446 THE POPULAR SCIENCE MONTHLY 



tainly does not apply. That it is inapplicable to their phylogeny I am 

 not prepared to say, although I am unable to think of any non- 

 parasitic insects that show evidence of descent from parasitic species. 

 There can be no doubt, however, that parasites are still able to give 

 rise to new specific forms. This capacity is without doubt very feeble 

 or languishing in the permanent parasites of the tape-worm and 

 Sacculina types as compared with that of the insects. Indeed, there is 

 much evidence to show that in insects, parasitism, far from interfering 

 with the process of species formation, may actually have a tendency to 

 favor or accelerate it. Sharp estimates the number of species of para- 

 sitic Hymenoptera on our globe at 200,000, and of this vast number 

 probably 80,000 belong to a single family, the Chalcididse, of which 

 only some 6,000 species have been described. Another parasitic family, 

 the Tachinida?, belonging to the great order Diptera, seems to be in 

 such an active stage of species formation that the most diligent and 

 thoughtful students of the group flounder about in it with a dazed and 

 almost ludicrous helplessness. And not only is practically the whole 

 enormous group of moths and butterflies to be regarded as parasitic, 

 but the same is true also of untold legions of plant-lice, scale-insects 

 and beetles. Hyperparasitism, which may be regarded as a kind of 

 permutation of parasitism, must also be mentioned in this connection, 

 because it gives us a glimpse of the virgin fields which the holometabolic 

 insects, owing to their peculiar method of development, are beginning 

 to invade. 



I believe that the foregoing discussion of the peculiarities of insect 

 parasites adequately supports the view that these organisms are emi- 

 nently fitted to function in controlling the depradations of injurious 

 insects. That they can not be regarded as instruments of extermina- 

 tion is obvious from the fact that under natural conditions the com- 

 plete extinction of the host species involves the destruction of the para- 

 sitic species, unless the later is able to live on more than one host. Al- 

 though it is not improbable that during geological time such joint ex- 

 termination of host and parasite has repeatedly occurred, we are unable 

 to cite any case that has fallen under the observation of the entomol- 

 ogist. Purely local extermination of injurious hosts by their para- 

 sites has, however, been observed. 



Before bringing my lecture to a conclusion two matters must be 

 briefly discussed. One of these, which is mainly of theoretical interest, 

 relates to the development of the parasite's association with its host, 

 the other, of more practical significance, to the methods of greatest 

 promise in the study of insect parasitism. We need not stop to con- 

 sider cases of the tape-worm type which reach their hosts by chance. In 

 the two other types which I have distinguished, we have the associa- 

 tion with the host established through the initiative of the larval para- 

 site itself (Sacculina type) or through the parasite's mother (insect 

 type). While the former type does not seem to call for any special ex- 



