INTERRELATIONS OF INSECTS 275 



or twice a day, for about twenty days. Thereafter they became in- 

 different to the violet rays." "The plasticity of the ants is remarkably 

 shown in their gradually learning to stay where they were never disturbed 

 by me, under rays from which their instincts at first withdrew them." 



Ants are sensitive not only to the different colors of the spectrum 

 but also to the ultra-violet rays, which produce no appreciable effect 

 on the human retina (though they induce chemical changes) . If obliged 

 to choose between the two, ants prefer violet to ultra-violet rays, as 

 Lubbock found. If, however, the ultra-violet rays are intercepted, by 

 means of a screen of sulphate of quinine or bisulphide of carbon, the ants 

 then collect under the screen in preference to under the violet rays. 



From lack of experience we can form no adequate idea as to the range 

 of sensation in ants or other insects. Ants can taste substances that we 

 cannot, and vice versa. They show no response to sounds of human 

 contrivance, yet many of them possess stridulating organs and organs 

 that are doubtless auditory; whence it may be inferred that ants can 

 communicate with one another by means of sounds. In rare instances 

 the stridulation of an ant can impress the human ear, as in a species of 

 Atta mentioned by Sharp. 



Experiments show that ants, as well as bees and wasps, find their 

 way back to the nest, not by a mysterious " sense of direction," but by 

 remembering the details of the surroundings, and in the case of ants, by 

 means of an odor left along the trail. 



In studying the habits of ants, the greatest care must be exercised in 

 order to discriminate between actions that may be regarded as purely 

 instinctive and those that may indicate some degree of intelligence. If 

 any insects show signs of intelligence, the social Hymenoptera do; but 

 in the study of this recondite subject, false conclusions can be avoided 

 only by observation and experimentation of the most critical kind. 



Hunting Ants. Some ants, as Formica fusca, live by the chase, 

 hunting their prey singly. The African "driver ants" (Anomma arcens), 

 although blind, hunt in immense droves, consuming all the animal refuse 

 in their way, devouring all the insects they meet, and not hesitating to 

 attack all kinds of vertebrates; these ants ransack houses from time to 

 time and clear them of all vermin, though they themselves are a great 

 nuisance to the householder. The Brazilian species of Eciton (Fig. 287, 

 B, C) have similar habits and are likewise blind, or else have but a single 

 lens on each side of the head. These insects hunt in armies of hundreds 

 of thousands, to the terror of every animate thing that they come across. 



