Insect Gills 83 



For Record include: 



1. Sketches of Spaerium, an adult and the largest young found inside it, side by side, 



and to the same scale, 



2. Diagram of a side view of Anodonta with one valve removed, foot, body, and gills 



uncovered, to show brood pouch. 



3. Diagram side by side of two chambers each of normal and gravid gills of Anodonta 



from slides. 



4. Sketches of glochidia in two positions. 



5. Explanation of figures, and notes adequate to indicate the contrast in reproductive 



methods in the families of Unionidae and Sphaeriidae. 



Study 19 

 A LABORATORY STUDY OF THE GILLS OF INSECTS 



Materials needed: (A) Living larvae having gills of the following types: 



1. Blood gills (as in Chironomus, Culex, or Corethra). 



2. Tracheal gills. 



(a) External: filiform (as in Hydropsyche or in Perla); lamelliform (as in Enal- 



lagma or Bsetis). 



(b) Internal (as in LibeUula, Anax, Gomphus). 



(B) Preserved specimens: 



1. Of gill-bearing larvae (in variety). 



2. Of pupas showing tube gills. 



Work Program: 



1. Mount a living larva having blood gills in a copious supply of water; cover and 



study the gills directly, noting their number, position and relations. Focus care- 

 fully upon one gill to see the outhne of its internal cavity, and to see the leuco- 

 cytes that drift about in it. 



2. To study the external tracheal gills, snip off a few gills with fine scissors and mount 



them in water; cover and examine at once, to see the tracheoles before the pene- 

 tration of the water into them has rendered these invisible. While filled with air 

 they appear as sharply defined black lines. They are not visible in preserved 

 specimens. Study especially the division of the large tracheae into fine tracheoles, 

 and the disposition of the latter, and their inter-communications. 



3. The internal gills of a dragonfly are arranged in rows upon the inner walls of a gill 



chamber that is made out of the posterior third of the alimentary canal. It is 

 so fine a piece of respiratory apparatus, so unique in plan, and it exhibits such 

 delicacy and refinement of structure, it is well worth a careful examination. 



It will be well, first, to see the external evidences of its operation. Regular 

 respiratory movements of the abdomen can usually be seen in a nymph that 

 lies quietly in a shallow dish of water. They may often be seen intensified if 

 the nymph be turned over on its back. With the expansion of the abdomen, 

 water is slowly taken in through the anal aperture, to be expelled with its con- 

 traction. The currents of the water may be demonstrated by placing some colored 

 fluid in the water close beside the anal opening. This is best done by holding the 

 point of a copying (indelible) pencil until its color is imparted to the water. The 

 forcible ejection of water from this gill chamber as an aid to propulsion may be 

 seen while the nymph is swimming about. Some idea of the force of the expulsion 



