530 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 52. 



the exuviae are usually found along the edges of the deeper pools. 

 Nymphs of severus from Satus Creek, Washington, indicate a three- 

 year period for their Ufe cycle. See pp. 544-549 for nymphs. 



Mites rarely infest the species of this genus, and as imagoes they 

 have few enemies except the birds which attack them while tenerals. 

 OpMogomphus morrisoni on Donner Lake was seriously attacked 

 by robins while emerging. As with many western species the most 

 serious cause of premature death among imagoes seemed to be the 

 occasional cold rains which come even in desert regions. On Satus 

 Creek I have seen severus practically wiped out for the first day or two 

 after a rain and regaining its numbers only after more had emerged. 

 Very old imagoes show a condition which might be called the "stag- 

 gers." They appear weak or drunk and on alighting will commonly 

 fall over on their sides and regain then feet with difficulty. 



The following notes cover the species of OpTiiogomplius I have taken 

 in California and Nevada. With them I have included OpMogomphus 

 severus montanus and a new species from Arizona, though neither has 

 been recorded from this territory. Hagen's reference of severus in 

 Nevada is probably nevadensis. It is probable that severus will be 

 found in the northern part of this area and that the new species, ari- 

 zonicus, will be found in the southern part. 



The forms listed fall into the following four groups: 

 Females without occipital spines. 



Group I. a>. Size small (abd., male, 34-36 mm.); humeral suture with a narrow 

 stripe preceded by an oval spot; posterior hamules broadly truncate. 



6' . Pale, markings brown severus. 



W. Markings black montanus. 



a^. A double hiuneral stripe (the anterior sti-ipe sometimes reduced to an 



oval spot in nevadensis); posterior hamules pointed. 

 6'. Size small (abd., male, 35-36 mm.); superior appendages of male 

 usually shorter than inferior; markings intense black; hu- 

 meral stripe double, broad and black morrisoni. 



b^. Size larger (abd., male, 37-39 mm.); pale colors ashy; markings re- 

 stricted; lateral stripe widely interrupted on segments 3 and 4; 

 himieral stripe double, but each stripe narrow or the anterior 



reduced to an oval spot nevadensis. 



Females with two occipital spines. 



Group II. o'. Occipitalspinesshortand widely spaced; male with inferior append- 

 age one-half length of superiors; humeral stripe reduced to a line 



and an anterior oval spot arizonicus. 



Group III. a'. Occipital spines slender and close together; male superior append- 

 ages long and cylindrical, the inferior tlu-ee-foiu-ths length of super- 

 iors; hiuneral stripe broad, heavy, and usually double bison. 



Females with foiu- occipital spines. 



Group IV. a^. A black, broad, double humeral stripe; entire pattern heavy; male 

 inferior appendage with inferior profile almost semicircular. 



occidentis. 



a^. Same as for a^ but paler, the entire pattern brownish instead of 



black californicus. 



