16 U.S. NATIONAL MUSEUM BULLETIN 2 79 



There is probably only one generation per year, with the adult as 

 the usual overwintering stage. It is possible that some species also 

 overwinter as tliird instar larvae. For example, the writer received a 

 group of T. plicatus from N. McFarland which were collected on August 

 5, 1963, near Portal, Ariz. Larvae reared from these adults did not 

 pupate until April 1 964. 



The timing of the life cycle appears to vary with the species and 

 climatic conditions. Those species occurring in Oregon appear to be 

 restricted to spring and early summer in their reproductive activities. 

 Certain Southv/estern species are active during mid and late summer, 

 but others such as T. frontera appear to have an early spring activity 

 period and then oversummer as quiescent adults deep within the soil. 



It appears that spermatogenesis starts late in the adult life of Trox 

 and Omorgus. '^ Mating of these beetles probably occurs, therefore, at 

 the onset of their next period of activity; early spring for those species 

 overwinterinsr as inactive adults. 



General Description of Immature Stages 



The immature stages consist of the following: egg, first stage larva, 

 second stage larva, third stage larva, and pupa. The eggs are soft and 

 shining milk white when first deposited, but decrease in luster with 

 advancing development. As with the eggs of other scarabaeids, the eggs 

 of Trox and Omorgus expand in size after oviposition. They vary in size 

 from 1.10 mm. by 0.85 mm. for T. scaber to 4.0 mm. by 7.0 mm. for 0. 

 texanus at a time shortly before hatching. The eggs of Trox retain their 

 spherical shape during development, but those of Omorgus take on a 

 reniform shape late in their development (fig. 23). 



First stage larvae have the fewest setae per given area of all three 

 larval stages. Second stage larvae have many more setae per given 

 area, but still slightly fewer than the definitive number. Figures 16, 17, 

 and 18 show the relative sizes of the three instars and their general 

 form. The larvae vary in color from white to straw yellow, with the 

 color becoming more pronounced with age. Color is most easily as- 

 certained in late third stage larvae which have voided the contents 

 of their alimentary tracts. The larvae of Trox and Omorgus are quite 

 variable even within the species with respect to body length. A tentative 

 range for body length may be arrived at, however, if one multiplies the 

 head width of the Omorgus species by a factor of 3.5 and 5.0 and the 

 head width of the Trox species by a factor of 4.5 and 7.5. 



' Personal coinuiuiiication from Dr. N. Virkki, University of Puerto Rico, Agri- 

 cultural Experimental Station, Februai'y 14, 1964. 



