GENERAL ZOOLOGY 



Fig. 15.19. Schematic diaa;ram of the s;eneral mechanism of uric acid excretion in the Mal- 

 pia;hian tubules of insects, based on the blood-sucivine; bus; Rhodmus. (Adapted from V. B. 

 Wie;s;lesworth, 1931, Journal of Experimental Bioloay^ vol. 8, printed bv permission.) 



composed of large cells surrounding a tubular lumen. The tubule is closed at 

 its distal end, but the proximal end communicates with the lumen of the 

 intestine. It is generally spirally wound about by several strands of muscle 

 fibers. The tubules lie in the hemocoel, bathed by blood, from which the 

 cells extract wastes, chiefly in the form of salts of uric acid (Fig. 15.19). 

 These salts are passed through the wall of the tubule into the lumen and move 

 downward, in solution, toward the intestine. Cells in the more proximal 

 portions of the tubule extract water and certain inorganic constituents from 

 this "urine," resulting in the precipitation of uric acid as crystals or 

 concretions in the lumen of the tubule. By gentle muscular pulsations of the 

 tubule, the precipitated mass, in the form of a paste, is emptied into the lumen 

 of the hind-gut. Here more water is extracted, and the wastes are eliminated 

 from the body with the feces. This excretory mechanism may be interpreted 

 as an adaptation for the conservation of water, by eliminating wastes with 

 the smallest possible loss of water. 



The development of the tracheal system has relieved the blood of a function 

 which in most other animals is of major importance, that of distributing 

 respiratory gases. Consequently, it is not surprising to find that the circulatory 

 system is not extensively developed, and that the flow of blood is relatively 

 sluggish. The heart of the locust is a slender, pulsatile tube extending along 

 the dorsal midline of the abdomen (Fig. 15.18). In each segment throughout 

 its length, the heart is provided with a pair of valved openings, or ostia. The 

 heart is supported from the dorsolateral body wall on each side by fan-shaped 

 groups of alary muscles and lies in a division of the hemocoel termed the 

 pericardial sinus. This space is separated from the perivisceral hemocoel by a 

 perforated membrane, the dorsal diaphragm. Blood enters the heart from the 

 pericardial sinus through the ostia and is driven forward, into the thorax and 

 head, through the aorta, which terminates near the brain. From the head. 



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