22 
BULLETIN 640, U. S. DEPARTMENT OF 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 larvae 
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 
larvae 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 larvae 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 larvae. 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 larvae 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 therag. (Authors' 
illustration.) 
