Wasps, Bees, and Ants 
481 
their case is always limited. Still, from a single scale-insect hardly more 
than J inch long a dozen and more tiny parasites have been bred. 
A question of interest is that regarding how many individuals of a single 
host-species may, in a given locality, be parasitized. For the effectiveness 
of any parasite in keeping an injurious 
insect pest in check depends, of course, 
on its relative prevalence. Touching 
this may be quoted Fiske’s estimate 
that less than 20 per cent of the Ameri¬ 
can tent-caterpillars, which are at¬ 
tacked by a total of twelve species 
of parasites, are destroyed annually 
in the vicinity of Durham, N. H. 
On the other hand I have found 
a constant parasitization of about 
two-thirds of all the pupating indi¬ 
viduals of the California oak-worm 
moth (Phryganidia californica) in 
years of its abundance in the vicin¬ 
ity of Stanford University, and this 
by the single ichneumon-fly, Pimpla 
behrendsii. 
The success of any form of para¬ 
sitism in any one locality in a given 
Fig. 677.—Larvse of certain curious hymen- 
opterous parasites; at left, Platygaster 
instricator; at right, P. herricki , which 
live in the alimentary canal of Cecidio- 
myid flies, ant, antennae; lb, labrum; 
md, mandible; li, labium; /, l 2 l 3 , legs; kr, 
clawed processes; /, lobe-like processes; 
hf, posterior processes. (After Kulagin; 
much enlarged.) 
season brings up also the interesting matter of host and parasite “ cycles.” 
It is obvious that in the face of a scarcity of host individuals the dependent 
parasitic species are bound to find difficulty in 
maintaining themselves; and conversely, that 
with the increase of the host in numbers “ good 
hunting” arrives for the parasites. But the 
good times bring hard ones in their train; 
for when hosts are abundant the parasites 
increase so rapidly in numbers (having usually 
several generations to the host’s one) as soon 
to overcome and sometimes almost extinguish 
in any given locality the host-species, which 
of course, means starvation for the parasite 
Fig. 6 :&.-Pimpla sp„ an ichneu- d new j of life f or the host. Thus are 
mon-fly. (Twice natural size.) 
brought about succeeding “cycles” of host 
and parasite abundance intimately associated with each other. In the case 
of the California oak-worm moth already referred to, a serious pest (when 
abundant) of the beautiful live and white oaks of California, the cycles are 
