Motes on scientific research. 127 



organs are degenerated, for instance it has been proved in reference to such keen- 

 sighted birds as vultures that the sense of smell plays no part in the search of food. 

 For many species of mammalia, the sense of smell is of paramount importance, some 

 of them even being provi,ded with glands in which odoriferous substances are secreted. 

 Bats are more dependant on their sense of touch than on smelling organs, their wing- 

 membranes and the external parts of their ears being especially susceptible. Moles, 

 hedgehogs, martens and some other small mammalia can be designated as smelling 

 animals par excellence. Dogs, of course, must also be mentioned in this category and 

 in point of fact, they seem to have a special sensitiveness for substances which are 

 not easily volatilized as well as for adhesive and putrid odours. Training experiments 

 have proved that they can even distinguish similar smells which are beyond their general 

 experience, for instance nitrobenzene from benzaldehyde. 



Henning paid special attention to ants which he was in the habit of observing in 

 their natural surroundings and mode of life in distinction to the majority of other 

 scientists who were wont to study their habits after enclosing them in confined narrow 

 spaces so to speak in "internment camps." His researches were restricted mainly to 

 the habits of the red wood ant formica rufa, L. in the forests of Taunus, Spessart, Black 

 Forest and, the Vosges. 



It goes without saying that such investigations must lead to better and more accurate 

 results when such experiments concerning their smelling sense are performed in the 

 open air than in narrow cages which are quickly permeated by odours. However, one 

 must, in making tests as to sense of smell in ants always pay attention to the fact 

 that their sense of sight plays a part thereby. It was already known that ants during 

 their course automatically leave an odoriferous spoor behind them by dropping a secretion 

 on the ground out of their anal glands. The odour of this substance proves it to be 

 formic acid, and Henning's experiments with artificially produced formic acid trails 

 showed that ants during their wanderings are mainly influenced by their sense of smell. 

 However, other odours also, especially such as resemble formic acid in the smelling 

 prism, were apparently suitable as a trail-smell for ants, for instance formic aldehyde 

 and formiates in general. Scents, however, which belong to other groups of the smelling- 

 prism efface the spoor, for instance camphor, benzaldehyde, rosemary and nicotine. 

 If pine needles or other nest-building materials are painted with odoriferous substances, 

 they are either carried to the nest or left alone or removed from the nest, according 

 to the odour of the compound. A single ant is not capable of producing a sufficiently 

 strong-smelling spoor for others to be able to follow it. Several ants are required in 

 order to produce spoors enabling them to find their way home. This is the origin of 

 their state formation. It is an outcome of the structure of their antennas and is easily 

 understood without any theories concerning social instincts, or special intelligence. 

 The sole explanation is that ants show a strong reaction for formic acid which they produce 

 themselves to which may be added the fact that this compound has a penetrating odour. 



Ants recognise eack other neither by sounds nor by rhythms of touch or the sense 

 of sight, but solely by their smell. If an ant is painted with a strange perfume, not 

 belonging to their range of experience, for instance with pineapple oil, it is treated as 

 an enemy by its companions or even killed, and then dragged into the nest as a prey. 

 The painted insect often tries to escape or to purify itself from the strange odour. 

 Animals which have been infected in the same fashion do not fight against each other. 

 Odours which are not far removed from the formic acid range of the odour prism, 

 such as putrid or empyreumatic ones, and also of turpentine and pine needle oils have 

 in diluted concentrations the effect that such insects are avoided by their fellows. 



