4io 



GENERAL BIOLOGY 



External 



Study 5J. A preliminary examination in living specimens 

 of the principal gill types of aquatic insects. 



Materials needed : Living larvas to illustrate : 



1. Blood gills (larvae of Culex, the mosquito, Corethra or 

 Chironomus) . 



2. Tracheal gills: 



Filamentous (larvas of a caddis fly, etc.) : 

 Lamellif orm (nymphs of a damselfly or mayfly) . 

 Internal (nymphs of the dragonfly, Libellula). 



Mount a larva 

 having blood gills in 

 a copious supply of 

 water, cover and 

 study the gills 

 directly, noting their 

 number, position and 

 relations. Focus 

 carefully upon one 

 gill to see the outline 

 of its internal cavity, 

 and to see the leuco- 

 cytes that drift about 

 in it. 



To study the ex- 

 ternal 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 penetration of the water 

 into them has rendered these invisible. While filled with 

 air they appear as sharply defined black lines. They are 



FIG. 240. Dragonfly nymph (Celitheini 



