576 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.52. 



April and the first two weeks of May, for on May 31 I found the 

 exuviae very abundant and no mature nymphs were found. The 

 exuviae were found about the roots of the alder trees, especially 

 where these overhimg pools, and usually occmTed from one to two 

 feet above the water. On Zyante Creek, a tributary of the San 

 Lorenzo near Santa Cruz, California, frequently a dozen exuviae 

 were found on one tree root. 



After emergence the imagoes appear to migrate upstream, not a 

 migration in numbers but each individual probably gradually work- 

 ing up to the swifter waters. This same upstream migration is observ- 

 able in the associated species, Cordulegaster dorsalis. My observa- 

 tions are as follows : On vStevens Creek, where, because of its nearness, 

 I have been able to observe Octogomphus at various seasons, exuviae 

 were very abundant in the 2 miles from the trout farm up to the 

 soda spring but occurred less commonly above this point. Imagoes 

 were not found below the soda spring m this region of greatest 

 emergence, though they were common above the spring, even to 

 the head of the creek on the divide 8 miles distant. On the lower 

 stretch of Zyante Creek on July 9, the exuviae were very abundant 

 but not a single imago was observed. Tlie same was true for the 

 Chico River where it emerged from the canyon east of the city of 

 Chico (Butte County, California) . 



As the nymphs apparently do not burrow in the bed of the stream 

 but live in the loose organic trash that collects in the pools and 

 eddies, this upstream migration is probably to offset the washing 

 down of the nymphs duiing the whiter rainy season when the streams 

 are foaming mill races. Theoretically, some such compensation 

 should occur or this species would gradually spread to the lower 

 portions of these streams, where it is certainly not now found. 



As with most gomphines, the males of this species stay near the 

 water while the females are seldom seen there. The males are usually 

 found in the sunlit openings of the streams where they perch on stones, 

 driftwood, or on the foUage of the surrounding alders. But while 

 prefering the simny spots they do not hesitate to hunt up and down 

 stream through the shade. The four females I have taken were found 

 along a road on the side of the gorge several hundred feet above the 

 stream. They appear to resort to the stream only to oviposit. 



After having spent various days wading down mountain streams 

 observing Octogomphus more often than catching thorn, I was rewarded 

 on July 7 by seeing a female oviposit. She came volplaning down 

 through an opening in the canopy of alders and, while going through 

 evolutions involving several figures, 8's and S's, she touched the sur- 

 face of the pool hghtly with the tip of her abdomen at intervals of 2 

 to 6 feet. After 20 seconds of this she airily spiraled up and out into 

 the sunshine, where she alighted on a bush on the hillside above the 

 creek. 



