481 



hf. 



their case is always limited. Still, from a single scale-insect hardly more 

 than I 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 calijornicd) in 



, . , , ..i . . FIG. 677. Larvae of certain curious hymen- 



years of Its abundance in the vicin- ojjterous parasites; at left, Platygaster 

 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 

 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 

 and a new lease of life for the host. Thus are 

 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 



instricator; at right, P. herricki, which 

 live in the alimentary canal of Cecidio- 

 myid flies, ant, antennae; Ib, labrum; 

 md, mandible ; li, labium ; /, 1 2 l y legs ; kr, 

 clawed processes; /, lobe-like processes; 

 hf, posterior processes. (After Kulagin; 

 much enlarged.) 



FIG. 678. Pimpla sp., an ichneu- 

 mon-fly. (Twice natural size.) 



