720 



ECOLOGY AND EVOLUTION 



categories of trophobiont, synechthran, 

 symphile, and parasite. 



The staphyHnid beetle, Termitonicus 

 mahout (Fig. 257), rides on the heads of 

 worker termites, Velocitermes beebei, and 

 imbibes nutritive liquids passed by mouth 

 between the termites. This unusual type of 

 termitophile may be classified as a syn- 

 oekete, although most synoeketes are 

 scavengers, with little direct contact with 

 their hosts. 



Fig. 258. Larva of a histerid beetle from 

 the nest of Velocitermes beebei in British 

 Guiana. While termite workers were licking 

 the abdominal exudatoria, the larva was feed- 

 ing on young termites. 



The adaptive evolution of myrmeco- 

 philous symphiles is illustrated by the con- 

 vergent appearance of trichome glands ir. 

 separately evolved groups of beetles, includ- 

 ing the Staphylinidae, Clavigerinae (Pse- 

 laphidae), Paussidae, Histeridae, and other 

 insects (Wheeler, 1926). Through some 

 sort of stimulation, perhaps odor, these red 

 or golden setae at the openings of clustered 

 unicellular glands are licked and sucked by 

 the ants (O. Park, 1932). 



The symphiles in termite nests exhibit 

 convergent evolution of glandular struc- 

 tures that secrete a fatty exudate (Mcln- 

 doo, 1923). In some cases, particularly in 



larval insects (Fig. 258), but rarely in 

 adults (Fig. 259), the "exudatoria" ma) 

 be numerous outgrowths from the body 

 wall (Silvestri, 1920). Especially in adult 

 symphiles, the exudate glands are usually 

 distributed over swollen body surfaces 

 (Emerson, 1935; Seevers, 1937; see Figs. 

 256, 260). Insects showing swollen soft 

 whitish bodies are termed physogastric. 

 Physogastry has appeared convergently 

 many times within the staphylinid beetles, 

 and also in several other groups of insects, 

 particularly among the termitophilous flies 

 (Fig. 260). 



The true symphiles are commonly 

 monoxenous, each species hving only in the 

 nests of one host species. Speciation of the 

 termitophiles often parallels speciation of 

 the hosts (Emerson, 1935). 



Some ants procure a large part of their 

 food from trophobiotic aphids and scale 

 insects. In a nimiber of instances an aphid 

 species is dependent upon the ants for its 

 existence, but in no instance is the ant de- 

 pendent upon a given species of aphid or 

 other trophobiont. 



Ants and termites benefit even less from 

 their symphiles. The symphiles are always 

 dependent upon their hosts, often during 

 different stages in their fife cycles, but 

 there is no reason to believe that these 

 social insects could not survive in the ab- 

 sence of their mutualistic symphiles. A de- 

 gree of mutuahsm has evolved between 

 the hosts and their myrmecophiles and 

 termitophiles, but the adaptations and the 

 benefits pertain much more to the guests 

 than to the hosts. The social life of the ants 

 and termites evolved in large part independ- 

 ently of the guests, which exploit the social 

 mechanisms and the socially controlled en- 

 vironment with only a moderate return to 

 their hosts. Again we find that the organ- 

 ism and the social supraorganism exhibit a 

 greater degree of cooperative integration 

 than is to be found in the interspecies 

 system. 



In drawing conclusions from our brief 

 survey of mutualistic relations, it is appar- 

 ent that evolution has guided numerous 

 organisms into a mutually beneficial coop- 

 eration, occasionally approaching the de- 

 gree of cooperation characteristic of the 

 parts of an individual organism. In addi- 

 tion to the more obvious and more extreme 

 cases discussed, innumerable organisms 



