EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



713 



Carteria or Chlamijdomonas) . The organs 

 of nutrition degenerate, so that the flagel- 

 late functions as an organ in the animal. 

 The flatworms live on the products of the 

 symbiotes, but in a later stage digest the 

 flagellates, lay eggs, and die. Thus, in this 

 instance, the animal is dependent upon the 

 symbiotes for its existence, but the flagel- 

 lates are also free-living, and do not seem 

 to have evolved toward dependence upon 

 Convoluta. 



Gilbert (1944) demonstrates that alga- 

 inhabited eggs of Ambystoma maculatum 

 have lower mortality, develop more 

 rapidly, and hatch earlier than do alga- 

 '^ree eggs. The larvae from alga-inhabited 

 eggs are both larger and more advanced. 

 The algae, in this case, multiply within the 

 egg envelopes, and apparently benefit from 

 the association. Gohar (Yonge, 1944) has 

 given experimental evidence that the corals 

 belonging to the family Xeniidae (Alcyona- 

 cea) are incapable of predatory activity 

 and starve in the presence of animal food 

 in the dark. In the light, animals harboring 

 zooxanthellae are healthy and vigorous, 

 even without animal food. 



Mutualism between these symbiotes and 

 their animal hosts is a possibility, but it is 

 questionable whether the symbiotes have 

 undergone evolutionary adaptation to their 

 hosts. The possibility of preadaptive mutua- 

 lism has been demonstrated between algae 

 (Chlorella pyrenoidosa) and chick connec- 

 tive tissue cells in tissue culture (Buchs- 

 baum and Buchsbaum, 1934; Buchsbaum, 

 1937a; see also p. 246). 



Fungi are actively cultivated by several 

 groups of insects, including the ambrosia 

 beetles (some Scolytidae and all Platypodi- 

 dae), the fungus-growing ants (Attini), 

 and the fungus-growing termites (Macro- 

 termitinae) (Wheeler, 1907, 1923, 1928b; 

 Hegh, 1922; Heim, 1940; Weber, 1941, 

 1945; Brues, 1946). The ambrosia beetles 

 are cosmopolitan, the fungus-growing ants 

 are found only in the tropical and warm 

 temperate regions of the New World, and 

 the fungus- growing termites are confined to 

 the warm regions of the Orient and Africa. 

 The fact that the genera within each of 

 these insect groups are phylogenetically 

 related indicates that the insects have 

 evolved in relation to the fungus-growdng 

 behavior (p. 246). 



The ambrosia beetles burrow in wood. 



but do not eat it. These fungus-eating 

 beetles have weak maxillae, in contrast 

 with the strong maxillae of their bark-eat- 

 ing and wood-eating relatives. They are 

 somewhat social in their behavior, adults 

 and young living together in connected 

 burrows. The adult beetles feed the larvae 

 and remove larval excrement. Large larvae 

 sometimes protect small larvae. In certain 

 genera, the larvae are located in small 

 pockets in the sides of the burrows. The 

 beetles "plant" the spores of the fungi in 

 the new burrows after carrying the spores 

 from the parental burrow. Brushes of hairs 

 on the front of the head are a common 

 adaptation for transporting the spores. 

 Some beetles carry fungal material in their 

 stomachs. After "planting," the fungi grow 

 upon the sides of the chambers in the sap- 

 wood. 



The fungi are reported to be specific for 

 particular beetles, irrespective of the kind 

 of wood in which the burrows are made 

 The fungi are partly nourished by the ex- 

 crement of the larvae. These facts indicate 

 that the fungi have also evolved in rela- 

 tion to the beetles, and an evolution of 

 mutualism may be presumed (Chamber- 

 lin, 1939). 



The attine ants indicate a phylogenetic 

 sequence in the development of their fun- 

 gus-growing behavior. The more primitive 

 genera use the gathered excrement of 

 caterpillars and beetles for a substrate in 

 which the fungus mycelium grows, and 

 from which they harvest mycelial tufts 

 or spherical bodies (bromatia). The 

 more specialized genera (i.e., Atta, Fig. 

 253) cut pieces of leaves from growing 

 trees and transport these leaf fragments to 

 their underground nest, where minute 

 workers, which never leave the nest, make 

 a spongy substrate from the leaves upon 

 which the fungus grows. 



A pellet of fungus mycelium is trans- 

 ported by the new queen in an infrabuccal 

 pocket. After digging a burrow in the 

 ground after her nuptial flight, the female 

 expels the fimgal pellet. Hyphae soon de- 

 velop, and the growing fungus is manured 

 by anal droplets. The worker ants develop 

 in about forty days or more and soon take 

 over the care of the fungus bed. The first 

 matiire workers, a few days after their ap- 

 pearance, break out of the nest and imme- 

 diately bring in pieces of leaves which are 



