16 SCHOOL ENTOMOLOGY 



down with itself and remain submerged until the supply 

 is exhausted. This is usually accomplished by means of 

 hairs on the un<l<-r side of the body which retain the air 

 bubble. The air supply is sometimes imprisoned under- 

 neath the wings. Next to this in simplicity is the tube 

 which reaches to the surface of the water and into which 

 the tracheae open. The insect can remain under water 

 indefinitely at the depth which corresponds to the length 

 of this tube. Mosquito larvae and pupae and water- 

 scorpions are so equipped. 



Insects that are most truly aquatic are provided with 

 tracheal gills, structures which can take up from the 

 water the oxygen necessary for the insect. These dif- 

 fer from the true gills of fish and crayfishes in that the 

 oxygen thus secured is carried to the tissues through 

 tracheal systems exactly similar to those of ordinary in- 

 sects while in the case of the true gills the blood is carried 

 to the gills as it would be to lungs. 



Tracheal gills show many forms. Dragon-fly nymphs 

 have gills that consist of a large number of tracheal tubes 

 in the lining of the rectum. Water is drawn into this, the 

 air taken up and the water expelled, often with some force 

 so that it serves generally as a means of propulsion. 

 Damsel fly nymphs have gills in the leaf-like plates at the 

 tip of the abdomen. Other forms, notably the hel- 

 gramite or larva of Corydalis, have tracheal gills consist- 

 ing of tufts of hair-like tracheae projecting from the body, 

 the location varying with the insect. 



Organs of special sense have many special structures. 

 The sense of feeling is often dependent upon hairs con- 

 necting with nerve fibers at different places on the body. 

 The antennae are, primarily, the main organs of feeling. 

 The sense of taste has organs located partly within the 



