206 A BOOK OF INSECTS 



problem, adduced physiological evidence which indicates 

 that the agamic generation originally possessed males ; 

 but why complete parthenogenesis has supervened, and 

 for what reason alternation of generation became estab- 

 lished (in preference to the simple double-brood principle 

 which obtains commonly among other insects) are problems 

 for the future. 



The associations of ants with plants are sometimes so 

 intimate that they suggest actual symbiosis, or mutual 

 benefit. Belt found several species of Acacia in Nicaragua 

 and the Amazon valley which are admirably adapted to 

 the needs of certain small ants of the genus Pseudomynna. 

 The trees in question bear large, hollow thorns which the 

 ants penetrate by boring a hole near the apex ; and in 

 these thorns the insects habitually live and breed. Further, 

 the tree supplies food as well as shelter for its insect 

 population. Glands at the bases of the leaf-stalks secrete 

 a sugary fluid, while many of the leaflets are tipped with 

 small sausage-shaped masses known as " Belt's bodies." 

 These are rich in albumen and are easily broken off. 

 They are systematically collected and eaten by the ants, 

 which appear to subsist exclusively upon the two kinds of 

 food which the tree provides. Other ants are similarly 

 attached to young Cecropia plants in Central America 

 and Brazil. In these trees there is a small depression just 

 above the insertion of each leaf- stalk where the rind is 

 much thinner than elsewhere. A female ant effects an 

 entrance at this weak spot, and sets up housekeeping 

 within an intemodal chamber of the stem. Meanwhile 

 the hole becomes closed by an excrescence from its 

 margin, and the ant is held prisoner until she has reared 

 about a dozen workers. The latter then re-open the orifice 

 from within, and keep it open, so that the members of 

 the community may come and go as their needs dictate. 



