INSECT PARASITISM 443 



lids. Dimly foreshadowed in this method of development are the more 

 complete modern types of insect metamorphosis, which have their 

 morphological origin, as we now know, in a doubling of most of the 

 rudiments of the organs in the embryo. On hatching, one set of these 

 rudiments develops immediately into the larval body, while the other 

 set remains in abeyance in the form of minute germinal centers, or 

 histoblasts, from which the body of the adult will be fashioned during 

 the quiescent pupal stage. The higher insects are therefore beautiful 

 examples of double personality, much more perfect examples of this 

 phenomenon, in fact, than any that has been discovered in man. The 

 larval insect is, if I may be permitted to lapse for a moment into an- 

 thropomorphism, a sluggish, greedy, self-centered creature, while the 

 adult is industrious, abstemious and highly altruistic, concentrating its 

 activities on reproduction and the dissemination of the species. Unlike 

 ourselves, who are Mr. Hydes and Dr. Jekylls in varying degrees, for 

 brief alternating periods in our lives, or even simultaneously, the youth- 

 ful insect sows its wild oats with a vengeance as a glutton or even as an 

 assassin and then experiences a change of heart and reforms for good 

 and all. 



Parasitism must have been very easily grafted on to such a sharply 

 dichotomic method of development as that of the holometabolous in- 

 sects, for the larvse of the predators are already much inclined to sloth 

 and gluttony when the food supply is abundant, and comparatively 

 little modification would be required to convert them into parasites. 

 But the same peculiarities of metamorphosis have also made the holo- 

 metabolic insects ideal hosts. We have already seen that insects, as a 

 rule, are themselves not only parasitic during larval life, but also pre- 

 fer larvae as hosts. It is not improbable that this is the primitive, and 

 that parasitism on the egg, pupa or adult is a secondary, or derivative 

 condition. The real secret of both host and parasite being larvae lies 

 in the peculiar significance of anabolism in this stage. The host ac- 

 cumulates great quantities of fats and proteids as a so-called " fat-body," 

 which is of little or no immediate use to the organism itself, but is stored 

 up to be utilized during metamorphosis. This fat body may, therefore, 

 be devoured by the parasite and converted into its own fat-body with- 

 out seriously injuring the host. Furthermore, the fact that the parasite, 

 too, stores up its food in the form of a fat-body instead of at once turn- 

 ing it over to its gonads and becoming reproductive, accounts for the 

 striking differences between the insect parasite, on the one hand, and 

 the tape-worm and Sacculina, on the other. The few exceptions among 

 insects, such as the female Strepsiptera, in which the food taken by the 

 larval parasite from its host is soon turned over to the gonads and used 

 for reproduction, leads to a permanent parasitism resembling that of 

 the tapeworms or the adult Sacculina. The larva is at once arrested in 

 its development and begins to reproduce by pasdogenesis. We may con- 



