268 THE BIOLOGY OF INSECTS 



to the outer world. Those students of life-relations who 

 doubt that such insects are blind because they dwell in 

 caves might be willing rather to believe that they are found 

 in such haunts because they are blind. 



Though seeing Httle or nothing, springtails are, however, 

 often provided with organs for receiving other kinds of 

 sense-impression. Their bodies are frequently clothed 

 with long tactile bristles ; the impression conveyed by 

 means of these must be of value in guiding the steps of 

 insects living in darkness and obscurity. On the feelers 

 are peg-Hke or bladder-like organs probably adapted to 

 receive chemical stimulation and to guide the creatures in 

 the search for food. On either side of the head, between 

 the eyes and the base of the feeler, there is found in many 

 springtails a problematical '' post-antennal " organ con- 

 sisting of a set of deUcate areas or processes of cuticle, often 

 arranged in form of ^ circle, an ellipse, or a rosette. To this 

 goes a branch of the optic nerve, the fibres whereof may 

 receive through it impulses due to vibration or chemical 

 stimulation (Fig. 70, h). 



Springtails as a group are very small, and their body- 

 structure is remarkable among insects, because the number 

 of abdominal segments is reduced from the usual ten or 

 eleven to six. This contraction and shrinkage brings about 

 a decrease in size w^hich enables the insects to subsist on a 

 relatively small food-supply ; hence they can survive and 

 propagate their race in apparently unpromising underground, 

 arctic and alpine haunts where larger and more elaborately 

 organised insects would speedily perish. They are able to 

 hold their own under conditions which would be fatal 

 to the large, strongly built and dominant butterflies and 

 dragon-flies that we were previously considering as occupiers 

 of wide territory. 



It is now time to turn to examples of insects adapted 

 to special kinds of environments, and the first feature for 

 discussion in this connection is the general form of the 

 creature. An insect's body is made up of a number of 

 segments, arranged in series, one behind the other, and the 



