FORM, GROWTH, AND CHANGE 49 



fascinating bypaths of nature-study. The dragon-fly larva 

 possesses, like insects generally, a system of air-tubes (Fig. 

 23 E d, 25 B), but while it lives submerged in water there is no 

 direct access to this system from without ; the abdominal 

 spiracles (Fig. 25 B sp.) are closed, and those on the thorax 

 alone may remain open, to become possibly functional on 

 occasions when the larva will leave for awhile its watery home 

 and make preliminary trials of the atmosphere, on aquatic 

 plant-stems or half-submerged rocks. But while the insect 

 is submerged it depends for its supply of oxygen on the air 

 dissolved in the water, and this is made available by means of 

 special sub-aqueous breathing organs (or gills). As just 

 mentioned these gills are found, in the larger and more robust 

 dragon-flies, on the inner wall of the large central chamber of 

 the hind-intestine known as the branchial basket (Fig. 23 E br). 

 Outgrowths of the wall in form of undulated plates or finger- 

 like, thread-like, or leaf-like lobes (Fig. 27 A-F) project into 

 the cavity, and these enclose fine air-tubes which are 

 connected (Fig. 25 B, R) with the great paired dorsal (DT) and 

 ventral (VNT) trunks. Into these oxygen is diffused from the 

 dissolved air of water taken into the hind-gut, and the periodical 

 forcible ejection of this water assists the larva in locomotion 

 by urging it forwards as by the working of a jet-propeller. 



The larvae of the slender, delicate damsel-flies also are able 

 to breathe through the wall of the hind-gut, though it has no 

 specialized gill-outgrowths. These larvae are provided how- 

 ever, with other organs of respiration in the tail-process and 

 the paired appendages (cerci) at the tip of the abdomen, which 

 are modified into external gills (Fig. 27 G H), circular or 

 compressed in cross-section, and traversed by numerous fine 

 branching air-tubes. 



Thus the dragon-fly larva lives in its underwater world, 

 growing towards the adult condition through its successive 

 moults as regards the increase in size of its eyes and of its 

 wing-rudiments, so that during its later stages it is often 

 spoken of as a nymph. But the characteristic larval structures 

 the mask and the gill-system persist in the penultimate 

 instar, so that the final moult which results in the emergence of 

 the winged adult involves a real transformation. The ripening 

 nymph suffers atrophy of its gills, and thrusting the front 



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