1977] 
Porter — Mesostenines 
85 
rather minute spatial and size differences in host selection. Each 
species exploits a series of hosts that may be taxonomically diverse 
but which occurs in a particular micro-habitat and falls within defi- 
nite size limits. 
Reference to the preceding discussions of abundant species, such 
as Lanugo picta, Compsocryptus texensis, Acerastes pertinax, and 
Agonocryptus discoidaloides, shows phaenological variation from 
year to year as well as from month to month within any given year. 
Such fluctuations probably result from the variable climate of the 
Valley (as discussed in the Introduction to this study). Sporadic 
“killing frosts,” which occur about once a decade, devastate some 
of the Valley’s subtropical biota but probably have little effect on 
ichneumonids. Indeed, mesostenines often become strikingly more 
abundant in the first warm days after a freeze, suggesting that low 
temperatures may be necessary to break their diapause. Further- 
more, Valley ichneumonids are perfectly adapted to the numerous 
4-10 day periods of cloudy, drizzly, cold weather (8-10 degrees C.) 
triggered in winter by passage of wet cold fronts, since they begin 
to fly immediately and abundantly within minutes after the sun 
finally breaks through and temperatures exceed 15 or 16 degrees C. 
Therefore, it is probably the Valley’s unpredictable and often lengthy 
droughts which exercise the most rigorous density independent con- 
trol on populations of these hygrophile insects. 
Finally, we have some data that permit comparison of Valley 
mesostenine phaenology with that of other New World subtropical 
communities. 
My three year net and Malaise survey of Mesostenini in the 
northwest Argentine Subandino (Porter, 1975a) produced 21 spe- 
cies and 38 specimens for January, 14 species and 20 species for 
February, 8 species and 14 specimens for March, 8 species and 12 
specimens for April, no records for May, 2 species and 2 specimens 
for June, 3 species and 3 specimens for July, 1 species and 1 speci- 
men for August, 7 species and 32 specimens for September, 8 spe- 
cies and 20 specimens for October, 7 species and 9 specimens for 
November, and 17 species and 32 specimens for December. Sub- 
andean mesostenines thus peak in summer (December-February), 
decrease gradually during autumn (March-April), practically dis- 
appear in late fall and winter (May-August), and then build up 
more or less progressively in spring (September-November). The 
Subandean summer is warm and most of the year’s rain falls be- 
