30 THE HESSrAN FLY IN THE UNITED STATES. 
contain them, these larvae are always lodged in the interior of the nervous system 
of the host larva, and there they bring about alterations and proliferations of a very 
curious character. The most frequent position is at the posterior of the extremity 
of the nerve chain, where the cyst of the parasite is formed. This extremity spreads 
out into an enormous bouquet of club-shaped, giant cells, which alone fills the 
larger part of the body cavity of the host. The larva of the parasite is lodged in a 
cyst filed with liquid, the cellular structure of which, with broad, polygonal con- 
tour, seems to indicate an amniotic envelope in a condition of retrogression. All 
around this membrane the giant cells are grouped. These exist not only in the 
immediate neighboihood of the cyst, but all the surrounding region of the nerve 
chain seems to have undergone the same degeneration and growth of giant cells. 
The youngest cells are hyaline, and present a fibrinous, longitudinal structure. The 
oldest cells are filled with fatty globules, and become entirely opaque. The giant 
cells increase and isolate vesicles, which separate and fall into the body cavity in 
the form of protoplasmic spherules, which are absolutely characteristic. When one 
dissects a Cecidomyid larva under the microscope he can be sure, if he sees these 
spherules floating in the liquid, that there are in the preparation one or more larva? 
of this parasite. The localization of the larva? of the Trichasis in the nerve chain 
or in the nerves of the larvae presupposes that the parasite pierces the egg or the 
young larva upon the median line at the time when the nervous system has not 
begun to branch and is concentrated in a single ventral band. The mass of giant 
cells evidently accumulate in themselves the nutritive material necessary to the 
parasite. They are a kind of internal gall, developed by the presence of the para- 
site. The Trichasis, in the condition of the cyclops-like larva, waits in its cyst 
until the tissues which surround it have submitted to the transformations by which 
it profits later for its food; then, when the host larva, exhausted by its presence, is 
transformed into a sort of sac filled with giant cells, it issues from its cyst to devour 
the accumulated material, which, probably, has nutritive qualities nearly identical 
■with those of the vitellus. After undergoing successive transformations into three 
larval forms the adult insect finally issues from the puparium of the host, only one 
adult finally making its appearance from an individual puparium, although in the 
cyclops stage four larva? may be present. There seems in this first stage to be a 
physiological competition between Trichasis larvae, only the oldest surviving to 
take on- the second stage. An interesting point is that there appear to be definite 
molts from the first to the second and from the second to the third stage, and that 
the dead bodies of the cyclops larvae which succumb do not interfere with the devel- 
opment of the survivor. 
Another species to which Marchal has given careful study is the 
Polygnotus minutus Lindni., and which he has localized in the stomach 
instead of the nervous system. As many as ten or twelve were found 
grouped together and developing simultaneously, and destined, all or 
nearly all of them, to complete their development. 
Quoting again from Howard: 
The group of young larvae forms a mass situated in the interior of the stomach. 
It is surrounded by a hyaline and perhaps adventitious membrane. Each para- 
sitic embryo is also surrounded by a membrane of its own. The larva is elliptical, 
somewhat attenuated at its posterior extremity, and provided with rather well- 
devoloped mandibles. They fill the gastric cavity, which is generally distended. 
The second and third larval forms follows The host is almost entirely devoured 
and reduced to a cutaneous sac. When ready for pupation they occupy the entire 
body cavity of the host, the skin being distended and showing by impressions 
the positions occupied by the contained parasites, thus appearing full of minute 
