22 



BULLETIN 640, U. S. DEPARTMENT OE AGRICULTURE. 



Fig. 20.— Ripe grapefruit showing copious gummy secretions that 

 may, though more often do not, follow attack by Mediterranean fruit 

 fly. (Original.) 



that they fall to the 

 ground. Why, then, 

 are Chinese oranges 

 and tangerines easily 

 infested with larvse 

 in the pulp whereas 

 lemons, grapefruit, 

 and oranges ward off 

 fatal attack either 

 entirely or until after 

 they are overripe ? 



The reason is that 

 a great mortality oc- 

 curs among the eggs 

 and newly hatched 

 larvse in citrus fruits 

 having a thick peel- 

 ing or rind. In Chi- 

 nese oranges the peel is so thin that the fruit fly can lay her eggs 



through it into the pulp itself or between the pulp and the rind, 



so that the larvse on hatching can at once begin 



to feed on the pulp, As a result the pulp of 



the Chinese orange (fig. 18) is almost always 



infested with larvse. The case is different with 



lemons (fig. 19), grapefruit (fig. 20), and ordi- 

 nary seedling oranges. In these fruits the peel 



is so thick that the fly must deposit her eggs in 



the outer part of the white rag as illustrated in 



figure 21. In making the puncture she often 



ruptures an oil cell in the rind, and the oil thus 



liberated kills the eggs. But if the eggs are laid 



between oil cells, the young larvae have difficulty 



in making their way through the rag to the pulp, 



and a very high percentage of them die in the 



attempt. 



Then, too, a gall-like hardening develops quite 



rapidly about the egg cavity in oranges, grape- 

 fruit, and lemons, as indicated by the darkened 



area about the egg cavity in figure 21. This 



hardening often makes of the cavity a prison 



from which the young larvse can not escape and 



in which they are literally starved to death. 

 It thus happens that the larvae that succeed 



in entering the rag of the peel from the egg 



cavity are able to reach the pulp of grapefruit 



Fig. 21. — Section of grapefruit 

 rind, showing two egg cavi- 

 ties, one in cross section. 

 Drawing made one week af- 

 ter fruit was picked. Note 

 conical elevation about egg 

 cavities left by withering of 

 rind; also thickened walls of 

 egg cavity and single larval 

 channel in the rag. (Authors 1 

 illustration. ) 



