482 Saw-flies, Gall-flies, Ichneumons, 



well marked, and we have come to rely on the effectiveness of the parasite spe- 

 cies, Pimpla behrendsii, in overtaking by rapidly succeeding generations the 

 increasing hosts of the pest, and in checking it before the actual realization 

 of what is not infrequently threatened, the killing of all the live-oaks in 

 certain regions of the state. 



An interesting phenomenon in the biology of these parasites is that of 

 hyperparasitism. It frequently happens that the parasites of a given host 

 are themselves parasitized by other (usually smaller) parasitic Hymenoptera, 

 while even these secondary parasites are not infrequently parasitized in 

 their turn by still other species. Indeed some 

 cases are known in which the tertiary parasites 

 are infested by a fourth or quaternary species. 

 An excellent example of hyperparasitism is re- 

 vealed by Fiske's careful study, already referred 

 to, of the hymenopterous parasites of the Ameri- 

 can tent-caterpillar. Twelve species of parasitic 

 hymenoptera infest these caterpillars; of these 

 twelve, six are themselves attacked by parasites 



(secondary), of which as many as six species may 



FIG. 679. Ophion purga- v . '/ , . , , . 



turn, an ichneumon-para- attack a single species of the primary parasites. . 



site of army-worms. (After Among these secondary parasites are not only 

 Lugger: natural size.) ,. ,. , f .-, i 



species distinct from the primary parasites, but 



some of the primaries parasitize each other as well as the caterpillars. Of 

 the secondary parasites, four species are in turn parasitized by other (ter- 

 tiary) parasites, of which three species have been noted, one occurring also 

 as a secondary parasite; and finally, one of these tertiary parasites is 

 infested by another of the tertiary group, which in this instance becomes 

 a quaternary parasite. Thus the old rhyme of 



"Great fleas have little fleas 

 Upon their backs to bite 'em, 

 And little fleas have lesser fleas, 

 And so ad infinitum," 



is often realized in the biology of the parasitic hymenoptera. 



Most interesting questions are suggested when we consider the unusual 

 life-conditions that may, and often do, obtain in parasitism. Lying immersed 

 in the blood-lymph of the body-cavity of the host, how does the parasitic 

 larva breathe, excrete, moult, etc. ? The process of feeding consists prob- 

 ably for the most part simply in the taking up of the food from the host's 

 blood, in many cases probably as much through the skin, by osmosis, as through 

 the mouth itself. With some species, however, there seems to be a definite 



