March, 1913.] . OSBURN : OdONATA. 9 



ODONATA IN RELATION TO THE HYDROPHYTIC 



ENVIRONMENT. 



By Raymond C. Osburn, 

 New York, N. Y. 



All species of dragonflies are aquatic in the larval stage, and there- 

 fore, the Odonata as a group belongs to the fauna of the hydrophytic 

 area. 



The eggs are deposited in the water, either by the process of 

 " dipping," by which the eggs are washed from the tip of the abdomen 

 of the female, or, in some cases, the female rests with the tip of the 

 abdomen beneath the surface of the water while the eggs are being 

 laid. In the latter case, the eggs are usually deposited singly in in- 

 cisions made in the stems of aquatic plants by means of the genital 

 valves. In other cases the eggs fall to the bottom singly, as a rule, 

 though the species of Tetragonenria and Epitheca deposit an egg-mass 

 in the form of a gelatinous string. A few of the tropical species lay 

 eggs in water caught in the cups formed by the leaves of Bromeliads. 

 The time required for hatching varies from a few days to several 

 weeks, depending upon the temperature of the water, and the young 

 larvae emerge to begin their aquatic life. They are predaceous, feed- 

 ing upon various kinds of aquatic animals, chiefly other insects which 

 they capture by means of the peculiar extensile labium. The larger 

 larvae often devour young fishes, tadpoles and even smaller indi- 

 viduals of their own species. 



Two distinct types of respiration are represented among the 

 dragonfly larvae. In the small species known as demoiselles (sub- 

 order Zygoptera) there are thin, flattened, leaf-like, terminal append- 

 ages of the abdomen which are adapted to the absorption of oxygen 

 from the water. In the larger, more robust forms, dragonflies (sub- 

 order Anisoptera), a unique method of respiration has been evolved. 

 The rectum is greatly expanded, the walls convoluted to form a sys- 

 tem of gills in which there are thousands of tracheal endings. By 

 rhythmic pulsations the water is alternately drawn in and expelled 

 from the rectum, and the rectal gills serve to extract the oxygen from 

 the water. This same apparatus also serves as a locomotor organ in 

 swimming by the members of this same group. Forcible expulsion 



