A. Porter 
349 
Unfortunately, in the desire to fill up the many gaps in our know¬ 
ledge of the life-histories of disease-causing agents such as Trypanosoma 
gambiense, the occurrence of natural parasites of the fly or other 
arthropod has sometimes been overlooked, and as a result of this, phases 
of the life-history of probably harmless parasites of insects have been 
mistaken for those of the deadlier organism, acquired by the host more 
or less casually in its food, and later transferred, also casually, to some 
vertebrate host. 
The study of the natural flagellates of arthropods therefore is most 
important, and it is necessary to study both the distribution of the host 
and of the flagellate it harbours. 
The subject of this paper is a flagellate, Crithidia allied to Trypano¬ 
soma, occurring in Gerris paludum 1 , a common British water-bug, 
popularly known as the “ pond skater.” A nearly allied species of 
Gerris , G. fossarum, common in Madras, also harbours a Crithidia, and the 
similarity between C. gerridis as described by Patton (1908) and the 
parasite that I have found in G. paludum is such that it is impossible to 
consider it other than the same organism. As the host occurs in a non- 
tropical region, there are slight differences between its parasite and that 
of the Indian form, so that I propose to describe, in detail, C. gerridis, 
occurring in a British species of Gerris. 
Materials and Methods. 
The Gerris paludum used in this investigation were collected from 
many localities in the south and east of England. Thus, specimens of 
the insect were examined from many places in Sussex, Middlesex, 
Surrey, Bucks, Gloucestershire, and around Cambridge, and Crithidia 
were found in the water-bugs from every one of these districts. 
During this investigation all stages of Gems paludum, from very 
young nymphs to adults, and also a large number of embryos were 
examiued, but while many of the young nymphs were found to be 
infected, I never found an infected embryo. 
The Gerris were carefully dissected, and the unruptured gut was 
divided rapidly into crop, stomach, intestine, and rectal portions, 
each portion being teased in 0'8°/o NaCl solution. Preparations of 
the Malpighian tubes and ovaries were also made, and the faeces of the 
bug were examined. For observation of the living organisms, wet 
1 I have much pleasure in thanking Mr W. L. Distant for kindly identifying the species 
of Gerris for me. 
