372 Relations between Species 



berlin, 1939). Sometimes the development of the fungi is sufficiently 

 luxuriant to clog up the tunnels completely, and the beetles are killed 

 unless they can eat their way out faster than the fungi grow! 



The perpetuation of this mutualistic relation between insects and 

 fungi is assured by elaborate structures and reactions by which the 

 fungi are transferred to the habitats of new generations. The fungus- 

 growing beetles, for example, possess certain external or internal 

 structures by means of which they carry spores or fragments of the 

 fungi from the old burrow, where the larvae were hatched, to the site 

 of a newly founded colony. Among the leaf-cutting ants the virgin 

 queen carries a pellet of fungus in a pocket below the mouth and 

 deposits the inoculum in her new bridal chamber. These arrange- 

 ments by the host for transmission are strong evidence that the guest 

 species are beneficial, and hence are symbionts. With parasitism, the 

 problem of transmission is always arranged for by the parasite in one 

 way or another and is resisted by the host. A more detailed consid- 

 eration of these relationships among insects will be found in the sum- 

 mary by Steinhaus ( 1946 ) . 



The pollination of flowers by bees, moths, and butterflies, and oc- 

 casionally by hummingbirds, is another manifestation of mutually 

 beneficial symbiosis, but one in which the species concerned may be 

 in contact for only a few seconds. The insect derives food from the 

 nectar, or other product of the plant, and in return carries pollen from 

 the anthers of one flower to the stigma of another, thus ensuring cross 

 pollination. The coordination of the elaborate behavioristic and 

 anatomical adaptations that have been evolved in the insect and in 

 the flower in relation to this cooperative activity is truly remarkable. 

 The flowers open, producing odors and displaying colors that attract 

 suitable insects, only at the time when they are sufficiently mature for 

 fertilization. In various ingenious ways the flower is so shaped that 

 the insect cannot get its food without dusting the stigma with pollen 

 carried from another flower and then picking up fresh pollen to be 

 carried to the next flower. The reactions and the structure of the 

 pollinating insects are correspondingly adapted. Bees, for example, 

 continue to visit the same species of flower as long as a supply of 

 mature blossoms is available and thus avoid mixing pollen from dif- 

 ferent species. 



The mutual dependence of insect and flower is frequently highly 

 specific and sometimes involves the reproductive cycle of the animal 

 as well as that of the plant. The peculiarly enclosed flowers of the 

 commercial fig are pollinated only by wasps of the genus Blastophaga, 

 and special floral structures called caprifigs provide the only place in 



