexes) reaches beyond the 
middle of the wing, the first costal division is at most double as long as the 
second, and the fourth vein is rather strongly curved at the base." 
Phora domestica Wood has “4 dorsocentral bristles in the female and 2 in 
the male and the male palpi with normal armature.. . “the costa in both 
sexes does not reach quite to the middle of the wing, the first costal division 
is more than double as long as the second and the fourth vein is less curved 
at the base; further, domestica seems always to have pale hind margins to the 
abdominal segments not present in bergenstammi .” 
