PSYCHE. 



ON THE PRESENT STATE OF OUR KNOWLEDGE CONCERN- 

 ING CONTAGIOUS INSECT DISEASES. 



BY STEPHEN ALFRED FORBES, CHAMPAIGN, ILL. 



[Annual address of the retiring president of tlie Cambridge Entomological Club, 14 January 18S7.J* 



It seems to have been from the begin- 

 ning characteristic of the Cambridge 

 Entomological Club that, without under- 

 valuing taxonomic work, our members 

 have chosen for themselves the field 

 of biological entomology. Not con- 

 tent with the mere orderly arrangement 

 of the facts of insect structure in the 

 form of a comparative anatomy or of a 

 classification, we have been especially 

 interested, as a rule, I think, in the 

 attempt to philosophize such facts ; to 

 trace them to their causes and to follow 

 them to their effects. Sympathizing 

 heartily with this tradition of tlie so- 

 ciety, I have selected as the principal 

 topic of my address a subject requiring 

 us to consider the insect as a living 

 organism, in active vital relation to the 

 living organic world, — a subject which 

 frees us in great measure from the 

 technical harness of a classification, 

 and must even lead us quite beyond the 

 borders of entomological science. It 

 is one of those outlying subjects which 

 come within the range of a fro?itier 

 patrol^ interested in the foreign rela- 

 tions of insect life, whether those of 



* For bibliography accompanying this address see 

 the Bibliographical rbcord, p. 15. 



peaceful commerce or of depredation 

 and defence. Topics of this sort are 

 the food relations of insects and, allied 

 to this, the captivating subject of their 

 relations to flowers. Hei"e also belongs 

 the complex subject or group of sub- 

 jects included under economic ento- 

 mology ; and here comes, of course, the 

 special topic of this address, — that of 

 contagious insect disease. 



Contagious disease, wherever it has 

 been traced to its origin, has proved to 

 be a phenomenon of parasitism ; and 

 is included, consequently, under the 

 general head of the interactions of or- 

 ganisms. 



Rejecting the many cases of parasi- 

 tism which have no very serious effect 

 on the insect host, whether because the 

 parasites are in their nature insignifi- 

 cant, or because — as in the case of the 

 termites and many wood-eating spe- 

 cies — the organism seems to have 

 adjusted itself to continuous and 

 extraordinary parasitism ; and further 

 excluding — since the outlines of our 

 subject must at best be arbitrary — 

 parasitism by other insects, I shall 

 limit myself to cases of true disease of 

 an epidemic and uncommonly destruc- 



