The excretory system of Rhodnius consists of four Malpighian tubes discharging into the rectum. Each tube consists of two segments:

(i) An upper (distal) segment, comprising two-thirds of the tube, made up of very granular epithelium, and with only clear fluid in the lumen. The striated border of this segment is a rigid structure of the type called "honeycomb border" ("Wabensaum").

(ii) A lower (proximal) segment, comprising one-third of the tube, in which the epithelium is almost free from granules and the lumen filled with uratic spheres. The striated border of this segment is a typical "brush border" ("Bürstensaum").

Below, each tube opens into the gut by way of a button-like ampulla. The four ampullae are lined by large cells which send long processes into the rectum; and around their opening is a zone of large cells, in the wall of the rectum, which constitutes a rectal gland.

After feeding, all the tubes are distended and clear while the watery urine is being excreted. Apart from this temporary distension, the cells of the upper segment show no definite changes. But the filaments of the striated border of the lower segment become very elongated, and, when the passage of the watery urine has ceased, uratic granules appear among them in increasing numbers. Later, these filaments retract, and the granules become free in the lumen.

The processes of the ampulla cells extend far into the rectum during excretion, and float in the fluid urine. Later they retract again.

Well-marked "vesicular secretion" is visible in sections of the Malpighian tubes, as well as in isolated tubes in Ringer's solution, but it is shown that, in Rhodnius, these changes are artefacts.

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