98 Helicosporidium parasiticum 
Walk, and (2) a horse-chestnut standing between the School of Agriculture 
and Downing College. 
In addition to the new .parasite, the larvae of Dasyhelea harbour several 
other parasites, two of which—(1) a gregarine, Allantocystis dasyhelei Keilin, 
and (2) a parasitic yeast Monosporella unicuspidata Keilin—have already been 
dealt with in my previous papers (1920 a and b). 
To find the larvae parasitised with Helicosporidium , one proceeds in the 
following manner: to a small quantity of the semi-fluid exudate collected from 
the wounds of trees, is added ordinary tap water sufficient to cover the bottom 
of a wide dish. The mixture is then thoroughly stirred and is left for an hour 
or more to settle. The numerous larvae of various sizes found at the bottom 
of the dish are then transferred by means of a pipette to a dish containing 
clean water and standing on a black or dark coloured surface. On careful 
examination of the contents of this dish it is noticeable that while the normal 
Dasyhelea larvae present a whitish but translucent appearance, a few individuals 
are usually seen which are white and opaque. Isolated on a slide and examined 
under the microscope, all these opaque larvae are found to be parasitised 
either with Monosporella, or with the new parasite Helicosporidium, or very 
exceptionally with both organisms together. As the proportion of larvae 
parasitised with Monosporella is very small, almost all the opaque and milky 
specimens of Dasyhelea larvae are found to be infected with the new parasite. 
If we examine under the microscope a parasitised larva, compressed 
between slide and coverslip, sufficient to prevent movement, we see that its 
entire body cavity is filled with small round corpuscles, 5 or 6 /x in diameter. 
These corpuscles occupy all the spaces between the organs of the larva and 
extend through all the segments including the head. 
Being free in the body cavity of the larva, the parasitic corpuscles are 
always seen circulating or even gushing from one segment to another. This 
movement is purely passive and is produced either by the contraction of the 
segments of the host, by the more or less regular movement of its internal 
organs, or by the rhythmic contractions of the heart and the consequent 
plasma circulation. These passive movements of Helicosporidium are very 
easily seen in the head of the larva, where the parasites are less densely 
crowded on account of the restricted spaces between the strongly developed 
muscles of the mouth-parts. The post-abdominal segments on the contrary 
are filled to such an extent with the parasitic corpuscles that these form a 
solid mass occupying the whole cavity of these segments. The posterior portion 
of the larva becomes thus very turgid, loses its mobility, and becomes very 
fragile. By pricking such parasitised larva with a fine needle or even by 
gentle pressure on the coverslip a milky fluid gushes from the wound, and this 
fluid when examined with the microscope is seen to be a pure suspension of 
parasitic corpuscles (PI. V, figs. 1, 2 and 3). 
All the larvae which were recognised from their external appearance as 
being infected, had already arrived at such an advanced stage of infection, that 
