THE R1':SP]R.\'J"()RV ()K(,AXS OF DRA( , OX-FLY 



LARV.^. 



Bv Stephen lioxxHEiL Rich, M.A.. B.Sc". 



The larvae of dragonflies, as we all know, are aquatic, living 

 among the weeds or burrowing in the mud at the bottom of still 

 ixx)ls. They all possess that form of respiratory system which 

 Pantel, in 1879, called " the closed tracheal system." This system 

 is unique in animal respiration, in that it provides for the passage 

 of gases as such from the state of solution in water to a free 

 state in air-chambers within the body of the animal, and 2'icc 

 versa. This accomplished, the respiration of each takes place in 

 the way normal for insects: i.e., fine tracheal branches provide 

 for direct access of air to all cells, obviating the use of haemo- 

 globin or similar blood-substances as efi^erents of carbon dioxide 

 v>r afferents of oxygen. 



The apparatus for extracting air from water and passing car- 

 bon dioxide into it, as found in the dragon-fly larva, is the most 

 perfect mechanism of its kind known. This statement applies 

 to the Anisoi:)tera. or large dragonflies. and only in a limited de- 

 gree to the Zygoptera. which are the smaller fluttering species. 



Let us examine the Anisopteran respiratory organs. If we 

 remove the dorsal body-wall of the larva of Ana.v. wc sec 

 tracheal trunks and branches passing int(^ all ])arts of the body, as 

 in every insect. Peculiar to this grouj), however, is the enlarge- 

 ment of the tracheal trunks Iving dorsal to the gut. as they pass 

 caudad. In the last four abdominal somites they give ofi^ a series 

 of re-dividing branches which pass to a much enlarged rectiun. 

 This rectum is. in fact, a gill-chamber. Internall\- it carries six 

 longitudinal rows, each a double row, with twenty to twenty-four 

 low gills in it. These gills are set diagonally, have a thick " cush- 

 ion " of (■])ithclium on each side, and arc filled with fat. j'^ach 

 gill carries on its crest eight to cle\cn \ilb, making a total of 

 roughly 3,000 villi. 



These villi are the functional respiratory parts. The tracheal 

 branches, repeatedly dividing, pass into them, and ultimately form 

 loops, rejoining each other. Sadones, 1895, showed that these 

 loo])s are in acttial contact with the chitinous ciuicle of the villi. 

 I shall explain, below, the functioning of this mechanism. 



To ciU ofl' this gill-chamber from the digi-stiw i)art of the 

 gut. a muscular band crc^sses the ileum in somite 5 of the abdo- 

 men, and is fastened at each end to the ventral body-wall. It 

 can thus be made to act as a ])incli-cock at the animal's will. 



The rectum has six longituditial bands of muscle, ])etween 

 (he gill-row^. It lias over its whole outer stu-face the usual 

 insectean loose bands of circular muscles. Thus the rectinn mav 

 l)e contracted forcibly — a movement used In- the animal to ])rope! 

 itself forward by recoil from this anal jet of water. 



The rectum has a short anal canal at its caudal vni\. ter- 

 niin:ucd by three o\erlapi)ing flaps :\ni\ \\\x^ s])inv a])pendages 



