a1 Agricultural Research Council, Unit of Insect Physiology, London School of Hygiene and Tropical Medicine
Chloride regulation. Following the ingestion of a blood meal the tick excretes about half the ingested water in the coxal fluid. The mean haemolymph chloride concentration before feeding is 1.00% and after feeding 0.96% NaCl; and that of the coxal fluid, 0.80% NaCl. These results confirm the values given by Boné (1943).
Morphology of the coxal glands. These comprise the coxal glands proper, which elaborate the bulk of the coxal fluid, and the smaller accessory glands of unknown function. Each flask-shaped coxal gland consists of two distinct regions, an outer ‘filtration chamber’ and an inner system of tubules. The latter lead to the external opening. The filtration chamber, which communicates with the tubules at only one point, is highly folded into an elaborate series of pockets and fingers which closely invest the tubules; numerous small muscle fibres inserted in these pockets pass outwards from the gland to attachments on the body wall. The histology of the two regions is entirely different: the filtration membrane is only 1–2μ in thickness and its cellular origin is much obscured: the tubule walls are 5–30 μ in thickness, are composed of cells with a dense, deeply staining cytoplasm, and are richly supplied with tracheae. In Ornithodorus delanoei acinus the walls of the sac-like coxal glands are lined both by ‘filtration membrane’ and by large irregular groups of ‘secretory’ cells; there are no true tubules.
Function of the coxal glands. The production of coxal fluid is under muscular control. It is believed that the contraction of the coxal gland muscles enlarges the filtration chamber and sets up a sufficient pressure difference across the membrane to initiate filtration into the gland. In the subsequent passage of the fluid down the tubules threshold substances such as chloride are reabsorbed. That the coxal fluid is primarily an ultrafiltrate of the haemolymph is suggested by (a) the rapid passage of dyes and even haemoglobin into the coxal fluid after injection into the haemolymph and (b) the very high rate of liberation of fluid. Serum albumen sometimes passes into the coxal fluid after injection but casein (and the normal haemolymph proteins) are fully retained.