526 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.52. 



The abdomen is creamy yellow with the following intense black 

 markings: The upper half of segment 1, an irregular triangular spot 

 on side of segment 2, an approximately triangular spot on the side 

 of each of segments 3-7; the apex of each of these is at the anterior 

 end of the segment, and the two spots on the opposite sides of any 

 segment meet dorsally across the apical end; sides of segments 

 8-10 heavily mottled with black; superior appendages yellow; inferior 

 black. 



Female. — Colored as is the male, but the side of segment 2 with 

 two horizontal black stripes and segment 10 largely yellow. (See fig. 

 148, B; 149.) 



Five Chico males measure as follows : 



Abdomen, 42-44 mm.; hind wing, 30. Two females from Los 

 Angeles measure, abdomen, 43 mm., hind wing, 33-34. 



The nymph of this species is more highly specialized for burrowing 

 than any other odonate nymph with which I am familiar. It is the 

 most frequently occurring form of large insect life found in the sand 

 beds of the rivers of central and southern California. 



All the rivers of California originate in the mountains as swift, 

 clear streams, but nearly all before reaching the sea have almost 

 level lower courses where they lose their swiftness, become warm 

 and meander over beds of shifting white sands. These sand beds 

 are so unstable that they actually flow with the water though at a 

 much slower rate. Perhaps this needs further explanation. A good 

 example is that of the Kern River at Bakersfield. Above the intake 

 of the irrigatmg canal in the latter part of summer this river was on 

 an average a foot deep and one hundred feet wide. Its bed was of 

 pure, loose sand. This was formed by the current into sand ripples 

 which faced with their steep slope down stream. The sand was in 

 constant movement as the current carried sand from the upper 

 gradual slope, up over the crest of each ripple to let it drop down the 

 steeper downstream slope. Where there were eddies and turns 

 these sand ripples became waves in size, being frequently six inches 

 to a foot high though they were usually less than 6 inches in height. 

 In such a stream the bottom inch of water is turbid with moving sand. 



It is in such an unstable environment that Progomphus flourishes. 

 Only once or twice have I actually found nymphs in the flowing 

 streams but in any side channel where the water has ceased to flow 

 the sand bed is marked in aU directions by the curving burrows of 

 these wandering nymphs. They burrow in the loose upper stratum 

 of sand and just deep enough to cover the back. Usually the tenth 

 segment protrudes. I have traced burrow tracks that were from 

 ten to fifty feet long. If one arrived at the fresh end of the burrow 

 before some water fowl had found it the nymph would be found 

 spasmodically burrowing forward. These nymphs could burrow as 

 rapidly as the average nymph can walk. Only such vigor and speed 



