DRAGONFLIES OF DOUGLAS LAKE REGION. 57 



ninae are found almost entirely in and on the submerged aquatic vegeta- 

 tion or on objects not covered with mu<l. The nymphs were almost 

 without exception clean, brightly colored and active. The Gomphinae, 

 on the other hand, were usually found in numbers on a mud bottom 

 in the top layers of the ooze. Specimens were, however, occasionally 

 captured on vegetation, logs or other floating objects, a fact which might 

 be explained as a preparation for the moulting attendant to trans- 

 formation. Gomphid nymphs were almost alwa3's dully colored and 

 more or less incrusted with mud and algae. The Libellulids were usually 

 found in the trash and debris on the bottom of the stream or among 

 tlie lower parts of the vegetation. Their preference for such situations 

 is also sho^^^l by the large number of mud-incrusted specimens of this 

 group collected. Some species of this family, for example, Tramea 

 lacerata, were taken in water less than two feet deep at Bryant's Bog. 

 The great depth of the water in the center of this bog, where it exceeds 

 twenty-five feet, maj' account for these nymphs being nearer to the 

 surface, clinging to the aquatic vegetation. 



Without exception the Zygoptera of the region were found in aquatic 

 vegetation rather near the surface of the water. The nymphs of some 

 species, for example Agrion aequabile, frequent the swiftest parts of 

 small streams. The preference of the Zygoptera nymphs for surface 

 situations or rapidh' moving streams may be a corollarj- of the oxygen 

 relations. In this connection it was observed that the Gomphid nymphs 

 have a very characteristic way of holding the posterior end of the ab- 

 domen turned up at a very decided angle. This habit, together with 

 their mud-burrowing habit and the length of the tenth abdominal 

 segment, which segment in many cases is much longer than the ninth 

 segment, might be interpreted as an ada])tation for securing a better 

 supply of oxygen. The top layers of ooze through which the nymphs 

 crawl generally contain decaj'ing animal and vegetable matter from 

 which more or less carbon dioxide is discharged; thus a nymph woulil 

 be able to get more water and hence more oxygen if the end of the 

 abdomen could be extended upward above the ooze. 



University of Colorado, 



October 24, 1914. 



BIBLIOGRAPHY. 



Muttkowski, R. A., Catalogue of the Odonata of North America, 

 Bull. Public :Mus. City of Milwaukee, Vol. I, pp. 1-208, 1910-11. 



Needham, J. G., and Hart, C. A., The Dragonfiies (Odonata) of Illinois, 

 Pt. I, Bull. 111. State Lab. Nat. Hist., Vol. VI, Art. I, pp, 1-94, 1901. 



Needham, J. G., Aquatic Insects in the Adirondacks, Bull. 47, N. Y. 

 State Mus., pp, 383-591, 1901. 



