73 



was consistently near maximum. The upper thermal limit 

 of MB spore development is below 32 C. Thus, optimal MB 

 spore production occurred near the upper thermal limit of 

 spore production. This is reminiscent of the activity curve 

 of heat-liabile enzymes and the growth curves of poikilo- 

 therms over a range of temperatures. Very possibly the 

 upper thermal limit of MB spore development is the tempera- 

 ture of inactivation of an enzyme (s). 



The B. dimorpha MB spore production is also inhibited 

 by low temperature (20-22.5 C) . In contrast, V. necatrix MB 

 spores are produced at temperatures at least as low as 16 C. 

 This difference in cold sensitivity may reflect different 

 requirements for enzyme stability due to host behavior. 

 Loss of quaternary enzyme structure at low temperature is 

 relatively common in homeotherms and their microbial S3rmbiotes 

 The homologous enzymes of poikilotherms do not, at least 

 in some cases, lose their quaternary structure at low temper- 

 ature (Hochachka and Somero, 1973). The hosts of both V. 

 necatrix and B. dimorpha are, of course, poikilotherms; 

 however, the host of the latter microsporidium is a subtrop- 

 ical, social insect that actively tends its brood, moving 

 them in a subterranian environment to regulate temperature 

 and humidity. The tumulus of the nest is a solar heating 

 device, and there is evidence that metabolic heat production 

 is significant (Seeley and Heinrich, 1981). The various 

 lepidopteran hosts of V. necatrix , however, are exposed 



