(1) Crithidia gerridis occurs in the alimentary tract of Gerris fossarwm, Microvelia and Perittopus sp. found in Madras. It is now recorded from the British water-bug, Gerris paludum, for the first time.

(2) The parasite occurs throughout the alimentary tract, in the ovaries and in the faeces of its host.

(3) There are three phases in the life-history of Crithidia gerridis, a pre-flagellate stage (Pl. IV, Figs. 1–10), a flagellate stage, and a post-flagellate stage (Figs. 64–69), the latter being adapted for life outside the body of the host, and for cross-infection.

(4) The adult flagellate has an elongate body, and possesses a large oval or round nucleus. There is also a smaller, usually rod-like mass of chromatin, the blepharoplast, near to which the flagellum arises. The flagellum may be as long again as the body, and is attached to it by a narrow, undulating membrane, in which myonemes are present (Figs. 22–46).

(5) The movements of the flagellated forms are characteristic; both the body and flagellum take part in the motion.

(6) The pre-flagellate forms are small, usually oval bodies, 3μ to 7μ long, and from 2μ to 4μ broad. Their nucleus often lies to one side and is round, the blepharoplast is rod-shaped. The flagellum grows out as a thin, delicate thread, in close contact with the body. The pre-flagellate phase of the parasite is found in the crop of the nymphs particularly, and also in the crop of the adults.

(7) The flagellate form has the general structure outlined in (4). The undulating membrane has indications of myonemes (Pl. IV, Figs. 41, 44, 45). The flagellum arises near a chromatic dot, the basal granule (Figs. 22, 30, 43). In the nucleus sometimes eight large chromatic masses may be present, or the grains may be very small. The nucleus on the whole is of the vesicular type. The blepharoplast is usually anterior to the nucleus, only very occasionally is it lateral. Chromidia are scattered in the endoplasm of the parasite (Figs. 26, 39, 46).

(8) The post-flagellate stage is a preparation for extra-corporeal life. The parasites divide in the rectum, lose their flagella, round themselves off and form a thin gelatinous cyst wall that rapidly hardens (Figs. 68, 69). These small cysts pass out with the faeces of the host.

(9) Longitudinal division is the chief method of multiplication. It may occur in all phases of the life-history and may be equal or sub-equal. The blepharoplast usually constricts first and division of it and of the flagellum follow one another very rapidly, division of the membrane follows, and then that of the body. The daughter halves gradually diverge and finally separate. In pre-flagellate division, rosettes may be formed by several rapid, repeated, longitudinal divisions (Fig. 20). The fully flagellated individuals first divide into two and repeated division may result in rosettes, but these very rapidly break up. Aggregation rosettes of mature flagellates are, however, extremely common (Fig. 47).

(10) The mode of infection is a casual one, the young nymphs taking up faeces containing crithidian cysts from the leaves of water plants.

(11) The parasite is purely a parasite of insects, occurring in two species of Gerris, G. fossarum and G. paludum. The systematic position of the parasite is near the Trypanosomes in the family Trypanosomatidae.

(12) There is much variation of form exhibited by the adult flagellate. Some of the very long parasites (Pl IV, Figs. 39, 44–46). appear to be peculiar to the Crithidia found in the gut of G. paludum, and have not been figured before so far as I know. This polymorphism needs careful attention, and is very confusing if only isolated stages of the parasite are studied. A similar remark applies to other crithidial and herpetomonad forms.