92 JOURNAL OP ECONOMIC ENTOMOLOGY [Vol. 3 



Total absence of any such instinct makes the prevalence of super- 

 parasitism wholly dependent upon and governed by the law of chance. 

 If it could be shown to be the rule, a calculation of the probabilities 

 of the occurrence of superparasitism would be a comparatively simple 

 problem in mathematics. 



In so far as those parasites are concerned which, like certain of the 

 Tachinids and apparently of a few among the Hymenopterous para- 

 sites, deposit their eggs or young larvae upon the food or the food 

 plant of the host, it must be conceded that the laws of chance apply 

 with scarcely any modification. A Taehinid, which deposits its eggs 

 upon the foliage of trees infested by caterpillars of its favored host 

 t(. be eaten by them, is trusting wholly to chance and to nothing else. 

 The fact that the parent fly is attracted to the vicinity of the host 

 before depositing her eggs affects the matter not at all, because no par- 

 ticular individual is selected or can be selected for the attack. 



In the case of those which attack a selected host it is easily conceiv- 

 able that a highly developed instinct might enable the parent to gov- 

 ern her selection, but that such is not the universal rule is as easily 

 demonstrated. Until very recently it could have been stated without 

 reserve that "not a single parasite among the many studied at the labor- 

 atory indicated in any way the possession of such discretionary powers. 

 At the present time the statement cannot be made thus unrestrictedly, 

 but it can still be said of the vast majority of the species which have 

 been the subject of more or less thorough study. In a very few species, 

 relatively, faint indications of such prescience on the part of the par- 

 ent females are apparent. 



In the opinion of some, mathematics and entomology exemplify, 

 respectively, the most exact, and one of the least exact among the sci- 

 ences, and the writer is well aware of the dangers which attend any 

 attempt to combine the two, but if, as must be conceded in some in- 

 stances, and as may yet prove to be the fact in all, the prevalence of 

 superparasitism is determined by chance alone, it is interesting to 

 speculate a bit on what might occur under fixed conditions. 



Given 100 insects, suitable as hosts, and equally attractive to a 

 given species of parasite, inhabiting a restricted territory (call it an 

 island), and let each of them be equally exposed to parasitic attack. 

 Let one fertile female parasite capable of depositing several hundred 

 eggs be given free access to her favored host under these conditions. 

 Suppose, what is perfectly possible, that the parasite is unable to 

 choose between hosts which are healthy and those which are already 

 parasitized. 



With the deposition of the first egg, a parasitism of 1 per cent re- 



