116 IVAlt TAAGARDII — ON THE CHEMOTROPISM OF INSECTS 



decomposition of albuminous substances. Furthermore, he lured Stomoxys 

 calcitrans into laying eggs upon a cotton rag drenched with valerianic acid, a 

 substance found in decomposing vegetable matter. 



Howlett then goes on to discuss the theoretical significance of these experi- 

 ments. He points out that our own sense of smell appears to be limited to 

 substances that possess a molecular weight of about 30 or more, whilst we are 

 unable to perceive any smell in substances with a less molecular weight, even if 

 they exercise an irritating influence upon our nasal mucous membrane. 



It is probable that the sense of smell in insects has been developed only in 

 certain directions, and that they are extremely sensitive to the odour of certain 

 substances, whereas others do not make any impression on them. In other 

 words, we may assume that every species is adapted merely to react to the odour 

 of certain organic substances, either positively or negatively, and that these are 

 probably substances which under normal conditions play a part in their life. 

 There ought, therefore, to exist a certain correlation between specialising with 

 regard to food and the sense of smell. The odour of organic matters again is 

 due to the occurrence in them of certain specific chemical combinations, e.g., 

 organic acids, amines, terebenes, phenols, glycosides, etc., which are characterised 

 by a certain structure and stratification of the atoms. 



Howlett did not know of Verschaffelt's work, but we discover immediately 

 how remarkably well the two researches complete each other, and even if the 

 chain of evidence is not fully perfect as to which factor determines the females of 

 herbivorous species in the selection of the food-plant for their larvae, we may 

 yet be permitted to draw the following hypothetical conclusions. The ovi- 

 positing of the females is guided, even in those cases where the larva has a diet 

 different from that of the adult, by chemotropism, and this latter is correlated 

 with the food of the larva in such a manner that, if the larva is a univor, it is only 

 one specific organic union in the food-plant which attracts the female. If the 

 food of the larva consists of several species of one and the same genus, or of 

 different genera within one or several families, then it is an organic union or group 

 of such, common to all these, to which the species reacts positively. 



As an example of how strongly specialised the sense of smell can be in insects, 

 we may mention the ants. It is a well known fact that the members of the same 

 species attack each other with the greatest fury if they only come from different 

 nests, and this has been proved by experiments to be due to each member of a 

 community of ants possessing its special nest-odour, by the aid of which it is able 

 to distinguish friend from foe. 



After this short account of what we know of the chemotropism of insects, we 

 may pass on to the question as to whether it can possess any importance for 

 economic entomology. From a theoretical point of view, we ought, if the above 

 conclusions are valid, to be able to discover in the chemotropical reactions of 

 insects in many cases a superb weapon in the fight against noxious species. For 

 it has always been considered that prevention is better than cure, and of all 

 methods in preventing devastation that is undoubtedly nearest to the ideal in 

 which we succeed in capturing the females ere they have had an opportunity of 

 ovipositing. And we shall probably be able to effect this if we succeed in 

 isolating the organic substances in the food-plants of the larvae, towards which 



