34 WHEELER— THE PARASITIC ACULEATA. 



and provisioning them with paralyzed insects merely elaborate the 

 same fundamental behavioristic theme or pattern, the main features 

 of which were also retained by the solitary bees even after they had 

 ceased to capture insect prey and had become pollenivorous and 

 nectarivorous. The social wasps and bees have merely modified 

 certain details in the behavior of the solitary species. Among the 

 ants the modifications are more profound, but the most primitive 

 subfamily, the Ponerinse, still exhibits many Sphecid traits. We 

 may assume, therefore, that the ancient parasitoid habits of the Ich- 

 neumonid ancestry still abides as a latent, phylogenetic memory, or 

 mneme, in the constitution of the whole Aculeate group. Hence it 

 is not surprising that this mneme can be revived in response to such 

 recurrent external and internal stimuli as- dearth of food and ur- 

 gency of oviposition. Under these conditions the solitary Aculeate 

 readily becomes parasitic and reverts to a type of behavior essen- 

 tially like that of the Mutillidae, Thynnidse and Chrysididse. In the 

 parasitic social Aculeates new behavioristic modifications have de- 

 veloped as the result of the complex and peculiar living environ- 

 ment presented by the social habit of the host, the trophallactic rela- 

 tions of the mother insect and her ofl^spring and the existence of a 

 worker caste. 



The general conclusions that may be drawn from the foregoing 

 survey of the parasitic Aculeata in particular and of insect parasites 

 in general may be stated as follows : 



1. We may distinguish two intergrading types of parasitism 

 among insects. One of these is true parasitism and is represented 

 by the lice, fleas, Mallophaga. many Diptera (CEstridse, Pupipara) 

 and some Hemiptera, which live on mammals and birds and do not 

 destroy their hosts. The other is parasitoidism, which is really a 

 refinement of predatism and is eminently characteristic of large 

 sections of the Hymenoptera and Diptera (Tachinidas). It leads 

 sooner or later to the death of the host. The difiterence between 

 the two types is largely due to dififerences in the size and vigor of 

 the hosts. 



2. Parasitoids are of two classes, one of which is best repre- 

 sented by the so-called Parasitica among the Hymenoptera and the 

 Tachinidje among the Diptera, which have no genetic relationship 



