84 Fresh-water Biology 



may be gained by tilting the abdomen of a swimming nymph upward until it 

 touches the surface of the water, when the water in the gill chamber will be shot 

 into the air. 



To study the structure of the gill chamber and of the gills themselves, the 

 following method will be found to be expeditious and satisfactory. Kill the nymph 

 by snipping off its head. Then snip off the abdomen at its base, trim off its 

 sharply triangular lateral margins for its whole length; pin it down to the waxed 

 bottom of a dissecting dish that is small enough for use on the stage of a dissecting 

 microscope or under a pocket lens; carefully lift off the roof of the abdomen 

 (already loosened at the sides by the trim-off of the margins), by seizing it in front 

 with the forceps. 



This will expose the gill chamber, which occupies the greater part of the ab- 

 dominal cavity, and terminates the alimentary canal. The severed posterior 

 end of the stomach will be seen in the middle in front, terminated in the rear by 

 a dense cluster of nephridia (Malpighian tubules), and followed by a slender, 

 white, ventrally curved and much concealed intestine, joining it to the gill 

 chamber. On either side of the stomach will be seen a large, silvery white air 

 trunk, which breaks up posteriorly into a great brush of lesser branches that 

 penetrate the walls of the gill chamber. This chamber itself, will be somewhat 

 collapsed; it may be distended by injecting air or water through the anal aperture 

 with a fine-pointed pipette; its longitudinal extent may be seen by lifting the 

 stomach with a forceps and drawing it forward. If turned to one side, a ventral 

 longitudinal tracheal trunk may be seen on either side of the body, breaking up 

 in the rear, like the dorsal trunk, into a multitude of branches, and entering the 

 walls of the gill chamber from below. 



Through the transparent walls of the gill chamber may be seen lines of the 

 black pigment that occupies the bases of the internal gill plates. Discovering 

 thus the location of the rows of gills, the chamber may be safely opened by in- 

 serting the point of a fine scissors and cutting the wall for its entire length between 

 two rows. The circular muscles of the wall will, by their contraction turn the 

 whole organ inside out, and fully expose the rows of beautiful, feathery, purplish 

 tinted gill plates. Then if a row be isolated with scissors and mounted on a slide 

 in water, a few individual gills may readily be isolated with needles under a 

 dissecting lens, covered, and studied with a microscope. 



4. Read in the text, Life of Inland Waters, pp. 273 to 281; Ward and Whipple's 

 Fresh Water Biology, pp. 876 to 880. 



For Record: 



1. Prepare sketches and diagrams illustrating the principal type of gills studied. 



2. Prepare a table of Gills of Insect Larvae writing the names of the insects in the left 



hand column, grouping them by orders, and having the following column head- 

 ings: 



Name (of the insect). 



Gill type (blood gill, tracheal gill, tube gill, etc., simple or compound). 



Number (total). 



Form* (in general, filiform, lamelliform, telescopic, retractile). 



Location* (on the body, whose divisions may be conveniently designated as 

 H— I, II, II— 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, for head, thorax, and abdomen' 

 respectively). 



Differentiation* (all alike or unlike on this insect; and if unlike, differing how). 

 * A diagram may express this best. 



