26 



pellet is expelled and placed on a specialized anteroventral 

 area, the praesaepium, of fourth- instar larvae. The prae- 

 saepium, which bears spines specialized for holding solid 

 food while the larva feeds, is absent from earlier instars, 

 which are fed only liquid. Because of this method of feeding, 

 the fourth- instar larva is the only stage which is vulnerable 

 to infection. Both spore types are ingested, but only the 

 NMB spore is infective. The MB spores are expelled unextrud- 

 ed in the meconium upon pupation. 



It is evident that B. dimorpha is indeed a dimorphic 

 microsporidium, since ingestion of NMB spores only resulted 

 in typical infection in which both spore types were produced 

 in normal ratios. 



The function of the MB spore remains unknown. A most 

 attractive hypothesis is that it either infects an alternate 

 host or is primed in the gut of a mechanical vector for 

 extrusion upon subsequent ingestion by ant larvae. Either 

 would explain the mode of intercolonial transmission of 

 the infection (fire ants are territorial and aggressive 

 towards conspecific ants) . Many candidate species exist 

 for the role of vector; a large and varied arthropod fauna 

 is associated with fire ants. Collins and Markin (1971) 

 listed 52 species of insects that have been collected from 

 fire ant nests ; other invertebrates also occur. At least 

 some of these organisms have symbiotic relationships with fire 

 ants and are known to travel between fire ant nests (Wojcik, 

 1975). 



