M. D. Haviland 
171 
in Stratiomys strigosa. In comparing the present species with the Braconidae- 
Flessili ventres, such as Aphidius , in which a trophic membrane is also formed 
round the embryo, there appears to be a tendency to lengthen embryonic at 
the expense of larval development. Thus in Aphidius , the membrane is cast 
off early, and the caudate larva moults twice before assuming the typical grub¬ 
shaped form; whereas in D. areolaris the larva begins to ingest food while 
still within the membrane, and after throwing off the latter and the first larval 
skin together, it at once appears in the typical form. Fig. 2 shows the only 
stage found in a large number of Phytomyzid larvae of all ages which were 
examined; and this suggests that, as the parasite oviposits when the host is 
very young, development before the metamorphosis of the latter does not 
proceed beyond a certain limit, although there is some increase in the size 
of the embryo, proportionate to the growth of the host. 
This adaptation may be correlated with the very complete metamorphosis 
of the host. In the early stages, the only nourishment taken is that which 
transfuses through the trophic membrane, but when the host forms its 
puparium, the parasite begins to ingest the surrounding tissues, and the mid¬ 
gut is filled with food. The mandibles of the hymenopterous parasites are 
better adapted for prehension than for mastication; and Aphidius and others 
break down the host’s tissues by a kind of “external digestion.” But if 
parasites of holometabolous insects can in any way delay growth until the 
metamorphosis of the host takes place, the organs of the latter will of them¬ 
selves disintegrate and undergo profound physical and chemical changes, 
providing what is presumably a suitable pabulum for the further development 
of the parasite. Unfortunately the literature of insect parasitism throws little 
light on this subject, but it has been suggested by me elsewhere (1921) that 
the early form of Charips, a Cynipid endoparasite of Aphidius , may be a kind 
of resting stage. 
It is obvious that it would be to the advantage of the parasite that the 
host should attain the full size before death or metamorphosis, in order that 
plenty of food should be available for its own subsequent development. The 
ectoparasitic Chalcids mentioned below probably rely partly on bacterial 
action for the disintegration of their food, and begin to feed immediately after 
hatching, no matter what the size of the host may be. As a result, there is 
considerable diversity of size of the imagos, and this may be suggested as the 
cause of the occasional disability of the parasite to complete its transformation. 
This diversity of size, owing to variability of nutrition, may lead to far- 
reaching results. 
Keilin (1915) observed that the size of the imagos of Pollenia rudis Fab. 
is determined by the dimensions of the host, or by its accidental death before 
the parasite larva is full fed; and he points out that if the difference in size 
should make mating impossible between large and small forms, distinct races 
might arise within the species. He cites the observations of Pantel, according 
to which the Tachinid, Meiyenia floralis, typically a parasite of Crioceris 
