423 



importance in classification. Two or more main tracheal trunks enter 

 each gill and send off branches towards the margins. The mode of 

 branching of the trachea is characteristic of many species, as is also 

 the degree of pigmentation. 



A closed tracheal system has been considered possible and even 

 probable in the Zygoptera, but thus far no connection has been traced 

 by me, with the highest magnification obtainable, between the ends of 

 the branches of the tracheae. The normal function of the gills is one of 

 respiration, the minute tracheae being supposedly able to take up the 

 oxygen from the water and to supply the animal with a sufficient Cjuan- 

 tity of the gas. Observations show, however, that complete loss of 

 gills does not injure the insect to any appreciable extent; and it has 

 been suggested that they also have cuticular and possibly rectal respira- 

 tion, the latter thought to have been demonstrated in the Agrionidae. 

 In young nymphs there is a pulsating movement in the region of the 

 rectum, but cross-sections of the abdomen of Ischnura vcrticalis and 

 several species of Enallagma show that there is no connection of the 

 tracheal system with the alimentary tract other than a few small 

 branches. What seems to be a more serious impairment of life activ- 

 ities in the loss of the gills is the decreased power of locomotion which 

 the insect suffers, the gills having the same importance as the tail of a 

 fish. Loss of gills frequently occurs, in which case new ones are pro- 

 duced ; but these appear only after the insect has molted, always remain 

 small, and are usually abnormal in figuration and tracheation (Fig. 

 77a). For different types of gills see Figures 48-72, 75-77a, and 80. 



Cerci. — Anal appendages corresponding to cerci are present dorso- 

 laterad of each lateral gill and vary in shape from tubercular to sty- 

 liform (Figs. 5, 18; ci). 



ADtJLT 



The adults of Odonata are distinguishable from all other orders 

 of insects by the type of their wing venation. The wing is character- 

 ized by the presence of a nodus and a stigma and a large number of 

 secondary cross-veins. The presence of accessory genitalia on the 

 second abdominal segment of the male is another unique feature. The 

 Zygoptera are for the most part separated from the suborder Anisop- 

 tera by the habit of folding their wings vertically when at rest. The 

 abdomen is much more slender than that of the Anisoptera, and the 

 wings are different in being contracted or petiolate at the base. 



Head. — In general appearance the head is wide and the eyes are 

 very prominent, and as the head moves on a point of the microthorax 

 its angle of rotation is very great. The epicranial furrow is present 



